Book - Manual of Human Embryology 11: Difference between revisions

From Embryology
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" In many mammals the sphenoid remains permanently divided into two parts, a presphenoid, which comprises the apical end of the body and the lesser wings, and a postsphenoid, which comprises the sella turcica, the great wings, and the pterygoid processes.
" In many mammals the sphenoid remains permanently divided into two parts, a presphenoid, which comprises the apical end of the body and the lesser wings, and a postsphenoid, which comprises the sella turcica, the great wings, and the pterygoid processes.


==OS ETHMOIDALB==
===Os Ethmoidale===


The ethmoid bone arises from one medial and two lateral primary and from several secondary centres in the cartilaginous nasal capsule. The ossification of the posterior cupola of the cartilaginous nasal capsule in man as  
The ethmoid bone arises from one medial and two lateral primary and from several secondary centres in the cartilaginous nasal capsule. The ossification of the posterior cupola of the cartilaginous nasal capsule in man as  
the ossiculum Bertini has been deibr™
the ossiculum Bertini has been described in connection with the sphenoid bone. In the quadrupeds this portion of the nasal capsule is ossified in conjunction with the ethmoid (Gaupp, 1906). ub^iUS  
scribed in connection with the sphe■■="'
In each lateral wall of the nasal capsule a centre appears in the fifth to sixth fetal month. It gives rise to the lamina papyracea, and in the eartiiaginoui.i  
noid bone. In the quadrupeds this portion of the nasal capsule is ossified in conjunction with the ethmoid (Gaupp, 1906). ub^iUS  
In each lateral wall of the nasal capsule a centre appears in the fifth  
to sixth fetal month. It gives rise n.cm.ri^'^^iiir'
to the lamina papyracea, and in the eartiiaginoui.i  
seventh and eighth months ossification „ ^'°^^'>— ('^f"^ ''""^'?i ^^  
seventh and eighth months ossification „ ^'°^^'>— ('^f"^ ''""^'?i ^^  
, . . .? , J IV 1 Renault. f torn Poiri«f.Poiner«.dCharpy,  
, . . .? , J IV 1 Renault. f torn Poiri«f.Poiner«.dCharpy,  
Line 2,586: Line 2,582:




Ossification begins late in the first year^^ independently in the superior portion of the nasal septum (lamina perpendicularis). It extends into the crista galli, the cribriform plate, and the lamina perpendicularis. Sappey and Poirier, following Rambaud and Renault, describe several centres on each side of the upper margin of the lamina perpendicularis at the base of the crista galli. From these centres ossification extends successively to the crista galli, the lamina cribrosa, and the lamina perpendicularis. In the crista galli in the second year a secondary nucleus arises. Ossification of the process is not completed before the fourth year. In the second year two accessory nuclei appear in the anterior part of the lamina cribrosa. By the sixth year the lateral parts of the ethmoid become united to the medial part (v. Spee and most authors). ^^ Ossification of the ethmoid is not completed until the sixteenth year. Synchondrosis exists between the lamina cribrosa and the sphenoid until toward puberty. About the fortieth to forty-fifth year the lamina perpendicularis becomes united to the vomer.  
Ossification begins late in the first year^^ independently in the superior portion of the nasal septum (lamina perpendicularis). It extends into the crista galli, the cribriform plate, and the lamina perpendicularis. Sappey and Poirier, following Rambaud and Renault, describe several centres on each side of the upper margin of the lamina perpendicularis at the base of the crista galli. From these centres ossification extends successively to the crista galli, the lamina cribrosa, and the lamina perpendicularis. In the crista galli in the second year a secondary nucleus arises. Ossification of the process is not completed before the fourth year. In the second year two accessory nuclei appear in the anterior part of the lamina cribrosa. By the sixth year the lateral parts of the ethmoid become united to the medial part (v. Spee and most authors). ^^ Ossification of the ethmoid is not completed until the sixteenth year. Synchondrosis exists between the lamina cribrosa and the sphenoid until toward puberty. About the fortieth to forty-fifth year the lamina perpendicularis becomes united to the vomer.
 


==CONCHA INFERIOR==
==CONCHA INFERIOR==

Revision as of 21:29, 26 August 2012

   Manual of Human Embryology I 1910: The Germ Cells | Fertilization | Segmentation | First Primitive Segment | Gastrulation | External Form | Placenta | Human Embryo and Fetus Age | Ovum Pathology | Integument | Skeleton and Connective Tissues | Muscular System | Coelom and Diaphragm | Figures | Manual of Human Embryology 1 | Manual of Human Embryology 2 | Franz Keibel | Franklin Mall | Embryology History

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XI. Development of the Skeleton and of the Connective Tissues

By CHARLES R. BARDEEN, Madison, Wis.


General Features

In the bodies of most living things certain tissues are differentiated for the purpose of passively supporting or protecting the physiologically more active structures. These tissues are characterized in the higher vertebrates by the predominant amount of •extracellular substance, usually fibrous in nature, which, in large part at least, is differentiated during embryonic development from the peripheral portions of branched anastomosing cells. according to the nature of the intercellular substance the supporting tissues are subdivided into white fibrous and yellow elastic tissues, reticulum, cartilage, and bone.


In early embryonic stages the branched anastomosing cells which compose the supporting tissue or mesenchyme, form an extensive continuous framework. Certain parts of this framework are differentiated into the definitive skeleton and other parts into connective-tissue structures which protect and support the parenchyme of the various organs of the body and attach these organs to the skeleton.


The development of the various connective-tissue structures may be considered from two aspects, that of histogenesis and that of organogenesis. The histogenesis of the connective tissues has been most carefully studied in the lower vertebrates, in which the cells are large and the conditions are relatively simple. Organogenesis has been more carefully studied in man than in any of the lower forms. The histogenesis pf the connective tissues is apparently similar in the different vertebrates. Study of the histogenesis of these tissues in man and the higher mammals in general serves to confirm the results found in the lower vertebrates. Organogenesis is peculiar for each species, although there are fundamental similarities to be observed in related forms. We shall first give a brief account of the histogenesis of the connective tissues with especial reference to man and then treat with more detail the morphogenesis of the human skeleton. The specific development of the intrinsic supporting connective-tissue framework of the various organs is most conveniently taken up in connection with each of these organs, and will therefore not be attempted liere.

PART I.

Histogenesis of the Connective Tissues

(a) Early Mesodermic Syncytium.

In the youngest human embryos which have been described there is present a well-developed layer of tissue composed of branched anastomosing cells. This tissue layer surrounds the amniotic and yolk-sacs and lines the chorionic vesicle (Fig. 219, A). It forms a continuous sheet between the epithelium lining the amniotic cavity and that lining the yolk sac. The origin of the tissue is imeertain. It evidently is homologous with the mesoderm which in many of the mammals is known to arise from the primitive streak and head process. In man the primitive mesoderm is apparently formed before the appearance of the medullary plate, the neurenteric canal, and the primitive streak. After these structures appear the mesoderm disappears in the mid-axial line anterior to the primitive streak, and a chordal plate is differentiated in the entoderm. (See Fig. 219.)

Keibel Mall 219.jpg

Fig. 219. — Dia«r«mmatic pectionn tlirough two young human embo-os dncribei) by Oral Bpee. A. IV. Sp«. ArFh. f. Anat. u. Fliysiol., Anal. Abt., 1S96. Taf. I. Fig. 3.1 llalf-acheniBtic xagiiul B«lJon thruugh Graf Spec's embryo v. H. B. (Kbcnda. Ts(. I. Fig. I.) [laLf-xchematio sugittiJ necuon (hrough Gnf [jpee-s embryo die. C. <Ebenda. 1SB9, Taf. XI, Fig. 14.) Tmnsvene i<ectioa through (he eDibo'onic C chorda aniage; f^,. chorion; en., caoalii neurentericus; cf., yolk^E p, blood ialBads; h., lieert; kp,, region o( primitive etrcak; M.,


(b) Formation of the Mesodermic Somites.


In Graf Spee's embryo Gle (Fig. 219, B and C) the mesoderm extends on each side of the neurenteric canal and of the medullary groove (Fig. 219, C) to the anterior extremity of the embryonic anlage where the mesodermic sheets of both sides become united (Fig. 219, B, h). Posterior to the neurenteric canal the mesoderm is intimately united to the tissue of the primitive streak, a region of active production of mesenchymal tissue. At the outer margin of the embryonic anlage the mesoderm is continuous posteriorly with the mesenchymal lining of the chorion, laterally and anteriorly with the mesodermic covering of the yolk sac and amnion (Fig. 219, B and C).


At a slightly later period the sheet of mesoderm on each side of the neural tube becomes longitudinally separated from the more laterally situated mesoderm (Fig. 220, A and C) and at the same time divided into a series of segments (mesoblastic somites). In the chick the first somites formed are the occipital somites (J. T. Patterson, Biological Bulletin, 1907, vol. xiii, p. 121), then follow in turn the cervical, thoracic, lumbar, sacral, and coccygeal. It is probable that the first somites formed in the human embryo belong to the occipital region. In the latter half of the first month of development in the human embryo there are found anterior to the cersdcal myotomes three incomplete occipital myotomes. The relations of these myotomes to the first somites differentiated have not yet been definitely determined. Etemod (Anat. Anz., 1899, p. 131) has described an embryo with eight somites, Kollmann (Anat. Anz., 1890, Arch. f. Anat. u. Physiol., Anat. Abt., 1891) one with fourteen (Fig. 220, A), and Mall (Journ. Morphol., 1897) likewise one with fourteen. Mall considers the first three somites in his embryo to be occipital somites. They probably correspond with the first three somites in the embryo described by Kollmann and possibly with the first three in the embryo described by Eternod.

The occipital somites are probably not completely divided off either from one another or from the lateral mesoderm (Figs. 229, 230, A).* The cervical somites, at least the more distal ones, on the other hand, become completely separated, and the tissue in


' Kollman states that in his embryo Biille, with fourteen somites, the segmentation externally appears well marked in the post-otic re^rion, but intemally^ is apparently incomplete. (Pei'sonal communication to the author.)

Keibel Mall 220.jpg

Fig. 220.— A. (After Kollmann) Human embryo with fourU«a Homitt section througli the region behind t^ Arch, f. Anat. u, Phy><ii>1.. Anst. Abt. o( MmiteB ot the embryo Bhowu in F.


", 2.5 mm. long. Mogn. 1891. Piste III, Fit. 3.1


ituogiienihichte des Mensthen, Fis. It9.) 30 t I. B. (Ebnula, Fig. 5S.) Transvene lOWQ in Fig. 220. A. C. (After Kollmann. Tiwuvene MCtion (hroujh the tenlh pair each assumes an epithelial character and becomes arranged about a central cavity or myoccel (Fig. 230, d). At the posterior end of the cervical region a solid column of cells marks for a short period the remains of the neurenteric canal. Bevond this in the axial region lies the tissue of the primitive streak which is continued into the mesenchyme of the allantoic stalk. In subsequent development mesoderm is differentiated from the anterior end of the primitive streak on each side of the posterior end of the neural groove. In this mesoderm successive somites are formed. Finally, as the differentiation of the body extends posteriorly, a definite primitive streak gradually gives way to a mass of mesenchymal cells situated between the ectoderm and entoderm, and then, in the caudal process, to a mass of cells entirely surrounded by ectoderm. From this mass of cells are successively differentiated the more caudal mesodermic somites.


(c) Axial Mesenchyme.


As the chorda dorsalis becomes differentiated (see below) marked changes take place in the somites. For a time these consist of epithelial tissue which surrounds a central cavity or myoccel (Fig. 220, C). Toward the end of the third week the cervical and thoracic myocceles become gradually filled with branched spindleshaped mesenchyme cells which come from the surrounding epithelium. The medial wall of the somite opens, and the mesenchyme cells wander out toward the neural tube and the chorda, and give rise to a tissue which ensheathes these organs (Fig. 221). The mass of mesenchyme derived from each somite represents a sclerotome. The successive sclerotomes soon fuse so as to give rise to a continuous mass of mesenchyme. The mesenchyme of the two sides becomes fused. Alter gi\'ing rise to the sclerotomes the somites become converted into myotomes, the further fate of which is described in the section on the development of the muscular system. In many of the lower vertebrates the lateral layer of the myotomes gives rise to dermis, but in mammals the dermis comes chiefly, if not wholly, from axial mesenchjTne. (Bardeen, Johns Hopkins Hospital Eeports, vol. ix, 1900.)


(d) Parietal and Visceral Layers of the Mesoderm.


During the formation of the embryonic coelom, the lateral tmsegmented mesoderm plates become divided into two layers, a parietal layer and a visceral layer (Figs. 220, C, and 221). The cells facing the coelom assume an epithelial character. The deep strata of the parietal layer give rise to scleroblastema, from which some of the skeletal apparatus and connective tissues of the trunk and limbs are derived. The deeper strata of the visceral layer give rise to the connective tissues as well as to the musculature of the thoracic and abdominal viscera.


(e) Meseochyme of the Head. The axial mesoderm of the trunk is continued forward on each side of the chorda dorsalia to the region of the base of the midbrain. From it arises a large part of the mesenchyme of the head, including most of that which gives rise to the skeletal structures of the cranium and the upper part of the face. The transformation of mesoderm into definitive skeletal structures is more direct in the cranial than in the spinal region. The formation of somites for the axial region of the head is restricted to the postotlc region, and even here it is, as mentioned above, less complete than

Keibel Mall 221.jpg

Fig. 221.— (AftiT Kollnmnn, Arch. f. AnM, 0. Pni-jiol.. Anal. Abt.. 1891, Plata III. FL(. 8.) Tran-veres ++++++++++++++++++++++++++

in the trunk. The cranial mesoderm apparently is largely converted into mesenchyme without going through that process of division into somites characteristic of the spinal mesoderm. The mesenchyme near the chorda in the occipital region shows no segmentation in the latter half of the first month. More laterally segmentation is indicated by the formation of myotomes from the dorso-lateral portion of the mesoderm. Near the myosepta the mesoderm may show a slight condensation.


In the prechordal mesenchyme of the head there are differentiated in many vertebrates vesicular cavities, "head cavities," lined by epithelium, from which musculature and mesenchyme arise. There are four such cavities in selachians and in reptiles. Their relation to the somites is undetermined. In man very transitory structures of this nature have been reported (Zimmermann, Ueber Kopfhohlenrudimente beim Menschen, Arch. f. mikr. Anat., 1898, vol. liii), but they are rare and play no essential part in development. The dorsal portion of the lateral mesoderm plate of the trunk is continued anteriorly into the branchial region, where it gives rise to the mesenchjTne of the branchial arches and partly also to that of the head. Ventral to the branchial arches the lateral mesoderm of the trunk is continued into the pericardial mesoderm. The coelom does not extend into the branchial region of the lateral mesoderm of the head.

(f) Origin of the Connective Tissues.

From the mesenchyme, derived in part directly from the primitive embryonic mesodermic tissue, in part from somites differentiated from this primitive tissue, and in part from the primitve streak, there arses a syncytial tissue which in turn gives origin to the various connective tissues and skeletal structures of the body as well as to some other structures, for instance, muscles and blood-vessels.


In the adult connective tissues the bulk of the tissue substance is usually described as extracellular.^ The chief problem for those who have studied the histogenesis of the connective tissues has been to determine whether the substances which are intercellular in the differentiated tissues have an intracellular or an intercellular origin. The weight of evidence seems at present to be decidedly in favor of the intracellular origin (Fleming, 1891, 1897, 1902, Retterer, 1892-1906, Spuler, 1897, Mall, 1902, and Spalteholz, 1906). Among recent investigators who believe that the connectivetissue fibres have an intercellular origin may be mentioned E. Laguesse (1903) and Fr. Merkel (1895, 1909).^ Golowinski, while contending that the fibres appear between the cells, admits that they rise close to the cell body. according to him, most investigators have described essentially the same phenomena, but some consider the mother substance in which the fibres arise as ectoplasm, while others consider it an intercellular substance. The majority of those who adopt the view that the ** intercellular" portions of the adult connective tissues are intracellular in origin describe the primitive mesenchymal cells as becoming differentiated into endoplasmic and ectoplasmic portions. In the ectoplasm the intercellular elements characteristic of each of the various kinds of connective tissue are differentiated while the endoplasm becomes <5onverted into the cells of the adult tissue. Retterer (1892-1906) gives a different description of the process. according to him the primitive tissue from which the various kinds of connective tissue are differentiated consists of a homogeneous syncytium in which nuclei are scattered about. This homogeneous syncytium becomes differentiated into two parts, a hyaloplasm and a granular chromophilic portion. The granular chromophilic portion surrounds the nuclei and gives rise to branching processes which anastomose so as ultimately to form an extensive network. The hyaloplasm lies in the meshes of this network. The fibres of recticulum, elastic fibres, and the branched anastomosing processes which fill the canaliculi of bone arise from the chromophilic network, while white fibrous tissue and the chief part of the ground substance of cartilage and of bone are differentiated from the hyaloplasm.


  • Spalteholz (Anat. Anz., 1906) has, however, shown tliat even in the adult many, if not all, of the fibrils have an intracellular position.


Recently still another view of the origin of the fibrils of the c'onnective tissues has been advanced. It has been known for some time that in the vitreous humor before the entrance of blood-vessels and mesenchyme cells there exists a fibrillar structure the components of which may be looked upon as branched anastomosing processes of cells of the retina and lens. From this fibrillar network the fibrils of the adult vitreous humor are probably derived. Aurel V. Szily (1908) has described a fibrous network filling in spaces throughout the embryonic body before the origin of the mesenchyme. The fibrils of the network are branched anastomosing processes of the epithelial layers bounding the various cavities. Szily thinks that when the mesenchyme cells arise they wander into meshes of this fibrillar network and enter into intimate relations with the component fibrils. The fibrils subsequently lose connections with the epithelial cells from which they arise. according to Szily the fibrils of the early embryonic syncytium are thus of epithelial origin, while the cell protoplasm is of the mesenchymal origin. Although the early connective-tissue fibrils are thus according to this view of epithelial origin, at a later stage connective-tissue fibrils are also differentiated in the ectoplasm of cells derived from the mesenchyme. according to Retterer (1904 and 1906) the syncytium of the cutis arises partly from the epidermis.

The following account of the origin of the connective tissues is based chiefly on the paper of Mall, who has taken up the problem in connection with the pig and man.


At an early stage there appear to be many individual cells in the mesenchyme which multiply rapidly, so that in certain regions the nuclei are closely packed together. Then the cells unite to form a syncytium and the protoplasm of the syncytium increases more rapidly in amount than the nuclei, so that the latter appear more widely separated from one another than at first. The nuclei at an early stage lie within the protoplasm of the syncytium, but gradually differentiation takes place. Immediately about the nuclei the protoplasm becomes granular and forms an endoplasm which is distinct from the rest of the syncytium or ectoplasm. From the granular endoplasm about the nuclei processes may extend into the surrounding ectoplasm. In the ectoplasm fibrillation becomes more and more distinct. The nuclei surrounded by the endoplasm come to lie in certain of the meshes of the network formed by the ectoplasm. In other of the meshes merely a fluid substance is seen. From this embryonic syncytium the various types of connective tissue are differentiated.


Reticulum. — Reticulum seems to be the least highly differentiated form of tissue which arises from the embryonic connective-tissue syncytium. The reticulum develops directly in the syncytial ectoplasm, while the nuclei and endoplasm are converted into cells which lie upon the reticulum fibres. In the liver the origin of the reticulum differs from that in other parts of the body in that it arises from Kupffer's endothelial cells instead of from mesenchyme. The endothelial cells form a syncytium in which the reticulum fibres are differentiated. according to Retterer the reticulum fibres arise from chromophilic processes of the perinuclear protoplasm.


White Fibrous Tissue. — ^In the development of white fibrous tissue from the embryonic syncytium Mall distinguishes two stages. In the first or prefibrous stage a tissue much resembling reticulum is differentiated, in the second or fibrous stage true white fibrous tissue appears. (Fig. 222, A and B.) In the first stage the syncytium grows very rapidly. The ectoplasm increases in amount much more rapidly than the endoplasm. The nuclei, however, multiply, and the endoplasm about each nucleus becomes drawn out spindle-like, giving rise to the well-known embryonic bipolar cells. The tips of these cells are extended into the ectoplasm, and here the endoplasm appears constantly to contribute to the ectoplasm. The ectoplasm becomes steadily more fibrillated. The strands of ectoplasm become more and more drawn out, in tendons and fascia? into parallel, in areolar tissue into interweaving bundles of fibres. In the fibrous stage the embryonic fibres are converted into true white fibrous tissue, their chemical nature meanwhile changing. The fibres at first occasionally anastomose, but during further development the anastomosing bridges begin to break down. according to Mall the larger fibres become split into the individual fibrils of white fibrous tissue. The embryonic spindle-shaped cells become converted into the adult connective tissue corpuscles. according to Retterer the fibres in the prefibrous stage belong to the chroraophilic processes of the perinuclear protoplasm. On the other hand, the collagenous fibres arise from the hyaloplasm {ectoplasm).

The body of the cornea is composed of a tissue the origin of which is similar to that of white fibrous tissue. It retains more features characteristic of the embryonic connective tissue than does the ordinary white fibrous tissue. It contains no elastic fibres.

Keibel Mall 222.jpg

(Fig. 12. Mail.) Section thraugh the skin of (

k plK 5 cm. long. White Gbrag

C. (Fig.


E. {Fig

++++++++++++++++++++++++++

Elastic Tissue. — With the exception of the tissue of the cornea probably all white fibrous tissue contains a greater or less number of elastic fibres intermingled with the bundles of white fibrils. The elastic-tissue fibres apparently are differentiated . directly in the same syncytial ectoplasm in which the bundles of white fibrils develop (Fig. 222, C). The youngest pigs in which Mall found elastic fibres were four centimetres long. these fibres were found in the aorta and neighboring arteries. Fenestrated membranes are formed by the coalescence of neighboring fibres. Spalteholz (1906) has found elastic fibres in the truncus arteriosus of pig embryos 9.2 mm. long. Ranvier held that elastic fibres arise from the fusion of rows of elastic granules. according to Mall, elastic fibres are never formed by the fusion of rows of such granules. Spalteholz has likewise found that the elastic fibres are directly differentiated. according to Eetterer, the elastic fibres arise in the perinuclear chroraophilic protoplasm and from the chromophilic processes which spring from it.


Adipose Tissue. — Adipose tissue appears in the fourth month in the human embrj^o. In the regions where the adipose tissue is formed the embryonic mesenchymal tissue becomes differentiated on the one hand into blood-vessels and a supporting fibrous-tissue framework, on the other into cells in the protoplasm of which granules of fat appear. The granules of fat in each cell gradually become consolidated, so that finally there arises a single large globule of fat which greatly distends the cell. The protoplasm of the cell now forms a thin covering for the globule of fat. The nucleus surrounded by a small amount of granular protoplasm lies at one side. The fat cells are arranged more or less definitely with relation to the blood-vessels and frequently form well-marked clusters. (See Bell, 1909.)


Cartilage. — In the formation of cartilage the ectoplasm of the syncytium becomes more and more dense. The nuclei surrounded by endoplasm come to lie in spaces in the ectoplasm, thus forming precartilage cells which in turn become converted into cartilage cells (Fig. 222, D). The syncytial ectoplasm undergoes chemical changes which make it exhibit the reactions characteristic of hyaline ground substance. Not infrequently the ectoplasm before becoming converted into hyaline ground substance becomes marked out into cell territories by the appearance of membranes between the cell units. These membranes appear as fine lines in cross section and have staining reactions similar to hyaline cartilage. When this condition is found, the cartilage has an epithelioid appearance (cellular cartilage).


The endoplasmic units or cartilage cells exhibit a differentiation into perinuclear and peripheral portions. From the peripheral portion hyaline substance is differentiated so as to form a capsule (Max Schultze). The capsule appears lighter than the surrounding tissue and has slightly different staining reactions. Meanwhile the endoplasm increases in amount, the nuclei multiply, and from time to time cell division takes place in the endoplasmic units, but this division does not extend into the surrounding ectoplasm. When cell division takes place, the line of separation between the two daughter cells usually becomes marked by a fine septal membrane composed of a substance that has some of the staining qualities of the cell capsules. This septum then becomes divided into two lamellae, each of which together with half of the old capsule surrounds a daughter cell. Sometimes the capsules of several successive generations of cells remain distinct for a considerable period, so that a capsule which first surrounded a single cell comes to surround several groups of daughter cells, each group and each daughter cell having in turn a capsule of its own. Usually, however, the primitive capsules become indistinguishably fused with the surrounding matrix, so that capsules about single cells or pairs of cells alone remain distinct. Growth of cartilage is in part interstitial, in part perichondral. The interstitial growth is due (1) to the direct increase in amount of the ectoplasm or ground substance, (2) to the formation of cell capsules at the periphery of the cells and the fusion of these capsules with the matrix, and (3) to cell multiplication. Perichondral chondrification is due to the formation of new cartilage beneath the perichondrium. The ground substance increases in amount faster than the cells multiply.^


In white-fibrous cartilage bundles of fibrils develop in the syncytium while the hyaline substance is being deposited. In elastic cartilage, according to Mall, elastic fibres are formed after the hyaline substance has been differentiated. according to Spalteholz (1906), however, elastic fibres appear before the hyaline ground substance in the ear cartilage of the pig. In the arytenoid cartilage clumps of elastic granules are deposited. While Eanvier held that elastic fibres arise from the fusion of rows of these granules. Mall, as mentioned above, believes that neither here nor elsewhere are the elastic granules fused to form elastic fibres.


Bone. — The histological structure of bone is still a matter of dispute. Most investigators seem to consider the ground substance to be composed of bundles of fibrils resembling those of white fibrous connective tissue embedded in a homogeneous ** cement" substance. V. Kolliker, who considered the cement substance to be slight in amount, believed the calcium salts to be embedded both in this and in the fibrils. V. Ebner, 1875, believed the calcium salts to be embedded chiefly in the cement substance. Eetterer, 1905 and 1906, believes the ground substance of bone to be composed of a chromophilic reticulum embedded in a hyaloplasm impregnated with calcium salts. It is well known that the ground substance of bone contains a collagenous substance similar to that of white fibrous tissue.

Bone, like other connective tissues, is formed from a blastemal syncytium. Ectoplasm becomes distinct from nucleated endoplasmic cell units. In the ectoplasm calcium salts are deposited. Two stages may thus be distinguished, — a pre-osseous, previous to the deposition of calcium salts, and an osseous, after these salts have been deposited. During ossification about two parts of inorganic salts combine with one part of organic matter. The cells which give rise to bone may appear similar to ordinary immature connective tissue cells or they may pass through a stage in which they appear epithelioid in character. Cells of the latter type are frequently found in regions where layers of bone are being applied to pre-existing bone or to calcified cartilage. The epithelioid cells, which Gegenbaur called osteoblasts, form a layer from the deep surface of which certain cells branch, anastomose, and give rise to an osteogenetic syncytium which becomes converted into bone (Fig. 223, A).

according to v. Kolliker (Gewebelehre) and to many other investigators, the osteoblasts secrete the ground substance, which, therefore, is to be looked upon rather as intercellular than as intracellular. To Waldeyer (1865) we are indebted for the first clear description of the differentiation of the ground substance of bone in the peripheral protoplasm of the osteoblasts.


  • For details concerning the development of cartilage see Retterer (1900).


The endoplasmic units, or bone corpuscles, have branched processes which anastomose freely through the canaliculi with those of neighboring cells. Before birth (Neumann) the periphery of the bone corpuscles becomes differentiated into a resistant cuticle which has staining reactions similar to elastic tissue (Retterer) and which is resistant to strong acids and alkalies. Brosike (1885) considered this cuticle (bone-cell capsule) to be composed of keratin, but Kolliker has shown it to be soluble in boiling water. according to Eetterer the protoplasm of the branching processes which lie in the canaliculi is converted into a similar substance.


In the human embryo bone arises chiefly in connection with a transitory cartilaginous skeleton which it gradually in large part replaces. Thus the vertebrae, ribs, sternum, the skeleton of the extremities, and most of the base of the skull are first formed of cartilage, and the cartilage is later replaced by bone (substitution bone). Centres of ossification may appear within the cartilage (endochondral ossification) or beneath the perichondrium (subperiosteal ossification). On the other hand, most of the bones of the face and the flat bones of the skull are formed directly in membranous tissue (intramembranous bone).


When bone is first formed in the embryo, it consists of a eoarse plexiform or spongy framework, in the meshes of which lies a vascular embryonic marrow. To the walls of the spaces in this primitive spongy bone successive layers of bone are added by osteoblasts, so that the spaces come to have lamellated walls. Similarly beneath the periosteum lamellae of bone are laid down, so that the surface of the bone comes to consist of a series of successive lamellae. The formation of definite lamellae of compact bone is not, however, well marked until after birth. Previous to this period the vascular spaces in the bone are relatively large, so that the coarse spongy structure mentioned above is long retained. In long bones Schwalbe found compact lamellar bone formed about the marrow cavity and in the Haversian canals in the sixth month after birth, but beneath the periosteum not until the fourth year. Kolliker (Gewebelehre) found lamellar subperiosteal bone as early as in the first year after birth.


During the period of the growth of bone new bony tissue is being constantly added in some regions, while in other regions the bone already formed is absorbed to make way for new vascular marrow cavities. In this process of bone absorption large cells, osteoclasts, containing, according to v. Kolliker who first described them, from one to sixty nuclei, play a chief part (Fig. 223, B). These osteoclasts vary in size, being from 43 to 91 /* long, 30 to 40 M wide, and 16 to 17 fi thick. They apparently have the power of dissolving bone or calcified cartilage. The depressions which they cause in bone are called Howship's lacunae. according to Kolliker, they arise from osteoblasts, and may again divide up into osteoblasts or after remaining for a greater or less length of time in the bone marrow they may disappear. The nuclei within the cell multiply by direct division. The changes of form which bones undergo through the process of growth by apposition of new layers of bone to pre-existing layers and the absorption of bone previously laid down are well illustrated by comparing the jaw of the infant with that of the adult (Fig. 224, C).


Under the term Sharpey's fibres, according to Eetterer (1906), several distinct structures have been described: (a) prolongation of the periosteum into the bone; (b) granular elastic protoplasmic processes of the lamellar system; (c) portions of the bone in which calcium salts have disappeared from the hyaloplasm and fibrous tissue has been differentiated. The true Sharpey's fibres are probably prolongations of the periosteum left behind as successive layers of bone are differentiated beneath the periosteum. To this brief description of the general nature of the process of ossification we may add a short account of the special features which characterize intramembranous, subperiosteal, and endochondral types of ossification.


Intramembranous Ossification (Fig. 222, E, Fig. 224). — ^In this type of ossification bone first appears in the form of a network of spicules interwoven with a network of blood-vessels. Ossification begins at a centre from which it radiates peripherally. As one passes from the centre towards the periphery in the early period of ossification, one finds all stages from fully formed bone to an undifferentiated embryonic connectivetissue syncytium. In ossification in very young embryos the connectivetissue syncytium appears to be directly transformed into bone. The transformation is marked first by the fibrils of the ectoplasm becoming more clearly marked, and then by the appearance of a basophilic substance in the ectoplasm. In older embryos the ectoplasm is, according to Mall (1906), transformed into prefibrous tissue and the latter is transformed into bone. The diameter of the embryonic bone corpuscles, according to v. Kolliker, varies from 13 to 22 f^.


The primitive plexiform bone is thickened by deposit of osseous substance beneath the periosteum. The latter appears soon after bone-formation has commenced. The spaces in the plexiform network of bone at an early stage become converted into canals containing blood-vessels and primitive marrow. In bone of membranous origin cartilage may subsequently be developed beneath the periosteum. Examples of this are to be found in the temporomandibular joint.

Keibel Mall 224.jpg

Fig. 224.— A.

File:Keibel Mall 225.jpg

Fig. 225. A.— (After Siymonowici, Text-book ol Histology, tmnslated by MiwCi>Uum, Fig. 103.) From B longitudinal neclion of * faigrr of & tlirBe-and-«-halt-montlia human fetus. Two-thirds oJ the second phftlbox are repreeeuted. At X a periosteal bud is to be seen. Magn. about 85 : I


Subperiosteal Ossification (Fig. 225, A and B). — Bone is formed in the deep layer of the periosteum (perichondrium) essentially as bone is formed in membrane which is not closely applied to cartilage. The bone formed beneath the periosteum has at first a coarse plexiform structure. The meshes of the osseous framework enclose vascular embryonic marrow. As mentioned above, dense subperiosteal lamellst are formed in human long bones in the first year after birth, according to Kolliker (Gewebelehre), while according to Schwalbe they are not formed until the fourth year. Subperiosteal ossification is the sole method of substitutioa of osseous for cartilaginous tissue in some of the bones (in the ribs, for example), while in others it is closely associated with endochondral ossification (diaphyses of the long bones). When it is the sole method of ossification, the underlying cartilage frequently undergoes changes similar to those preceding endochondral ossification (see below).

File:Keibel Mall 226.jpg

Fig. 226, B. — (Afor Siymonowic., Teit-book of HinWlogy, tmoJiliWd by MMC»llum. Fig. 196.) From » lonsitiidiaal nection of a tinger of a four-moDths huioBD fitiu. Only tbfl diaphysis of the saccnd phalanx is rtpreMnt«d. Mien, about 86 : 1.


Endochondral Ossification

In endochondral ossification processes from the osteogenic layer of the perichondrium, or periosteum, extend into the substance of the cartilage, and these give rise on the one hand to destructive activities which break down the cartilage and on the other to constructive activities which result in the formation of bone. Endochondral ossification is preceded by well-marked changes in the cartilage (Fig. 225). The cartilage cells first multiply rapidly in number and then enlarge so that the matrix becomes relatively reduced in amount. Neighboring cartilage cells may so expand that the matrix between them disappears. Meanwhile calcium salts are deposited in the matrix in the form of granules which may become confluent. The processes from the periosteum break into the cavities occupied by the cartilage cells, enlarge them, and thus give rise to primary marrow cavities. In the phalanges and other long bones of limited size, the cartilage at the centre of the shaft may be completely absorbed before the endochondral ossification begins. The primitive marrow is vascular and contains an embryonic syncytium not highly differentiated. Osteoblasts and osteoclasts and embryonic connective tissue, however, appear in it at an early stage, fat and marrow cells at a later period. About the primitive marrow cavities bone is quickly laid down and there thus arises spongy endochondral bone. The bone first laid down is later again absorbed during development of the larger central marrow cavities. From the cavities into which the marrow first penetrates it gradually extends into neighboring cartilage-cell spaces. As the osteogenic tissue spreads, the surrounding cartilage undergoes changes similar to those which took place at the primary centre of ossification. Thus, so long as the process of ossification continues, the cartilage farthest removed from the centre of ossification shows the least modification from the type of primitive hyaline cartilage, while as one passes toward the centre of ossification one finds the successive changes of cell multiplication, cell expansion, and calcification of the matrix. In long bones these successive stages are especially well marked. The multiplication of cartilage cells gives rise to groups which become arranged in long columns which are parallel to the long axis of the bone. The boundary between the zone of ossification and that of the highly modified cartilage is usually fairly sharp (Fig. 225, B). Capillary loops extend close to the limit of the advancing ossification. The extremities of these loops are often dilated.


The fate of the cartilage cells in the calcified matrix is still in dispute. Most modem investigators, including Kolliker ( Gewebelehre, 1889), seem to follow Sharpey and Loven in con<;luding that the cartilage cells are destroyed as osteogenic tissue derived indirectly from the periosteum enters the cell spaces. Numerous accurate observers, however, among who may be mentioned H. Muller (1859), Ranvier (1865), and Retterer (1900), T)elieve that the cartilage cells become converted into osteogenic tissue, each cartilage cell giving rise to several smaller cells and to reticular tissue. The old view, that cartilage may become directly converted into bone, seems to have few modern adherents.*


  • according to Strelzoff (1873), this metaplasia is constant in some regions, for example, in the lower jaw of human embryos.

In epiphyses centres of ossification arise at a comparatively late period. Blood-vessels, which spring from the periosteum and from the bone marrow, penetrate into the epiphyseal cartilage long before ossification begins. Friedlander (1904) gives good pictures of the blood-vessels in the epiphyseal cartilages of the long bones. In some cartilages the blood-vessels appear in the third fetal month. In the seventh all the larger cartilaginous areas show rich vascular plexuses.


Growth of Bone. — The question as to whether or not there is an interstitial growth of bone has given rise to extensive investigations. The evidence is fairly conclusive that there is no wellmarked interstitial growth in bone. Hales, Duhamel, John Hunter, and others showed, during the eighteenth century, that two pegs driven into a bone do not move apart during development unless there is a non-ossified region between the two pegs. Z. G. StrelzoflF (1873), however, brought forward a certain amount of evidence to show that under some circumstances there may be a slight interstitial growth of bone.^ Experiments made with madder go to show that growth of bone takes place entirely by apposition. Madder stains newly forming bone, and by feeding it to young animals the successive applications of layers of bone may be followed. Experiments along this line were first performed by Duhamel and J. Hunter. Duhamel also showed that a ring placed on the outside of a long bone of a young animal may eventually be found in the marrow cavity.


Regeneration. — In case of fractures union is effected by osteoblasts which give rise to new bone which unites the broken ends. These osteoblasts in young animals may apparently be derived either from the marrow or from the periosteum, but in the adult chiefly, if not wholly, from the periosteum. Bonome (1885) has, however, brought forward evidence to show that the bone corpuscles in certain conditions where they are supplied with abundant nutrient blood may give rise to osteoblasts. Not infrequently temporary cartilage is produced in places at the site of the fracture. In man fibrous tissue is often produced if the broken ends of the fractured bone are not closely approximated. The experiments of Oilier and others have shown that the bone-forming power of the periosteum may be exercised even when this is transplanted into the tissues at some distance removed from any bone. If the periosteum is preserved it has the power of restoring in nearly normal form large parts of bone.


See also Egger (1885) and J. Wolff (1885).

Addendum

Since the preceding section on the development of the connective tissues was written, there have appeared several important articles on the development of the connective tissues in mammals. Fr, Merkel (1909) brings forth new evidence in favor of the intercellular origin of the connective-tissue fibrils. He pays particular attention to the development of limiting membranes which in places sharply mark off epithelium from the underlying connective tissue. These membranes, according to Merkel, arise from the connective-tissue matrix independently of the connective-tissue cells. They may become fibrillated. Similar non-cellular connective-tissue substances are formed at an early stage in the septa between myotomes, and later between muscle cells of various types and in lamellated connective tissues. The sarcolemma of striated muscle cells has a similar origin, according to Merkel. Disse (1909), on the other hand, describes the osteogenetic tissue as arising from the cell protoplasm. Each osteoblast becomes divided into two parts, a perinuclear granular portion and a peripheral, usually basilar, hyaline portion. The hyaline substance derived from osteoblasts fuses to form a mass in which fibrils differentiate after the hyaline substance is separated from the perinuclear protoplasm. Bell (1909) gives a clear description of the development of adipose tissue. He supports the view of the histogenesis of the connective tissues adopted by Mall.

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PART II. Morphogenesis of the Skeletal System

A. General Features

The definitive skeletal system is composed of bones and cartilages united to one another at joints by means of ligaments. In the lowest vertebrates a cellular rod, the chorda dorsalis or notochord, situated in the mid-axial line ventral to the central nervous system, constitutes the chief part of the axial skeleton. In the higher vertebrates a chorda dorsalis is also formed during early embryonic development, though in mammals and man it lends little or no skeletal support to the embryo and mere derivatives of it are to be found in the adult. The definitive skeleton of the higher vertebrates, including man, is differentiated from the mesenchyme of the head, trunk, and limbs. The process of differentiation is somewhat complex. As a rule, the first visible step in the process is marked by condensation in the sclerogenous mesenchyme or scleroblastema. Thus, in the development of the skeleton of the inferior extremity, condensation begins in the vicinity of the future hip-joint and from here extends distally and proximally, so that there is produced a continuous mass of condensed tissue in which pelvic, femoral, tibio-fibular, and tarsal regions and five metatarso-phalangeal rays may be distinguished. The hard parts of the skeleton are developed from centres which appear in the scleroblastema. The joints are developed in the scleroblastema which intervenes between the hard parts.

The Bones

It has been mentioned in the section on histogenesis that most of the bones of the body are first formed of cartilage and then, during subsequent development, bone is gi*adually substituted for cartilage, substitution or cartilaginous bones (Figs. 277 and 278). Other bones are formed directly in the membranous scleroblastema, membrane or investment bones. The bones of the extremities, with the partial exception of the clavicle, the bones of the spinal column and thorax, and the greater part of those of the base of the cranium, have a chondrogenous origin. The greater part of the bones of the cranial vault and of the face arise directly in the scleroblastema.


It is to be noted, however, that during the formation of many of the typical substitution bones ossification may extend into membranes attached to the cartilage, so that certain processes on these bones are membranous in origin, and that, on the other hand, certain parts of bones of membranous origin may secondarily give rise to cartilage (temporomandibular joint). Several of the definitive bones of the skull have an origin partly cartilaginous, partly membranous.

Substitution Bones

As a rule, a centre of chondrification appears in the midst of condensed scleroblastema. (See femur, tibia, and fibula, Fig. 275.) It may, however, appear in tissue but slightly condensed, as in case of the vertebral bodies. Fig. 273. The cartilaginous centres expand rapidly, both by apposition from the surrounding blastema and by interstitial growth. Neighboring centres are thus soon brought into close approximation. Some of the centres fuse with one another in the region of approximation. Between other centres joints are developed. The fate of the cartilaginous centres, therefore, differs considerably in different regions.


The conditions in the skeleton of the limbs are the simplest. Here for each of the bones, including the pubis, ischium, and ilium, there is a single centre of chondrification (see Fig. 226 and Figs. 275 and 276). The clavicle forms an exception to the other bones in that the tissue at the centre of chondrification is not converted into typical embryonic hyaline cartilage (see pp. 380 and 388). The centres of chondrification for the pubis, ischium, and ilium soon fuse with one another so as to produce a continuous cartilaginous hip-bone, which gradually assumes definitive form (Figs. 276, 277, and 278). With the exception of a few cartilages in the wrist, the fate of which is treated elsewhere (p. 383), each of the other embryonic limb cartilages undergoes an independent development. In the region of the knee-joint, however, and possibly in some other articular regions of the limbs, independent skeletal elements become at an early period temporarily fused together by a kind of precartilage (Fig. 283). Temporary joints of this kind resemble the permanent joints of the shark's fin.


A centre of ossification appears ia the main body of each of the cartilages of the skeleton of the limbs; in most of them early in fetal development, but not until after birth in those of the ankle and wrist, with the exception of the calcaneus, talus and cuboid and in the patella and other sessamoid bones. These chief centres of ossification establish bone in place of cartilage as growth proceeds. In case of all the limb bones except those of the ankle and wrist secondary epiphyseal centres of ossification appear early in childhood in those portions of the bone still cartilaginous, and as maturity is approached become fused with the main part of the bone. Growth in length of bone, as stated in the section on histogenesis (p. 311), is dependent upon the growth of the cartilaginous matrix and ceases when the epiphyses become fused with the main body of the bone. In the adult limb skeleton the only cartilage remaining is that upon the joint surfaces of the bones.


In the vertebral columyi there are two bilaterally placed centres of chondrification for the body of each vertebra and one for each half arch (Figs. 239 and 249). The arch cartilages join the body considerably before they unite dorsally so as to complete the arch about the spinal cord. The ribs develop from separate centres of chondrification and do not fuse with the bodies. In the cervical, lumbar, and sacral regions there are more or less distinct centres of chondrification of costal elements which quickly fuse with the cartilage of the body. In the sacral region the various cartilages fuse to form a cartilaginous sacrum. The cartilaginous vertebral bodies are at first separated by thick blastemal discs, but as development proceeds the discs near the centre become thin and partially converted into a precartilaginous tissue, so that for a brief period there is a continuous vertebral axis composed of tissue of a cartilaginous nature but in which segmentation is clearly marked.

The cartilage of the sternum arises mainly from the cartilage of the ribs, from which it is secondarily separated by the formation of costosternal joints. There are primary centres of ossification for the bodies of the vertebrae, each half arch, the ribs, and some of the costal elements of the sacrum. In addition, there are many epiphyseal centres. In the cranial blastema numerous centres of chondrification appear (Figs. 310 and 311). These, however, fuse to form a continuous chondrocranium, in which no blastemal sutures remain to separate one cartilaginous element from another (Figs. 312 and 313). The incus and stapes remain distinct cartilages. The malleus is long continuous with Meckel's cartilage, the cartilaginous skeleton of the mandibular arch. The cartilage of the hyoid arch becomes attached to the chondrocranium. While the chondrocranium is being formed, centres of ossification begin to appear in various parts of the cranial scleroblastema. From these centres of ossification, partly by expansion and partly by fusion of neighboring centres, there are produced the membranous bones of the skull (Fig. 321). Meanwhile, centres of ossification appear in the chondrocranium and by expansion and fusion give rise to the substitution bones of the skull. In the definitive skull some bones, like the parietal, frontal, and maxillary, are purely membranous in origin. Some, like the ethmoid, hyoid, incus, and stapes, are fairly typical substitution bones, while many of the bones, like the occipital, sphenoid, and temporal bones, arise partly from centres which appear in membranous tissue, partly from centres which appear in the chondrocranium. In the membranous tissue in which the centres for the investment bones appear the definitive form of the skeletal part is much less clearly marked than in the chondrocranium (compare Figs. 310, 311, 312, 313, 321).

Cartilages

Not all the cartilage of the embryonic skeleton becomes replaced by bones. Some of the embryonic cartilages become reduced to fibrous tissue, as in the case of the stylohyoid ligament; some give origin to the cartilages of the definitive skeleton, such as the costal cartilages and parts of the nasal capsule; some merely disappear.

Joints

When first diiferentiated the fixed parts of the skeleton are united to one another by dense blastemal tissue in which little definite form is to be observed. In ease of synarthroses this intervening blastemal tissue becomes directly or indirectly transformed into fibrous tissue (syndesmosis), into cartilage (synchondrosis), or into bone (sjTiostosis). While, as a rule, the fibrous tissue of a syndesmosis comes fairly directly from the primitive blastema of the embryonic joint, it may arise as the result of retrograde metamorphosis of cartilage (lig. stylohyoideum). A synchondrosis is usually preceded by an embryonic blastemal syndesmosis. A synostosis is usually preceded by a sjnidesmosis or a synchondrosis.® In a diarthrosis the joint cavity, synovial membrane, and the various ligaments characteristic of the joint are diiferentiated from the dense blastemal tissue which unites at first the two embryonic cartilages entering into the joint. Disci articulares and menisci articulares are also diiferentiated from this blastema. In case of the few diarthroses formed between membrane bones, as for instance between the mandible and the temporal bone, the blastemal tissue has the power of giving rise to cartilage which covers the joint surfaces of the bones.


The various steps in the diiferentiation of a simple diarthrosis are well illustrated in the digital articulations (Figs. 226-228). In Fig. 226 are shown the cartilaginous anlages (a) of the three phalanges and the distal part of the metacarpal of a finger of an embryo 2.7 cm. long. These cartilaginous anlages are embedded in a dense blastema which shows lighter areas in the vicinity of the future joints (c). The term intermediate zone has been applied to the dense tissue lying between the two cartilages entering into a joint (6). As the cartilages expand they come into close approximation, as shown in the finger of a fetus 7 cm. long (Fig. 227). At this stage the cartilage is undergoing changes preliminary to ossification. The perichondrium about the joint surfaces of the cartilage entering into the joint is very dense. The joint cavity first appears at the periphery of the joint (Fig. 227). Gradually it extends in between the two cartilages entering into the joint and a variable distance over the head toward the shaft (Fig. 228, A, B, C). The form of the joint surfaces of the bones entering into the joint is highly diiferentiated before the joint cavity appears (Fig. 227).

In the more complex joints in which menisci or intra-articular ligaments are diiferentiated, as in the knee-joint and hipjoint


"The nucleus pulposus of the interv^ertebral fibrocartilage (disc) arises from the tissue of the chorda dorsalis (see p. 341).


Fig. 826-228.


(Figs. 281, 282, 285), the cartilages of the bones entering into the joint are less closely approximated at the time of the formation of the joint cavity than in simple joints, like those of the fingers. The external ligaments and the various intra-articular structures are differentiated directly from the intermediate zone of blastema, while the blastemal tissue next the joint surfaces of the cartilages entering into the joint becomes condensed into a dense perichondrium. The rest of the tissue becomes less dense in character and is converted into mucoid tissue with a few cells scattered through the matrix (Fig. 282). As in all diarthroses the formation of the joint cavity begins at the side and extends toward the centre of the joint. The definitive cavity may be formed by the fusion of several cavities which appear at various places in the periphery of the joint (knee-joint, p. 372). The mucoid tissue disappears as the joint cavity enlarges. The capsular ligament which is formed from the periphery of the intermediate blastemal zone is continuous on each side of the joint at first with the perichondrium and later with the periosteum. The synovial membrane is formed on the inner surface of the capsular ligament. Synovial villi arise in the latter part of fetal life


At the time of the appearance of the joint cavity the bones entering into the joint are composed of cartilage in the region of the articulation, although ossification may be well under way at some distance from the articulation (Figs. 227 and 228). After the appearance of the joint cavity the articulating parts undergo an elaboration in form (Fig. 228), which may be quite extensive (Figs. 286, 288). This elaboration of form is due not only to interstitial growth of cartilage, but also to the appositional growth of bone. As the result of the ossification, all the cartilage near the joint becomes entirely replaced by bone except on the joint surface, where, as a rule, a layer of hyaline cartilage remains throughout life. The thin, dense layer of blastemal perichondrium which for a short time covers the joint cartilage, as a rule disappears early, although it may give rise to a permanent film of tissue or the joint cartilage may become in part composed of fibrocartilage ( sternoclavicular, temperomandibular, costovertebral, sternoccstal articulations).


The relative positions of the articulating bones vary greatly in different regions at the time of the formation of the joints. The kneeand elbowjoints, for instance, are flexed at an angle of about 90"^ while the wrist-joint is nearly straight.

Sesamoid Bones

Tendons are closely fused to the joint capsule in many articulations of the extremities. In certain regions where this occours sesamoid bones are developed. The largest of the sesamoid bonesis the patella. Well-marked sesamoid bones are found regularly on the flexor side of the metacarpoand metatarsophalangeal joints, usually of the first and frequently of the other digits of the hand and foot. Dorsally placed sesamoid bones have also been seen in connection with the thumb. On the flexor surface of the thumb a sesamoid bone is frequently found at the inter phalangeal joint. Fibrous interphalangeal sesamoids have been found in connection with the fingers.


The sesamoid bones are better developed in some of the lower mammals than in man, and, according to Pfitzner, are more frequent in the human embryo than in the adult. They are developed at the periphery of the intermediate blastemal zone. The blastema becomes condensed, and then in the better marked sesamoid bones becomes gradually transformed into cartilage. Ossification takes place relatively late in childhood. On the intracapsular origin of the sesamoid bones see Bradley (1906). In some tendons not intimately connected with a joint capsule a sesamoid bone may be developed in a region where the tendon is subjected to stress against a bone about which it turns. An example is the sesamoid bone often found in the tendon of the peroneus longus where this plays over the tuberosity of the cuboid. according to Lunghetti (1906), the sesamoid bone in the tendon of the M. peroneus longus develops in fibrous connective tissue, not in cartilage. It is commonly stated that it passes through a fibrocartilaginous stage before becoming ossified.

Variations in the Development of the Skeleton

Variations in the bones of the adult human skeleton are frequent. Thus, for instance, skeletons with only eleven or with thirteen free ribs are not uncommon. Rosenberg, Pfitzner, Thilenius, and others would ascribe some of the variations found in the adult skeleton to the chance persistence of transitory conditions normally present in the embryonic or fetal skeleton and supposedly of phylogenetie importance.

The studies of Thilenius, Bardeen, Mall, and others have shown, however^ that the skeleton of the embryo is subject to fluctuating variations like those found in the adult. At present there are not sufficient data to determine definitely the relative frequency of skeletal variations in the adult compared with those in the embryo or fetus.

Abnormalities in the Development of the Skeleton

The form of the skeleton as a whole and of the individual bones which compose it depends partly upon heredity, partly upon the mechanical and chemical influences to which it is subject during growth. The variations which are a normal inheritance of the race, including such extreme forms as individuals with six toes or six fingers, are to be distinguished from the abnormalities of structure due to unfavorable environment either within or without the body. In the main the shapes of the bones and joints are inherited, but to some slight extent both bones and joints are moulded by the experience of the indiridual. Abnormal stress of muscular or other origin, and abnormal lack of stress, as in cases of muscle paralysis, both give rise to bones and joints abnormal in form.


During development the skeleton is markedly influenced by internal chemical conditions affecting the growth or general nutrition of the body. The skeleton in some cases seems to be the part primarily affected. The skeletal lesions vary all the way from a retardation in the time of appearance of centres of ossification to the failure of a part of the skeleton to develop or to hyperplasia and abnormal form-differentiation.


Agenesis, or failure of skeletal development, may be due either to primary lack of origin of a part or to an affection which destroys the skeletal anlage after it has begun to differentiate. It is most frequently found in the cranial vault and in the vertebral arches, less frequently in the vertebral bodies and the bones of the extremities. The osseous defect is usually, but not always, associated with other marked physical deformities.


Hypoplasia, underdevelopment, of the skeleton, whether generalized or eonfined to a part, may be due either to prenatal or to postnatal conditions. The failure of the bones to develop normally may be due (1) to lack of active proliferation of cartilage (characteristic of cretins), (2) to inactivity in the process of ossification, membranous, subperiosteal or endochondral (see Michel, 1903, Lindemann, 1903), (3) to a premature union of epiphyses with the main part of a bone, (4) to growth of connective tissue between the growing cartilage •of a bone and the region where ossification usually extends into the cartilage (micromelia chondromalacia, fetal rickets), and (5) to inflammation and other abnormal conditions affecting the growing parts of the bone.


Various congenital forms of hypoplasia are recognized, — microsomia, micromelia, micromelia chondromalacia (fetal rickets), cretinism, etc. In most instances while there is a general underdevelopment of the skeleton the long bones are especially affected and appear short and relatively thick; the pelvis and thorax are also usually abnormally smaU, and the root of the nose is broad and not infrequently sunken in. The causative factors of these conditions are obscure. In cretinism growth of cartilage is retarded and there is a delay in the appearance of centres of ossification and also in the fusion of epiphyses with the main parts of the bones (Wyss, 1900). In this disease there is good evidence that the failure of cjevelopment of the body, including the skeleton, is due to lack of normal secretion by the thyroid gland. It is not improbable that the secretions of other glands of similar type may affect the development of the skeleton. Some diseases involving both the skeleton and the hypophysis have led to the belief that there is a relation between this gland and skeleton development. This relation has, however, been disputed (Arnold, 1894). K. Bach (1906) has recently discussed the apparent influence of the thjrmus on the growth of bones.


Hyperplasia f overgrowth of the bones, is due (1) to an excessive activity of membranous or subperiosteal ossification or (2) to a prolonged persistence of actively growing epiphyseal cartilages, union of epiphysis with the main part of the bone being delayed, while endochondral ossification continues beyond the usual time. Hyperplasia may be local or general and may give rise to a well-proportioned or to disproportionate enlargement of the skeleton. It is stated that removal of the testicles early in infancy or congenital absence of the testicles may lead to an excessive prolongation of the activity of the epiphyseal cartilages and hence to gigantism (P. Launois and P. Roy, 1903, Poncet, 1903). Phosphorus and arsenic in small doses are said to promote bone growth. Partial hyj>erplasia is found most frequently in the skull and in the bones of the hands and feet. An irritative stimulus, such as a blow, may excite excessive growth of bone. In young people a small centre of inflammation (tuberculosis, osteomyelitis) in the diaphysis may excite activity in the processes concerned in ossification and induce abnormal growth in size of bone. If the centre of inflammation is near the epiphyseal cartilage, ossification is apt to be very irregular.


In congenital syphilis there are frequently, although not always, present characteristic irregularities in the deposition of calcium salts and in the formation of narrow cavities in the ossifying cartilage. This sometimes gives rise to marked ahnormality of form.


In rickets the process of bone absorption is abnormally active, while the formation of new bone is characterized by lack of deposit of the normal amount of calcium salts. In endochondral ossification there is no well-marked zone of calcification. The bones are abnormally thick, clumsy, and heavy and may be much distorted. In teratomata of various forms the skeletal abnormalities correspond with those of the rest of the body.

Bibliography

(On general features of the morphogenesis of the human skeleton.)

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Arnold, J. : Weitere Beitrage zur Akromegalief rage. Virchow's Arch. Bd. 135, S. L 1894.

Bach, K. : Zur Physiologie und Pathologie des Thymus. Jahrb. f. Kinderheilkunde. Bd. 64. 1906. Bade, P.: Die Entwicklung des menschlichen Skeletts bis zur Geburt. Arch. f. mikr. Anat. Bd. 55, S. 245-290. 1900.

Bardeen and Lewis: Development of the Limbs, Bodywall, and Back in Man. Amer. Joum. of Anat. Vol. 1, p. 1. 1901.

Bardeen, C. R. : Vertebral Variation in the Human Adult and Embryo. Anat. Anz. Bd. 25, S. 497. 1904. B^clard : Uber die Osteose oder die Bildung, das Wachstum und die Altersabnahme der Knochen des Menschen. Meckel's Arch. Bd. 6, S. 405-446. 1820.

Bernats, a.: Die Entwicklungsgeschichte des Kniegelenkes des Menschen mit Bemerkungen iiber die Gelenke im allegemeinen. Morphol. Jahrb. Bd. 4, S. 403. 1878.

Bolk: Die Segmentaldifferenzierung des menschlichen Rumpfes und seiner Extremitaten. Beitrage zur Anatomie und Morphologic des menschlichen Korpers. Morphol. Jahrb. Bd. 27. 1899. Bd. 28. 1899. Sur la signification de la sympodie au point de vue de I'anatomie segmentals Petrus Camper Desl 1. 1901.

Bradley, 0. C. : A Contribution to the Development of the Interphalangeal Sesamoid Bone. Anat. Anz. Bd. 28, S. 528-536. 1906.

Bruch, C. : Beitrage zur Entwicklungsgeschichte des Knochensystems. Neue De;ikschriften der allgemeinen SchweizeriseLen Gesellschaft fiir die gesamten Naturwissenschaften. Bd. 12, S. 16. Zurich 1852.

Corridi, G. : Dei principale nuclei di ossificazione che possono invenirsi alF epoca della nascita. Uanomalo Napoli. Vol. 3, p. 143, 179, 231. 1891.

de Coulon, W. : Uber Thyreoidea und Hypophysis bei Cretinen, sowie iiber Thyroichalreste bei struma nodosa. Virchow's Archiv. Bd. 147. 1897. Damany, p. le: L'adoption de Fhomme a la station debout. Journ. de TAnat. et de la Physiol, p. 135-170. 1905.

DoRRiEN, Ernst: Uber Riesenwuchs und Elephantiasis congenita. Diss. Leipzig 1905. Erissonius: Traite des os des enfants. Cited bv I^ Double, 1906.

Grawitz: Fetus mit kretinistischer Wachstumsstoruntr des Sehadels und der Skelettknoehen. Virchow's Arch. Bd. 100. S. 2-'>6-262. 1885. Hagen: Die Bildung des Knorpelskeletts bpim men<?^h lichen Embryo. Archiv f. Anat. und Physiol. Anat. Abt. S. 1-40. 1000.

Hekke u. Reyher : Studien iiber die Entwicklung der Extremitaten des Menschen, insbes. der Gelenkflachen. Sitzungsb. der K. Akad. der Wiss. Math.-Naturw. Klasse. Wien. Bd. 80, S. 217. 1874.

Hepburn, D. : The Development of Diathrodal Joints in Birds and Mammals. Joum. of Anat. and Physiol. Vol. 23, p. 507. 1889.

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Kaupmann, E. : Untersuchungen iiber die sogen. fetale Rachitis. (Chondrodystrophia fetalis.) Berlin 1892. Kerckring, Theod. : Spicilegium anatomicum continens osteogeniam foetuum. Amstelodami. 1670.

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SiLBEBSTEiN, A.: Ein Beitrag zur Lehre von den fetalen Knoehenerkrankungen. Arch. klin. Chir. Bd. 70. 1903. Thilenius, G. : Untersuchungen iiber die morphologiscbe Bedeutung accessorischer Elemente am menschlichen Carpus (und Tarsus). Morph. Arbeiten. Bd. 5. 1895. accessorische und echte Skelettstiicke. Anat. Anz. Bd. 13. 1897.

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B. Origin and Fate of the Chorda Dorsalis

The cervical region of the chorda dorsalis appears to arise from the dorsal wall of the entero vitelline sac beneath the medullary groove, although it is not probable that these cells belong primitively to the entoderm. In many mammals the tissue from which it is derived comes primarily from the mesodermal head or chordal process (see p. 47). Bonnet,^ however, has ascribed to the yolk entoderm the origin of the tissue for the anterior tip of the chorda in the dog and sheep, although the main part of the anterior portion of the chorda in these animals is derived from tissue which has become incorporated in the entoderm through fusion of the head process with the dorsal wall of the enterovitelline sac. In the human embryo, as mentioned above (p. 293), there is at a very early period a layer of mesoderm formed between the ectoderm and entoderm in the mid-sagittal plane (Fig. 219, A). At a slightly later period (Fig. 219, B and C) the mesoderm has disappeared in this region. Possibly it is incorporated with the entoderm which now beneath the neural groove presents a plate of tissue slightly thicker than the entoderm on each side. This is the chordal plate, the anlage of the anterior end of the chorda dorsalis. Kollmann (1890) has given an account of the origin of the chorda in a human embryo 2.11 mm. long with fourteen mesodermic somites. In this embryo (Fig. 220) the neurenteric canal has disappeared. Anterior to the primitive streak in the mid-sagittal plane a longitudinal ridge of cells projects dorsally from the entoderm (Fig. 220, B). This ridge of cells gradually becomes pinched oflF from the entoderm (Fig. 220, C). In a slightly older embryo described by Mall (1897) (Fig. 229) the neurenteric canal is represented by a solid column of cells. The chorda extends forward from this column of cells as far as the buCoopharjoigeal membrane. As Seessel's pocket develops, the chorda remains for a time attached to its posterior wall. This connection is lost at about the period when the buCoopharyngeal membrane is ruptured. 'Anat. Hefte, 1901.

In the embryo described by Mall the posterior end of the chorda lies opposite what is probably the eighth cervical somite. During the development of the thoracic, lumbar, sacral, and coccygeal regions of the embryo the chorda is gradually developed caudalwards. In this portion of its development the chorda is not first embedded in the entoderm and then again differentiated out, but is derived directly from the primitive streak and from the tissue which replaces the primitive streak caudalwards.


In an embryo described by His (L, 2.4 mm. long) the notochord has a distinct lumen. This is not present in older embryos.


At first there is no distinct membrane about the chorda (see Figs. 220, B, and 220, C). The cells are large, with clear protoplasm. By the end of the fourth week of development a thin structureless membrane encircles the chorda, which is now about at the height of its development. The chorda is cylindrical. The cells are polygonal and are filled with a fine granular protoplasm. During this period differentiation of the base of the skull and of the spinal column is marked by condensations in the axial mesenchyme (Fig. 231). Subsequently in the spinal region the intervertebral discs and the bodies of the vertebrae form about the chorda dorsalis. Between the chorda cells and the outer sheath of the chorda there appears an inner membrane, apparently mucoid in nature (Williams). according to Minot (1907), at the period when the axial mesenchyme begins to be differentiated into vertebras the notochord shows slight transient dorso-ventral segmental flexures. Just before ossification begins the chorda disappears in the vertebral bodies. In the intervertebral discs it becomes transformed into the tissue of the nucleus pulposus. In the cranial region the posterior and the anterior portions of the chorda dorsalis become embedded in the skeletal tissue of the base of the skull while the intermediate portion lies between the base of the skull and the dorsal wall of the pharynx (Fig. 266). Ultimately the cranial part of the chorda completely disappears. That part of the chorda which lies in the retropharyngeal tissue gives rise to numerous projections and swellings and is the first portion of the chorda to disappear. The posterior portion of the cranial part of the chorda comes to lie on the dorsal side of the basal occipital plate and disappears at the time of ossification of the basi-occipital. The anterior extremity of the chorda persists longer than the retropharyngeal portion, but usually disappears during the ossification of the base of the skull. A description of the changes undergone by the chorda is given in connection with the development of the vertebral column and skull. Traces of the cranial part of the chorda dorsalis may persist in the adult and give rise to tumors.

FiB.230b. Fie2300. Eli.230d.


FiQ. 229.— (AttCT Mall, Joliin. of Morpliology. vol. 12. 18»7. Fig. Ifl.) OuUine drawing of ■ mediki ngitul sectian of (he rnodd of Msll'ii embryo XII. Magn. 50: 1. The heavy line is the aorla. The Blriated. Am., amnion: a.. Ijorder between fore-brain and mid-brain; X. X'. exwnt of oiofure of gpinal eonai; 5. BeeBBel'ii pocket; CA., chorda; b'. b'. first and aecood bnmchial poekeU mouth: T., thyroid; II., pericardial npace: Ph., pharynx; ErU.. ■ ' '


The cvlom within the biMly is represented black. O' and 0>, fint and tl C, first and eilhth cen-ical myotomes; T, tint thoracic myotome; a., aoi I., thyroid; lliver: Ph.. pharynx; i., intestiae; ne.. neurent^ric canal: A

Entochorda

A hypochorda or entochorda arising fitwn the entoderm beneath the chorda dorsalis has been found in fishes, amphibia, birds, and reptiles, but apparently has not yet been described for the human embryo. In part the tissue of the hypochorda joins that of the chorda dorsalis. (See Ad. Reinhardt, Morphol. Jahrb., Bd. 32, 1904; Ph. Stohr, Morphol. Jahrb., Bd. 23, 1895; and S. A Ussoff, Anat. Anz., Bd. 29, 1906.)

Bibliography

The chief paper on the early development of the chorda dorsalis in man is that of Kollmann (1890). Important data concerning the chorda dorsalis are to be found in the various papers deseribmg human embryos with fourteen somites or less.

Bonnet : Beitrage zur Embryologie des Hundes. Anat. Hef te. 1897 und 1901. Eternod: Conununication sur im oeuf humain avec embryou excessivement jeune. Arch. Ital. de Biologies Vol. 22. 1895. See also Monitore Zool. Ital. Vol. 5, p. 70-72. 1894. Sur un OBuf humain de 16,3 muL avec embryon de 2.1 mm. Arch, des Sciences Phys. et Nat. Ann6e 101. 4 Periode. T. 2, p. 609-624. 1896. H y a un canal notochordal dans Tembryon humain? Anat. Anz. Bd, 16, p. 131-143. 1899.

Frassi, L. : Uber ein junges menschliches Ei in situ. Arch, f . mikr. Anat. Bd. 71, S. 667. 190a

Froriep, a. : Kopfteil der chorda dorsalis bei menschlichen Embryonen. Beitrage z. Anat. und Embryol. Als Festgabe fUr Jaccb Henle. 1882.

GiacX)minIt C. : Un CBuf humain de 11 jours. Arch. Ital. de Biologic. Vol. 29. 1898.

Heiberq, J.: Uber die Zwischenwirbelgelenke und Knochenkeme der Wirbelsaule. Mitt, a, d. Embryol. Inst, der K. K. Univ. Wien. I, S. 119-129. 1880.

His: Anatomic menschlicher Embryonen. 1880.

Janosik, J.: Zwei junge menschliche Embryonen. Arch. f. mikr. Anat. Bd. 30, S. 559. 1887.

Keibel : Zur Entwicklungsgeschichte der chorda bei Saugem. Arch, f . Anat. und Physiol. Anat. Abt. 1889. Kollmann: Die Entwicklung der chorda dorsalis beim Menschen. Anat. Anz. Bd. 5, S. 308-321. 1890. Die Rumpfsegmente menschlicher Embryonen von 13-35 Urwirbeln. Arch. f. Anat. u. Physiol. Anat. Abt. 1891.

Leboucq, H. : Recherches sur le mode de dispnrition de la chorde dorsale chez les vertebres sup^rieurs. Arch, de Biol., p. 718-736. 1880. Luschka: Die Halbgelenke, 1852; 2. Aufl. 1858. Die Altersveranderung der Zwischenwirbelknorpel. Virchow's Arehiv. Bd. 9, S. 309-327. 1856. Uber gallertartige Auswiiclise am Clivus Blumenbachii. Virchow's Arehiv. Bd. 11, S. 8-12. ia57.

Mall: A Human Embryo 26 Days old. Joum. of Morph. Vol. 5, p. 459-480. 1891. Human Coelom. Joum. of Morph. 1.S97.

MiNOT, C. S.: The Segmental Flexures of the Notochord. Anatomical Record, Amer. Joum. of Anat. Vol. 6. 1907.

MuLLER, H.: Uber das Voikomnien von Resten der chorda dorsalis bei Menschen nach der Geburt. Zeitschrift f. rationelle Med. Bd. 2. 1858.

MusGRAVE, James : Persistence of the Notochord in the Human Subject. Joum. of Anat. and Physiol. Vol. 25. 1891.

Robin, C. : Memoire sur revolution de la notoeorde. Paris, 212 pp., 1868.

RoMiTi, G. : Rigonfiamenti della corda dorsale nella porzione cervicale nelFembrione umano. Notizie Anat. Siena. 1886. Spee, Graf v.: Beobachtungen an einer menschlicben Keimscheibe mit offener Medullarrinne. Arcb. f. Anat. und Pbys. Anat. Abt. S. 159-176. 1889. Neue Beobachtungen iiber sebr friihe Entwicklungsstufen des menschlicben Eies. Arch. f. Anat. und Pbys. Anat. Abt. S. 1-30. 1896.


On the development of the notochord in the higher mammals see especially: Williams, L. W. : The Later Development of the Notochord in Manmials. Amer. Jour, of Anat. Vol. 8, p. 251. 1908.

C. VERTEBRAL COLUMN AND THORAX

The development of the vertebral column and thorax may be divided into three overlapping periods : a membranous or blastemal, a chondrogenous, and an osteogenous.

The Blastemal Pebiod.

The division of the axial mesenchyme into segments, sclerotomes, which correspond to the myotomes and spinal ganglia, is marked at an early stage by intersegmental arteries (Fig. 233, A. is.). The segmental differentiation extends into the region dorsal to the spinal cord, but ventrally it does not reach the chorda dorsalis. Each sclerotome becomes divided into two portions, a caudal half in which the tissue is condensed, and a cranial half in which the tissue is less dense (Fig. 234). In sections through hardened tissue a slight fissure, the intersegmental fissure (v. Ebner, 1888), may partially separate the two halves.®

From the condensed tissue of the caudal half there arises a primitive vertebra of Remak, or scleromere, with dorsal (neural) and ventral (costal) processes and chordal processes which unite these to the perichordal sheath, a dense layer of tissue forming a continuous sheath about the chorda dorsalis (Figs. 234, 237, 238, 240, 241, 242). From the tissue of the anterior halves of the sclerotomes arise * * interdorsal membranes" which unite the dorsal processes of the scleromeres (M. id., Figs. 236, 244, 245, 247), and *-interventral membranes" which unite the bases of the ventral processes {M. ii\, Figs. 235, 243, 244, 245). The chordal processes become hollowed out caudalwards by a loosening up of their tissue and strengthened cranialwards by a condensation of tissue immediately bounding the intervertebral fissure (Figs. 234, 235, 243,


  • Schultze (1896) has described in a correspondine: position in selachians and reptiles a diverticulum which communicates with the myoccel. The fissure is apparently to be looked upon as an offshoot of the myoccel. In birds the fissure is said to arise independently of and to fuse secondarily with the myoccel. In mammals it appears after the myotome has become independent and the myoccel has disappeared.

Fxa. 231.


Fig. 232. Fios. 231 and 232. — Diagrammatic outlines to represent the development of the skdetoo during the blastemal period. Fig. 231. Embryo II, length 7.5 mm. Fig. 232. Embryo CIX, length 11 mm. o, occipital; c», first cervical; 0, first thoracic; i*. first lumbar; »', first sacral; co^, first coccygeal vertebra.


engtri 3.S mn). Pig. 234. Eir 335 and 236. Embryo CoXl.I. Isnglh 6 mm. Fi(. 235 throu(li th( 23S through b more dons) plane. Fig*. 233. 236. 238 repre«nt iwcdons cut somewhat obliquely so that (h« right Bide of ihB Miction. » ventraJ to the left. In Fig». 234 and 236 on the right side the bodie. of several embryonic vertebne are reprewnled in outline. In Figa. 234 and 235, owing to artefncli, the myotomes ore pulled away from the iwlerolometi. ..4.i«., arteria interugmentalia; Cntt., ctelom; Chjl., chorda domlui; Der., dermis; F.v.E.. fissure of v. Ebner (intenertebntl Huure); M.id., membrans interdoTvalis; Af.tc.. membiaLimiiiteTventralia; M.ip.. spinal cord; Myo.. aiyatome: ^.fp.. nervua »pinalia; PiAS., perichontal nheath; Pr.c. proeewuj eoslalis; PrjA., proM9»u« chord»li>; Pr.n.. proee8.«uB Dcuralis; Stl„ sclerotome; V^., vena cardioaLifl.

244, 245). There is thus formed about the intervertebral fissure a primitive intervertebral disc.® The tissue lying between each two discs now becomes completely surrounded by a membrane of condensed tissue, which may be termed an interdiscal membrane (Fig. 246, M. iv.). Meanwhile the perichordal sheath between each two discs becomes extended ventrodorsal) y, so that it gives rise to a *' perichordal' ' septum which divides into two parts the space surrounded by the interdiscal membrane (Figs. 239, 246, 247, Pch.s.). During the earlier stages of the blastemal period the scleromeres are essentially similar throughout the length of the vertebral column. The differentiation of the scleromeres begins in the cervical region and extends caudalwards. At the end of the first month of development the scleromeres present the appearance shown in Figs. 231, 240, 241, and 242, although their margins are less sharply marked than it is necessary to represent them in the model. At this period of development the interdorsal and interventral membranes have begun to appear in the cervical region, but are not represented in Fig. 231. Soon after this period the thoracic region of the spinal column becomes distinguishable from the neighboring regions through the great development of the costal process of the thoracic scleromeres (Figs. 232 and 239). Meanwhile centres of chondrification arise. These are described below. ^^


The occipital Region. — In man, as pointed out above, the primitive axial mesenchjone in the head posterior to the otic region undergoes a partial segmentation. At the end of the first month of development there are three fairly well-marked occipital myotomes which afterwards disappear. The axial mesenchyme corresponding to these myotomes is not definitely divided into sclerotomes, although that opposite the last occipital myotome becomes divided like each of tiie spinal sclerotomes into a light •I have elsewhere (1905) called the united chordal processes of the scleromere a primitive intervertebral disc, but it seems better to restrict this term to the structure here described. according to Williams the primitive intervertebral discs are to be regarded as places in which the tissue remains dense while between them the differentiation of the bodies of the vertebras is marked by a loosening up of the tissue. according to Williams the scleromeres are not true morphological imits.


      • Charlotte Miiller (1906) has described a transitory, longitudinal ridge of cells which extends between the mid-ventral surface of the spinal column and the aorta. Opposite the primitive discs this ridge is connected to the anlages of the corresponding ribs by bands of tissue (hypochordal Spangen) which are not fused to the discs. Opposite the vertebral bodies the lighter tissue of the bodies is continued into the lighter tissue of the centre of the ridge of cells. The ridge extended from the second to the ninth thoracic vertebra in a 13 mm. embryo. There is no segmentation visible in the tissue of the ridge.


anterior and a condensed posterior half (scleromere). The lighter half is continuous apicalwards with the slightly condensed, unsegmented mesenchyme which lies in the region of the more anterior occipital myotomes. This in turn Is continued into a thin layer of dense tissue which is closely applied to the back of the pharynx. The chorda dorsalis surrounded by a perichordal sheath is continued from the spinal region through the centre of the occipital sclerotome and the tissue in front of this into the dense tissue on the back of the pharynx, in which it may be followed to Seessel's pocket. Fig. 231 represents the sclerogenous tissue of the occipital region at the end of the first month. The posterior portion of this, corresponding in form with the first cervical scleromere, is composed of very dense tissue. Anterior to this the tissue is much looser in texture. The occipital scleromere has fairly well developed neural and chordal but has no clearly marked costal processes. No clearly defined interdorsal and interventral membranes are developed from the light half of the first cervical sclerotome. For the subsequent changes which take place in this region see p. 343.

FtQ. Z4C. — (Afwr Banleen, i

FlQ. 247.


FiGB. 2«-247.— (Afwr Banleen, Ainer, Joum. of Anftt., vol. Lv, 1905.) \ie»a of modcla repre Fig9. 240-242. F.mbryo tl. length 7 mm. Ma(a.33):l. FiO243-245. Embn'O CLXIII, length . Magn. 23 ; 1. Figs. 246, 247. Embryo CIX, lengtli 11 mm. Magn. 2S : 1. Figa. 240. 243. 246. n from in front: Figi. 241, 244. 247. views from the nde: Figs. 242, 24S, \iewt! from briiind. A.U.. 'rii Inter^menlsUx: CA.(f.. rhiHila dorealis; Diar, mtervert«biAldi«: M.iif., manbrsasiDlenloraalia;

Chondrogenous Period

On each side of the blastemal vertebra three primary centres of chondrification appear at about the same time, one for the neural process, one for the costal process, and one for the vertebral body. Fig. 239 shows these centres as they appear in a cross section at an early period. Figs. 248 and 249 show, the early cartilages of an embryo slightly older (length 14 nma., age five and a half weeks).


The cartilages of the vertebral body are formed by a transformation of the loose tissue lying between the primitive intervertebral discs and surrounded by the interdiscal membrane. At first the cartilage of the left side is separated from that of the right by the perichordal septum. Soon this is broken through and the two cartilaginous anlages of the vertebral body become united about the chorda. In the thoracic region this union seems to take place at about the same time dorsally that it does ventrally. A sagittal section through the thoracic region of an embryo at this stage is shown in Fig. 250. The chorda dorsalis is surrounded by a perichordal sheath. The latter is encircled by dense intervertebral discs which alternate with light cartilaginous bodies, surrounded by perichondrium which is less condensed than the tissue of the discs. Ventrally and dorsally longitudinal ligaments have been differentiated from the surrounding mesenchyme.


according to Schultze (1896), the cartilages of the bodies lengthen at the expense of the discs and finally fuse to form for a brief period a continuous cartilaginous colunm. In human embryos between 20 and 40 mm. in length the discs become very thin near the chorda dorsalis, but the centre of the disc does not become so completely differentiated into embryonic cartilage that it is not possible to distinguish the boundaries between successive vertebral bodies. according to Charlotte Miiller (1906), the intervertebral tissue near the chorda so far undergoes chondrification that capsules may be seen about the tissue cells. This stage is quite transitory. At the periphery of the discs the annulus fibrosus is meanwhile differentiated more and more into a condition resembling the adult (Figs. 250, 253, 256, 259).


The chorda dorsalis at the period shown in Fig. 250 is of about the same size at the level of the discs as at the centres of the bodies. It may become slightly swollen in the bodies, but as the bodies increase in size at the expense of the discs the chordal canal becomes enlarged in the intervertebral areas and constricted at the centres of the bodies (Figs. 253, 256, and 259). The chorda loses its continuity and the chordal cells become clumped in the vicinity of the discs, spread out there in the form of a flat disc (Fig. 258), increase in number, and give rise to the nucleus pulposus. Meanwhile the chorda cells lose their cell membranes and form a syncytium similar to that of the mesenchyme. About the cells mucin is formed in considerable amounts (Williams). The chordal canal long remains in the vertebral body (Figs. 256 and 259). The chordal sheath remains in the canal until the period of ossification.

Fla. 26S. Flo. 256.


Fig. 257. Fig. 258. Fig. 259.

FioB. 248-269.— (After flardeen. Anier. Journ. of Anal,, vol. iv. 1B05.) FigB. 248. Z4B, 251, 252, 254. 255, 257, 258. Ventral, lateral, and doraal views of inodelj' made by vertebne during the chandrogenoui" period. On the left fjde the cartllaginouii, on the right the enveloping fibrous liwue i* shown. The latter is also shown on the eighth vertebra in Fig. 252. Fign. 248. 249. Embrj'o CXI.IV, lengtli 14 mm. M«gn. 17 : 1. Figs. 2.11. 252. Embryo XXII. length 20 mm. Magn. 13:1. Figs. 254. 255. Embryo CXLV. length 33 mm. Magn.QM. Figs. 2S7, 258. Embryo LXXXIV, lengtliSOmm. Magn. 9:1. Fig. 257. dorsal view, left half . Fig. 258. median view. Figs. 250, 253, 250. 259. flagittal -enions in the mid-line through the niilh, seventh, and eighth thoracic segments of a series of embr>-oi! from 15 lo 50 mm. long. Fig. 250. Embryo CXLI\'. length 14 mm. Fig. 25B. Embryo C\'1I1, length 22 mm. Fig. 256. Embryo LXXIX. lenglh 33 mm. Fig. 259. EmbryoCLXXXlV.lengIh50mm, C.e.. eorpus vertebra; Co«o, rib; CA.rf., chorda dorsalis; Z>t«, inteiw vertebtaldise; I,., lamina; L.v.. lig. ventrale; /'r.a.a.. proc.articulariaant. Isup,); Prji.p., proe. articularia post. {iaf.J; Pr.n.. proc. neumlis; Fr.rd., prae. radicularit; Pr.t.. prnc. spinaliK; PrJr., proc. transvecBus.


The cartilage of the bodies in embryos of the sixth week (Figs. 248, 249, and 250) is of an early, embryonic, hyaline type. At a slightly later stage (Fig. 253) two regions may be distinguished, a central and a peripheral. The peripheral cartilage resembles that of the preceding stage, while the central cartilage is more dense. Gradually the cartilage at the centre of the body undergoes further changes. The cells enlarge and become sharply set off against the matrix, and finally an invasion of vascular tissue takes place, chiefly from the posterior surface. These changes in the cartilage are preliminary to ossification.


During the development of the vertebral bodies changes have been active in the neural processes. At the period represented in Fig. 239 the neural cartilage is a small, flat body situated in the dorsal process of the blastemal scleromere; from this as a centre, radicular, transverse, cranial (superior) and caudal (inferior) articular, and laminar processes are rapidly developed. This structural differentiation may be followed in Figs. 248, 249, 251, 252, 254, 255, 257, 258.


The cartilaginous radicular processes are at first slender rods which grow out towards and finally fuse with the corresponding vertebral bodies (Figs. 249 and 251). Froriep (1883) has shown that in the chick these processes form a more essential element of the body than in mammals. In the atlas they form a part of the ventral arch, but in the thoracic region of mammals they fuse with the posterolateral portion of the corresponding vertebral bodies.


After their junction with these the radices of the arches increase in size but otherwise show no marked alterations of form. The transverse processes are at first short projections which lie at some distance from the corresponding ribs (Fig. 249). At the time tubercles are developed on the ribs the transverse processes grow outward and forward to meet them (Figs. 252 and 255). At first the developing cartilage of each rib and the corresponding vertebral transverse process are embedded in a continuous blastema, but before chondrification has proceeded far branches from successive intervertebral arteries become anastomosed in the region between the neck of the rib and the transverse process. Fig. 239, Aa, shows the loose tissue through which this arterial anastomosis will take place.


Between the extremity of the cartilaginous transverse process and the rib a joint cavity is developed, and the surrounding blastema is converted into the joint capsule and the costo-transverse ligaments. Similarly a joint is developed between the head of the rib and the corresponding intervertebral disc and vertebral bodies.


The articular processes develop slowly from the cartilage (Figs. 249, 252, and 255). Extension takes place anteriorly (Pr. a a.) and caudalwards {Pr. a.p.) in the interdorsal membranes. In an embryo of 14 mm. (Fig. 249) these cartilaginous articular plates are separated by a distinct interval. In one of 17 mm. they have approached each other very closely ; and in one of 20 mm. not only do the articular processes show distinctly more form (Fig. 252), but in addition the inferior articular process of one vertebra slightly overlaps the superior process of the vertebra next posterior. This overlapping of the articular processes is distinctly more advanced in an embryo of 28 mm. and still more so in one of 33 mm. (Fig. 257). In a fetus of 50 mm. (Fig. 258) conditions essentially like the adult have been reached.


The laminar processes scarcely exist in an embryo 14 mm. long (Fig. 249). In an embryo 20 mm. long (Fig. 252) they have begun to project dorsal to the region of the articular processes. The dense embryonic connective tissue covering the laminar processes at this stage gives attachment to a membrane surrounding the spinal cord, membrana reuniens dorsalis, and to another one covering the dorsal musculature. In a fetus of 33 mm. the laminar processes extend well toward the mid-dorsal line (Fig. 255) ; in one of 50 mm. (Figs. 257 and 258) they completely encircle the spinal canal and from the region of fusion of each pair of laminar processes a spinous process extends distally, though not so far as in the adult. The completion of the spinal canal takes place earlier in the thoracic than in the cervical and lumbar regions.


Alterations in the cartilage of the neural processes preliminary to ossification begin at about the time that they take place in the vertebral bodies. They are first seen in centres which correspond to those in which the neural cartilage begins in the blastemal neural processes. In a fetus of 50 mm. calcification may be seen in the arches of the first cervical to the sixth thoracic vertebrae. From the blastemal tissue surrounding the cartilaginous vertebrae are developed the various ligaments of the spinal column.


Summary. — Each cartilaginous vertebra is developed from four centres of chondrification. In addition a separate centre appears for each rib. In comparing these centres with the blastemal formative centres, we find that each primitive centre of blastemal condensation enters into union with tissue derived from the anterior half of the body-segment next posterior and then gives rise to three centres of chondrification, one for the neural arch, one for the rib, and one for a lateral half of a vertebral body. When ossification first takes place the centres of ossification of the neural arches and the ribs correspond to the original chondrification centres in the blastema, but the centres of ossification of the bodies in most of the vertebrae show little trace of the bilateral origin of the centres of chondrification.

Regional Differentiation, the Thoracic Vertebrae and the Thorax

The chief steps in the development of the thoracic vertebrae have been described in the preceding section. In the blastemal stage these vertebrae became differentiated from the cervical and lumbar by the greater development of the thoracic costal processes. During the chondrogenous stage this difference becomes still more marked. The distal ends of the thoracic costal processes grow rapidly forward (Figs. 260 and 261). The cartilaginous ribs which are differentiated in these blastemal processes do not fuse with the vertebrae, but are connected with them at first by dense tissue and later by joints and ligaments. The blastemal distal ends of the ribs at first take a nearly horizontal direction (Fig. 260), but later their course of development becomes altered by the heart and liver. (See Figs. 261-263.) The distal blastemal ends of the ribs become united by a blastemal tissue to form on each side a sternal plate (Fig. 261). This sternal plate proximally is fused with the anlage of the clavicle and distally extends to the blastema of the seventh rib. The fusion of the tips of the more distal true ribs into the sternal plate does not always take place in regular sequence. The eighth rib is connected near the sternal aniage with the blastema of the seventh and the ninth with that of the eighth rib. the tenth rib becomes similarly attached to the ninth. The attachment of eaoh of the three distal ribs to the one next anterior is apical up to the latter half of the second month. After this it is marginal (Ch. Miiller, 1906).


Fig. 262. Fia. 263. FiOB, 260-303.— (After Charlotte Holler, Morpholos. Jihrb.. 1906.) The developmeat of the nrti Fig. 260. Embryo 13 mm. long. Fig. 261. Embryo 17 mm. long. Fig. 262. Embryo 16 mm. looc. fig. 263. Embryo 32 mm. long. The sternal ends of the clavicles become united with one another by a dense band of tissue which probably represents the episternum of lower forms (Fig. 261). Later, when the heart has descended into the tlioracic cavity, the cranial ends of the sternal plates become united with one another, and the epistemal band becomes united to them and loses its intimate connection with the clavicles (Fig. 262). It normally disappears before the end of the second month. The sternal bands gradually become fused throughout their length to form the sternal anlage (Fig. 263). Sometimes fusion takes place distally before it is completed in the middle. The ensiform process is formed by the fusion of bands of tissue which extend distally from each sternal plate. The eighth pair of ribs does not enter into its formation (Ch. MuUer). Chondrification takes place in the sternal plates at the time these begin to fuse to form the unpaired sternum. according to Ch. Miiller, chondrification extends from the ends of the ribs into the sternal anlage, and the separation of the cartilaginous ribs from the sternum is a secondary process. The order of separation is not always regular. Some investigators assume that special centres of chondrification arise in the sternum.


LITERATURE.

The most important papers on the early development of the human stemmn are those of G. Ruge, Morphol. Jahrbuch, Bd. 6, 1880; A. M. Paterson, Joum. of Anat. and Physiol., Vol. 35, 1901; and Ch. Muller, Morphol. Jahrb., 1906. Not infrequently the epistemal rudiments, instead of fusing completely with the sternum, become ossified either as separate bones or as bony projections from the upper margin of the sternum. (H. Eggeling, Verb. Anat. Gesellsch., 1903.) Paterson describes a sternal anlage independent of the ribs. The existence of such an anlage is disputed by Ch. Muller. Krawetz (1905) describes in the mouse two sternal anlages which have an origin independent of the ribs.


Cervical Vertebrae and the Base of the Occipital Bone

During the earlier stages of development the cervical vertebrae resemble those of the thoracic region. The two regions soon become differentiated from one another bv the much greater development of the costal processes in the thoracic region (Fig. 232). The seventh cervical vertebra alone, as a rule, has large costal processes and these do not extend far beyond the transverse processes of the neural arches.


In the costal processes of the seventh cervical vertebra centres of chondrification are found at the period when similar centres appear in the ribs. Centres of chondrification in the rest of the^ cervical vertebrae appear much later, usually not until the embryo has reached a length of from 16 to 18 mm. according to Valenti (1906), there are normally no separate centres of chondrification in the costal elements of the cervical vertebrae, but chondrification extends into them from the cartilage of the bodies. There seems, however, good evidence of separate costal centres which arise near and quickly fuse with those of the bodies.


As in the thoracic vertebrae, there are two bilaterally placed centres of chondrification for each of the vertebral bodies. These soon fuse with one another ventral and dorsal to the chorda dorsalis. Tn the first two vertebrae the ventral fusion takes place before the dorsal fusion.

there are separate centres of chondrification for the neural processes. In the more caudally situated cervical vertebne these centres are similar to those of the thoracic vertebrje. In the more cranially situated cervical vertebra? each neural centre of chondrification appears as a basal plate lateral to the cranial end of the body of the vertebra. With this it soon fuses. From this


Fig. iOS. o{ the cervinl vertabifE and theoccipiulcftrtilace of ■» embryo plate-like base chondrification extends rapidly into the main part of the arch. From the neural arches are developed laminar, articular, and transverse processes. The cartilaginous costal centres become fused medially with the bodies of the vertebrae and laterally with the tips of the transverse processes. The dorsal growth of the laminar processes and the formation of the spinous processes of the cervical vertebrie take place in the main as in the thoracic. When fully formed, however, the cartilaginous cervical vertebrae have essentially the shape of the adult osseous cervical vertebrae. Even before the end of the second month of development distinct cervical characters may be distinguished.

Specific mention must be made of the mode of development of the epistropheus, of the atlas, and of the basioccipital.

Epistropheus. — The general mode of development of the epistropheus is like that of the other cervical vertebras. The dens represents the body of the first cervical vertebra. Union of the body of the first vertebra with that of the second takes place through the transformation of the intervertebral disc into hyaline cartilage, first lateral to the midsagittal plane, then later in this plane. Fig. 265 represents a stage where the lateral fusion is complete while the medial fusion has not yet taken place. The articulations between the superior articular processes of the epistropheus and the lateral masses of the atlas apparently are developed rather in the interventral membranes than in the interdorsal membranes. (See Fig. 264; compare with Figs. 249, 252, 255, 258.) Atlas (Figs. 264 and 265). — The base (radicular process) of each cartilaginous arch piece of the atlas becomes temporarily fused with the cartilage of the body (14 mm. embryo). This fusion is brought about by incompletely differentiated cartilage, and soon after it takes place the precartilaginous tissue between the arch and the body becomes transformed into a dense blastemal tissue in which ligaments and a joint cavity are later developed.


Meanwhile, during the period of chondrification in the arches and bodies of the cervical vertebrae, there takes place a condensation of tissue on the ventral margin of each of the more proximal cervical intervertebral discs near the cranial end of the vertebral body which lies caudalwards from it (Fig. 266). These condensed transverse bands of tissue connect the ventral ends of the blastemal neural processes with one another. They represent the hypochordal Spangen or braces of Froriep, and may appropriately be called hypochordal arches. In their intimate relations to the intervertebral discs they apparently differ from the hypochordal Spangen described by Charlotte Miiller in the thoracic region of the human embryo (see note, p. 334). In man the hypochordal arches are transitory in all except the first cervical segment. In the more distal segments the tissue composing them seems to become merged in the intervertebral discs without going beyond the blastemal stage. In the first cervical segment the hypochordal arch becomes chondrified at the time of the separation of the arches from the body after the temporary fusion mentioned above. The cartilage of the hypochordal arch becomes united on each side to that of the neural hemiarch. There are evidences of two bilaterally placed centres of chondrification in the hypochordal arch.


but fusion of these centres with one another and with the cartilage of the neural hemiarches takes place as soon as ehondrification is well under way. according to Froriep (1883), in the cow there is a single median centre in the hypochordal arch. In the white rat, according to Weiss, there are two bilaterally placed centres of ehondrification in the hypochordal arch of the atlas. Froriep reports in the cow temporary centres of ehondrification in the other cervical hypochordal arches, but no true cartilage formed, except


Stpl.niui Fia. 266.— Sa^tUl sectian throush the hud or an embryo 14 mm. Ions. Arcui AvpaA-. ■rcua hypoehordsliB, bypochordiJ bi»c« or "Spatige"; A, 6o«(., arteri* bluularis; Calv. mrmbr., a^vKi* BtanbrsnMeft; Cart, occ.. oartilBgooocipitaliB; Cart. «pft.. cartilage iphenoidalii; C i. fl, corpus vertfbiw sextK; CA. rf., thorda doraaliB: Lintr., Uogua; (Ewp*., aaophagus; Si^ii. noji. »«ptum nam; Tr., trache*. very temporarily in that of the epistropheus. Weiss found no ehondrification in any hypochordal arch in the white rat except that of the atlas." according to Sehauinsland (Hertwig's Handbuch, 1906), the neural arches of the mammalian vertebne contain elements of both the ventral and dorsal arches found in the lower vertebrates, and the ribs belong primitively to the ventral arches. In the mammals and man, however, the presence of ventral arch elements is manifest merely in the caudal and cen-ical regions. In the caudal region temporary hsemal processes are developed (see p. 352). In the cervical region the ventral arches are refiresented by the hypochordal braces or arches. In reptiles and birds the hypochordal arches are more extensively developed than in man.


"Ganfini (ISWO) has reported in a number of eases the apparent rudiments a hypochordal arch in connection with the basilar portion of the occipital.


The costal processes of the atlas become fused medially to the basilar part of the neural arches {Fig. 264). For a brief period (14 mm. embryo) the bases of the neural arches of the atlas and epistropheus together with the tissue intervening between the atlas and occipital bone become fused into a nearly continuous mass of precartilage {Fig. 267).'* Basioccipital. — It has already been pointed out that opposite the last occipital myotome the axial mesenchj-me is differentiated, like that of the spinal sclerotomes, into a light anterior half and a dense posterior half. the dense posterior half is called a scleromere. In the spinal region each scleromere joins with the light half of the sclerotome next posterior in giving rise to the body and arch processes of a spinal vertebra. In man the occipital scleromere is not thus associated with the light half of the first spinal sclerotome. On the contrary, it becomes associated with the lighter tissue of its own segment and with the tissue into which this is continued cranialwards. Fi:?. 266 may serve to illustrate F'oae:.— sagittai >»iion through the uieni p>rt ,1' Ti -111 Jill J of the cervical rapan of theepioal coJiuna of the embryo this. It will be noted that .bown in Fi(. zee. the anterior half of the first spinal sclerotome is composed of light mesenchjTnatous tissue, while the basioecipital and the bodies of the spinal vertebrse are composed of cartilage. Chondrification of the base of the occipital begins in two bilaterally situated centres in the posterior portion of the occipital anlage. The union of these centres takes place caudalwards ventral to the notochord and apiealwards dorsal to the notochord. The neural processes of the caudal part of the occipital anlage seem to have separate centres of chondrification, but these centres fuse almost immediately with the centres of chondrification of the body. Figs. 264 and 265 show the appearance of the occipital cartilage toward the end of the second month of embryonic development. For further details see the subsection on the development of the skull.


"Hagen (1900) gi\-es a aomewhal different account of the development of the atlsB and epistropheus in man. He concludes (1) that the dens epistrophei arisea from the region of the body of the epistropheus and a portion of the body of the atlas, (2) that the massie laterales of the definitave atlas arise from the rest of the primary anlag« of the body of the atlas, and (3) that the short piece which luiites them in front arises from the fusion of both neighboring septa. according to Weiss (1901), the liglit, cranial half of the first spinal sclerotome gives origin to a cartilaginous tip on the dens epistrophei. The caudal part of the occipital Weiss r^ards as arising from the neural processes of the last occipital scleromere. Robin (1864) gives several good pictures of early stages of the cartilaginous cervical i.'ertebrTE.

Ligaments and Joints. — The atlanto-occipital like the lateral atlanto-epistropheal diarthroses are apparently formed rather in the interventral than in the interdorsal primitive membranes. From the interdorsal membranes between the atlas and the occipital bone arises the membrana atlanto-occipitalis.

From the periphery of the perichordal part of the light anterior half of the first spinal sclerotome are differentiated the cranial extremities of the anterior and posterior longitudinal ligaments, of the tectorial membrane, and of the alar and the crucial ligaments of the atlas. About the chorda dorsalis in this region the lig. apicis dentis is differentiated, probably chiefly from the perichordal tissue. Cartilaginous and osseous nodules found occasionally in this ligament have been thought by some to represent remnants of the original tissue of the chorda (H. Miiller, 1858). Albrecht (1880) advanced the view that these nodules represent the vestige of a supplementary vertebra (pro-atlas), but this view has been disputed by Comet (1888), Chiarugi (1890), and others. Weiss states that in the white rat the perichordal tissue of this region gives rise to the tip of the dens epistrophei, but this appears not to be the case in man. The tissue between the apical ligament of the dens, and the anterior, alar and crucial ligaments of the atlas becomes converted into a fibro-adipose tissue. In this there is ventrally a slight extension of the synovial cavity between the dens and the atlas and dorsally a greater extension of the cavity between the dens and the transverse ligament. The ligaments in the vicinity of the epistropheus are developed from the periphery of the perichordal tissue and from the interdorsal primitive membranes. LUMBAR, SACRAL, AND CoccYGEAL VERTEBR2E. In the earlier stages of development the lumbar, sacral, and coccygeal vertebrae resemble the thoracic. The blastemal vertebrae arise each from the contiguous halves of two primitive segments of the axial mesenchyme. Each vertebra exhibits a body from which neural and costal processes arise. The neural processes are connected by ** interdorsal" membranes. As the blastemal vertebrae become converted into cartilage specific differentiation becomes more and more manifest. The cartilaginous vertebral bodies and the intervertebral discs are all formed in a manner similar to that of the thoracic vertebrae and except for size manifest comparatively slight differences in form. The more distal coccygeal vertebrae are, however, irregular. But the chief specific differentiation is seen in the costal and neural processes.

The development of the vertebrae of the distal half of the vertebral column may be followed in Figs. 27^278 (p. 368).

In the lumbar vertehrce radicular, transverse, articular, and laminar processes arise from the neural cartilages. The radicular processes resemble the thoracic but are thicker; the transverse processes are shorter, much thicker at the base, and remain bound up with the costal processes; the superior articular processes develop in such way as to enfold the inferior articular processes of the vertebra next cr anial wards ; the laminar processes are broad, grow more directly backward than do the thoracic, and on meeting their fellows in the mid-dorsal line fuse and give rise to the typical lumbar spines. The mammillary and accessory processes are developed in connection with the dorsal musculature and are not definitely formed in cartilage. In the sacral vertehrce the neural cartilages give rise to very thick radicular processes; to articular processes, the most cranial of which develop like the lumbar, while the others long retain embryonic characteristics ; to transverse processes which in development are bound up with the costal processes; and to laminar processes which are very slow to develop and of which the last fail to extend far beyond the articular processes.


In the coccygeal vertebrce the neural processes of the first, and rarely of the second, give rise to cartilaginous plates. From these only radicular and incomplete articular and transverse processes arise. The comua of the adult coccyx represent fairly well the form of the embryonic neural semi-arches. In the thoracic vertebrae cartilagino'iis ribs develop from separate centres in the blastemal costal processes. In the lumbar vertebrae separate cartilaginous centres probably also always arise in these processes, but they are developed later than those of the thoracic vertebrae and quickly become fused with the cartilage of the transverse processes. The transverse processes of the adult lumbar vertebrae represent at the base a fusion of embryonic cartilaginous costal and transverse processes, but laterally an ossification of membranous costal processes.


In the sacral vertebrae separate cartilaginous costal centres are developed, but they soon become fused at the base with the transverse processes of the neural cartilages. Laterally by fusion of their extremities the costal processes give rise to that part of the sacrum which articulates with the ilium. In the coccygeal vertebrae the costal processes of the first vertebra become fused with the transverse processes and develop into the transverse processes of the adult coccyx. It has not been determined whether a separate costal cartilage is developed in these processes or cartilage extends into them from the neural processes. The costal processes of the other coccygeal vertebrae have merely a very transitory blastemal existence.


For a brief period the more distal sacral and the coccygeal vertebrae have membranous hcemal processes. Schumacher (1906) describes a haemal arch on the first coccygeal vertebrae which he considers present in most human embryos 3-5 months old. Centres of ossification correspond in general with centres of chondrification, but, as in the case of the vertebral bodies and the more distal sacral neuro-costal processes, a single centre of ossification may represent two centres of chondrification.


Period of Ossification.


In the vertebrae, ribs, and sternum one may distinguish primary and secondary centres of ossification. Most of the primary centres appear early in intra-uterine life, while the cartilaginous vertebrae and thorax are assuming a definitive form. The secondary centres appear after birth. Vertebrae.


Primary Centres. — There are three primary centres, one for the body of the vertebra and one for each hemi-arch (Fig. 268, A). These centres begin to appear at about the same time, but in the cervical region the centres of ossification in the arches appear before the centres in the bodies, while in the thoracic and lumbar regions and, as a rule, in the sacral region, the reverse is true. In the fetuses studied and tabulated by Mall (1906) the first centres found were in the second to the eighth neural arches of a fetus 33 mm. long and about 57 days old. In a fetus 34 mm. long and about 58 days old centres of ossification were found in the arches of all the cervical and thoracic vertebrae, and in the bodies from the third thoracic to the first sacral. Centres of ossification in the bodies first appear in the more distal thoracic vertebrae and the first lumbar.


The centres of ossification in the arches extend from the cervical region distally in fairly regular sequence, although not with equal rapidity in different fetuses. In one fetus 53 mm. long and about 72 days old they had extended to the third sacral vertebra (twenty-seventh spinal vertebra). On the other hand, no centres were found in the arches of this vertebra in several other fetuses, 73-105 days old. The centres in the arches of the more caudally situated vertebrae arise generally in the fifth or sixth month.


The centres of ossification in the bodies of the vertebrae extend cranialwards from the thoracic and caudalwards from the lumbar region. In a fetus 70 mm. long and about 83 days old they had extended on the one hand to the epistropheus and on the other to the last sacral vertebra (Mall). There is considerable variation, however, in the rapidity with which the centres of ossification appear in the bodies of the cervical and sacral vertebrae.


The type of ossification in the bodies is endochondral (see p. 309). The cells at the centre of the body of the vertebra enlarge and become sharply set off against the intercellular substance. Finally an invasion of blood-vessels takes place, chiefly from the


Fig. 268.— (After R. Quain, Quam's Anatomy, 10th ed., vol. ii, Pt. 1, Figs. 19, 20, 21. and 22.) Diagrams to illustrate the development of various vertebnp. A, fetal vertebra; B, thoracic vertebra of child of two years; C, thoracic vertebra in the seventeenth year; D and E, lumbar vertebra of about same age; F, atlas before birth; G, atlas in first year; H, epistropheus in fetus of seven months; I, epistropheus shortly after birth; K, sacrum before sixth month; L, saonmi at birth; M. sacrum at about twenty-third year; N (after Allen Thomson), first sacral vertebra at fourth or fifth year. C, centre of ossification in the body; C\ coitre of ossification in the dens; Co, centre of ossification in costal element; Et epiphjrsis; N, centre of ossification in neural arch; V, centre of ossification in ventral arch of atlas. dorsal periosteum. Calcium salts are deposited in the cartilage and this is followed by actual ossification in fetuses about three months of age. In the arches the process of ossification is likewise endochondral.^^


The centre of ossification in the body of the vertebra gives rise to the greater part of the body of the definitive vertebra. occasionally the centre may arise as or become divided into two bilaterally placed centres, one for each half body. This division may persist in the adult. The centres in the arch give rise to the posterolateral part of the body of the vertebra and to the greater part of the arch with its various processes. It is in the cervical region that the centres in the arch contribute most to the body. At birth the bones arising from each of the centres of ossification of a vertebra are separated from one another by cartilage. During the first year the centres of ossification in the neural arches in most of the vertebrae become united dorsally. This fusion takes place first in the lumbar region. Between the third and sixth years the bony arches become united to the body. This fusion takes place first in the thoracic region. The neurocentral suture lies in a nearly sagittal plane in the' cervical, in an oblique plane in the thoracic, and in a frontal plane in the lumbar region.

"In enibr>'()s cleared by the Schultze method the complementary primitive centres described by Rambaud and Renault do not appear. It is probable that they were artificially produced by the methods of preparation employed by Rambaud and Renault (Mall). The primary centres for the neural hemi-arches are single, not double. See, however, the account of the lumbar vertebrsB (p. 354).


Secondary Centres; Epiphyses. — Toward the intervertebral discs the bodies of the vertebrae remain long covered by at layer of cartilage. About the seventeenth year a centre of ossification appears in the cartilage on each intervertebral surface. From each of these centres of ossification a thin epiphyseal disc of bone arises (Fig. 268, E). The discs fuse with the body about the twentieth year. The line of suture is visible usually for a year longer.


The tips of the spinous and transverse processes are covered during infancy by cartilage. In this cartilage epiphyseal centres of ossification appear between the sixteenth and the twentieth years and join the osseous arch after the twentieth year (Figs. 268, C and D). Similar secondary centres on the dorsal margins of the superior articular processes and on the costal facettes of the thoracic vertebrae have been described, but are not generally recognized. (See Poirier and Charpy, Traite d'Anatomie, vol. i, p. 342, 1899.)


Cervical Vertebrce. — ^In most of the cervical vertebrae, according to Leboucq (1896), the ventral limb of the transverse process is ossified by ingrowth at one end from the radix, on the other from the tip of the transverse process. In the seventh cervical vertebra frequently, in the sixth occasionally, and in the fifth and fourth rarely, there may arise during the second to the fifth month a separate centre of ossification for the costal element. While this costal element may remain free as a cervical rib, it usually becomes fused with the osseous projections from the radix and the transverse process (Figs. 269, A and B). Except in the seventh cervical vertebra the epiphyses of the spines are usually double.


Atlas, — The posterior arch and the lateral masses of the atlas are ossified from two bilaterally placed centres which correspond to the centres of the neural arches of the other vertebrae (Fig. 268, F). In the anterior arch one, or sometimes apparently two, centres of ossification appear during the first year after birth (Fig. 268, G). The dorsal union of the osseous neural arch pieces occurs between the third and fifth years. Often a separate centre of ossification appears in the spinous process before the neural arch pieces are united (Quain). The union of the posterior with the anterior arch occurs between the fifth and ninth years.

Epistropheus. — The neural arch and the body are ossified essentially as in the other cervical vertebras except that occasionally there are two bilaterally placed centres in the body. The odontoid process becomes ossified from two bilaterally placed centres which appear in the fourth or fifth fetal month and soon A


Fig. 269. — (After T. Dwight, Piereors Human Anatomy, 1907, Fig. 168.) Diagrams illustrating the homology of the costal elements. A, sixth cervical vertebra; B, seventh cervical vertebra; C, fifth thoracic vertebra; D, second lumbar vertebra; E, fifth lumbar vertebra; F, transverse section through sacrum. The costal elements are stippled.

fuse together (Fig. 268, H and I). These centres furnish material for a part of the superior articular processes. Between the fourth and sixth years the odontoid process becomes joined to the body and the radices of the arch, first laterally and then ventrally and dorsally. Between the centre of the base of the odontoid process and the body of the epistropheus a disc of cartilage remains till late in life. The apex of the odontoid process is formed from a separate centre of ossification, which appears in the second year and is joined to the main part of the process about the twelfth year. This apical piece probably represents an epiphysis. There are also said to be rudiments of a cranial epiphysis of the body of the epistropheus, but this statement is not generally accepted. A caudal epiphysis of the body is constant.


Lumbar Vertebrce. — The mammillary processes of the lumbar vertebra^, of the first sacral vertebra (Fawcett, 1907), and of the twelfth thoracic vertebra have special epiphyses, which appear about the time of puberty or a little later and join the rest of the vertebrae after the eighteenth year (Fig. 268, D). Somewhat rarely the costal element of the first lumbar vertebra has a separate centre of ossification, which appears early in fetal life. It may remain free as a lumbar rib. Sometimes in the fifth lumbar vertebra, and very rarely in some of the others, there are found two centres for the arch on each side; one for the radix, transverse process, and superior articular process, the other for the lamina, inferior articular process, and spine. according to Poirier and Charpy (Traite d'Anatomie, 1899), the fifth lumbar vertebra has a special epiphysis for the anterior tubercle of the transverse process.


Sacrum. — The usual primary centres are found for each of the five sacral vertebrae, one for the body and one for each neural hemi-arch. In addition there are separate centres for the costal elements of the first three or four vertebrae (Fig. 268, K and L). Sometimes, apparently, costal centres are found merely in the first two sacral vertebrae, sometimes in all five. Changes preliminary to ossification occur both in the bodies and in the neural processes of the sacral vertebrae at a period quickly following their appearance in the lumbar region. Actual ossification in these centres in the more distal vertebrae, as a rule, does not take place until a considerably later period, usually not until the fourth month in the bodies and the fifth or sixth in the arches. The centres in the arches join those of the bodies between the second and sixth years. The more caudally -situated join before the more cranially situated. Union of the laminae with one another takes place from the seventh to the fifteenth years. It takes place first in the more cranially situated vertebrae, frequently does not occur in the fourth and seldom in the fifth. The centres for the costal elements of the first three vertebrae arise usually, according to Posth, in the fifth, sixth, and seventh fetal months respectively. That for the fourth vertebra does not usually arise until tiie third month after birth. There are considerable variations in the time of origin. The costal centres unite with those of the neural arches between the second and fifth years. They unite with the bodies slightly later than with the arches. Rambaud and Renault (1864) describe special centres which are said to arise in the sixth month in the transverse processes. Posth could not confirm this.


In addition to these primary centres there are epiphyseal plates for each body and two for each lateral sacral margin, one for the auricular surface and one for the rough edge distal to this (Fig. 268, M). according to Poirier (Traite, 1899), the auricular epiphyseal plates arise from the fusion of the epiphyses of the transverse processes. Fawcett (1907) describes them as arising from four costal epiphyses belonging to the first two sacral vertebrae. The tuberosities he describes as arising from the costal epiphyses of the third and fourth sacral vertebrae and transverse epiphyses of the fourth and fifth. The epiphyses of the bodies begin to arise about the fifteenth year and those of the auricular plate between the eighteenth and the twentieth years. Epiphyses for each of the tubercles of the spinous processes are described by Rambaud and Renault, 1864, and Fawcett, 1907. Fawcett describes twelve costal epiphyses, eight epiphyses belonging to the transverse processes, two to the mammillary processes and three to the spinous processes.


The sacrum begins to be consolidated about the time of puberty. The costal processes on each side fuse with one another. This is followed by union of the epiphyses with the bodies and by ossification in the intervertebral discs. The process begins caudally and extends in a cranial direction. The bodies of the first and second sacral vertebrae usually become united about the twentyfifth year but the centres of some of the intervertebral discs may persist longer than this. The lateral epiphyseal plates unite about the twenty-fifth year. For the recent literature on the development of the sacrum see Posth (1897) and Fawcett (1907).

Coccygeal Vertebrce. — Ossification in the coccygeal vertebrae usually takes place after birth. Each is ossified from a single centre. The centre for the first vertebra usually appears in the first year but may appear much later, that of the second appears from the fifth to the tenth year, that for the third just before and that for the fourth just after puberty. The three more distal vertebrae usually become united with one another before being joined to the first. This latter union may not occur until the thirtieth year. The first coccygeal vertebrae not infrequently becomes united to the sacrum. In old individuals the whole coccyx is often united by bone to the sacrum, more often in men than in women. according to some authors, there are two epiphyseal plates for each of the bodies of the first four coccygeal vertebrae and in addition separate centres of ossification for each of the comua of the first vertebrae.^* Two centres of ossification for the fifth coccygeal vertebra, one for the body and one for an epiphysis, are also described as arising in the tenth year (Poirier and Charpy, 1899).


Ribs

Ossification begins in the ribs before it does in the vertebrae. Centres appear in the bodies of the sixth and seventh ribs toward the end of the second month and then rapidly come to view in the other ribs. The centre in the first rib usually appears before that in the twelfth. All are usually present by the end of the second month, but that in the twelfth may not appear until later. In two specimens out of 29 fetuses with an estimated age of 55 to 110 days, Mall (1906) found a centre of ossification in the costal element of the seventh cervical vertebra. The osseous nucleus arises near the angle of each rib and extends rapidly toward the head. At the end of the fourth month the osseous shaft of the rib bears about the same proportional relation to the costal cartilage which it has in the adult. About puberty epiphyseal centres arise, one for the articular surface of the head, one for the articular surface of the tubercle, and one for the non-articular surface of the tubercle. FVequently only one epiphysis seems to arise on the tubercle (Fig. 270). Usually no tubercular epiphysis is found on the eleventh and twelfth ribs. The union of the epiphyses with the shaft takes place after the twentieth year. The epiphysis of the head does not usually join before the twenty-fourth year.


"According to Poirier, the primary centres appear in the fourth or fifth year in the first vertebra, in the sixth to the ninth year in the second, third and fourth. The epiphyseal plates appear from the sixth to the twelfth year.


FiG. 270. — (After R. Quain, Qiutin's Anatomy, 10th ed., vol. ii, Pt. I, Fig. 31.) Diagram to illustrate the epiphyHes of the head and tubercle of one of the mid-thoracic ribs at about the twentieth year. 1 , body ; 2, epiphysis of the head ; 3, that of the tubercle.

The centres of ossification of the ribs are subperiosteal in character. In the adult the first costal cartilage may become partially or completely covered by a superficial layer of bone. Late in life the other costal cartilages may become thus covered, especially on the superficial surface. This process is more frequent in men than in women (Quain).


Sternum

Ossification in the sternum begins considerably later than in the ribs. The centres of ossification are variable in the time and place of their appearance (Fig. 271, B and D). About the middle of the sixth fetal month a centre usually appears in the manubrium. Often other accessor}^ centres appear (Fig. 271, D). Thus Mayet (1895) in fourteen stemums out of eighteen found one or more accessory centres in the manubrium; in ten instances one extra centre situated caudalwards from the main centre ; in four instances two or more accessory centres. In addition there are two epiphyseal centres next the sternoclavicular joints. These fuse with the manubrium between the twenty-fifth and twenty-eighth years. The body of the sternum is usually ossified from five or from seven centres. The segment next the manubrium is usually ossified from a single centre which appears in the seventh month. The next segment may be ossified from a single centre or from a pair of centres. The last two segments most frequently are each ossified from a pair of centres, but may be ossified from a single centre. As a rule, all the centres of ossification except those in the last segment are present at birth and these last appear during the first year after birth. By the sixth year the centres of each pair usually have become fused with one another. Glenerally the various osseous segments of the body of


Fig. 271.— (Att«R. Quain. Qudii.

'. ABBtomy,

imhed.

.vol.

CMaficatioD o! th« Btemum. A.




ium ■nd Gnt three etemsl iKcme.




of osulic eeolreBl E, example ol


BUraum;

•ep


i. Ft. 1. Fi|. 30.) Diasnun illustnt


the sternum become united in the 12-25 years, but lines indicating the boundaries between them remain till late in life. The manubrium and body rarely fuse; according to Gray, in about 6 or 7 per cent, of cases after 60 years of age. There may be a foramen in the sternum due to lack of fusion of a pair of centres of ossification or to failure of a centre of ossification to develop (Pig. 271, E). Four times out of twelve Mayet foimd the bilaterally placed centres of the body fused vertically with those of their own side before the fusion of pairs across the median line had taken place." The ensiform process is ossified from a single centre which appears late, usually not before the sixth year, and rarely transforms the whole process into bone. The centre of ossification arises at the base of the process. The osseous ensiform process is usually united to the body in middle life. " See Markowski, 1902, 1905, for a somewhat different description of the ossification of the slernum.


Relative Length of the Different Regions of the Spine during Development*

In 1879 Aeby contributed an important paper dealing with the length of the various regions of the spinal column at different ages, with the height of the constituent vertebrae and with the thickness of the intervertebral discs in man. He showed that in young embryos the cervical region is relatively much longer, the limabar much shorter than in the adult. These results have been confirmed by Moser (1889), Ballantyne (1892), and others. It has been shown by Bardeen (1905) that, in embryos during the second and third months of development, if the length of the thoracic region be taken as 100 the length of the cervical region is about 60, the lumbar from 40 to 50, the sacral from 33 to 42.5. In the adult the cervical region has been estimated at from 41.7 to 47.5, the lumbar from about 56.3 to 71.6, the sacroccCoygeal from 61 to 68. (See Ravenel, 1877, Aeby, 1879, Tenchini, 1894, Dwight, 1894 and 1901.) Curvature of the Spinal Column during Development.


During the first month of embryonic development the spine acquires a marked ventral flexion (see 2, Fig. 272). From this period until the time of birth the cervico-thoracc-lumbar region of the spine, at first rapidly and then more gradually, becomes straighter (109, 144, 108, 145, 184, Fig. 272). The sacral region also becomes much straightened during the second and third month of embryonic development (109, 144, 108, 145, 184, Fig. 272), but subsequently acquires a second ventral flexion (Ad, Fig. 272). During the latter half of embryonic development there takes place a marked dorsal flexion at the lumbosacral border (184, Fig. 272). After birth and the assimiption of the erect position dorsal flexion takes place in the cervical and the lumbar regions (I and Ad, Fig. 272).


Number of Vertebrae and Regional Differentiation*

At the period of greatest development of the caudal extremity of the human embiyo thirty-six vertebne usually are present. This stage is reached in embryos from 8-16 mm. in length. occasionally the number of vertebrsB may reach thirtyseven. Beyond the last vertebra the chorda dorsalis extends for some distance distally (Fig. 273). Regional differentiation, as already pointed out, becomes well marked toward the latter part of the blastemal period. The thoracic region is clearly demarcated by the great development of the costal processes of the thoracic vertebrsB. The sacral region becomes definitely marked when the blastema of the sacrum comes into contact with that of the ilium. according to Rosenberg (1877, 1899, 1906), the costal processes of the seventh cervical and the first lumbar vertebra at the period of chondrification are to be regarded as ribs, so that in subsequent development there is a reduction in the number of thoracic vertebrae. While each costal element of the seventh cervical vertebra has a centre of chondrification like a true rib, and near the body of the vertebra appears enough like a rib to be called a " rudimentary rib," one is not more likely to mistake it for the first rib than one would be to mistake the costal element of the seventh cervical vertebra in the adult for a true rib. This is even more true of the costal element of the first lumbar vertebra. Although this normally probably has a separate centre of chondrification, it has distinct characteristics which sharply demarcate it from the twelfth thoracic rib, characteristics of form as well as of size. (See Figs. 261 and 262, from an article which has been cited by Rosenberg in support of his hypothesis.)


According to Rosenberg, the saemm is composed at first of a more distal set o£ vertebrs than those belonging to it in the normal adult condition; in other words, the iliac attachment of the skdeton of the limb is supposed to advance cranialwards along the spinal column during ontogeny. The studies of Holl (1882), Paterson (1893), Bardeen (1904), and others have shown that the views of Rosenberg do not correspond with the conditions found in the majority of the human embryos and fetuses, at the period under discussion, which have been carefully studied.


Fig. 272.— DiAgram to show the ourvftture of the spinal oolumn, the proportional lengths of the various regions, the relations of the acetabula to the sacral region, and the direction of the long axis of the femur in a series of embryos and fetuses 7 to 50 mm. in length, in an infant and in an adult. Each curved line represents the choida dorsalis of an individual. The cervical, lumbar, and coccygeal regions are represented by the heavy, the thoracic and sacral by the light portions of the line. The approximate position where a line joining the coitres of the two acetabula would cut the median plane is r e pres e nted at a. For Embryo II, in which the skeleton of the inferior extrmnity is not yet differoitiated, the position of the future acetabula is deduced from Embryo CLXIII, length 9 mm. The line passing in each instance from a and terminating in an arrow point represents the long axis of the femur. For Embryo II this line is pointed toward the centre of the tip of the limb-bud. From a in each instance a perpendicular is dropped to a line connecting the two extrmnities of the sacral region. The numbers refer to the following embrsros and fetuses in the collection of Professor Mall: 2. II, length 7 mm.; 109, CIX. length 11 mm.; 144, GXLIV, length. 14 mm.; 108, CVIII, length 20 mm.; 145, CXLV, length 33 mm.; 1S4, CLXXXIV, length 50 mm.; I new-bom infant; Ad, Adult.


Variation in the number of vertebraB belonging to each of the regions of the spinal column occurs in the embryo as well as in the adult. Bardeen (1904) reaches the conclusion that the frequency of variation in the embryo is probably the same as that in the adult. Before this is definitely decided a much greater number of embryos must be studied than are at present on record. Regional variation in the embryo and fetus must not be confounded with the normal changes taking place in the development of the costal elements of the vertebrae of the cervical and lumbar regions. A separate centre of chondrification in the costal element of the first lumbar vertebra does not indicate a lumbar rib unless the costal process and the centre of chondrification resemble morphologically the twelfth thoracic rib so that there is no sudden change of form from the one to the oth^r.


Comparative Development of the Vertebrte.


For an account of the embryological development of the vertebne in the tower vertebrates and a summary of the literature relating to the subject the exeellent article by Schauinstand in Hertwig's Handbuch der Entwickelungsgesehichte der Wirbeltiere should be cousulted. In the anamniotes, the chorda dorsalis plays a relatively much greater part than in the amniotee, and in the sauropsida a mueh greater part than in the mammals. Primitively there are apparently four areh pieces developed on each side in each sclerotome, two dorsal and two ventral. As a rule, the cranially situated dorsal and ventral arch pieces of each sclerotome are incomplete while the caudally situated arches are firmly united to the sheath of the notochord. In the amniotes the cranial arch pieces of one sclerotome unite with the caudal of the sclerotome next cranialwards to form the definitive vertebral arches and arch bases. The bodies of the vertebra develop between the regions of attachment of the arches to the chorda. The neural and articular processes come from the dorsal arches, the transverse, costal, hiemal, and hypochordal (see p. 345) processes from the ventral arches.


In the development of the human vertebne the caudal dorsal and ventral arch aniages of each sclerotonie arise simultaneously and are soon united by a common base or cbordal proc«s6 to the mesenchymal sheath about the notochord. The cranial dorsal and ventral arch aniages arise later, the dorsal becoming the interdorsal membranes, the ventral the interventral membranes. The cartilages of the bodies develop about the notochord between the bases of the cranial and caudal ventral arch anlages. But one centre of chondrification appears to give rise to the definitive vertebral hemi-arch, although a separate centre arises for the costal process.



Fig. 273.


The ossification of the definitive vertebrae varies so in different classes of vertebrates that no comparison of the process will be attempted here.


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WiRTH, A. : Zur Anatomic und Entwicklungsgeschichte des Atlas und Epistropheus. Leipzig 1884.

D. SKELETON OF THE LIMBS

One of the most studied subjects in morphology has been the development of the vertebrate limbs. Since, fortunately, critical summaries of its literature have recently been given by several noted investigators, among whom may be mentioned Wiedersheim (1892), Mollier (1893, 1895, 1897), Gegenbaur (1898), Rabl (1901), Fiirbringer (1902), Ruge(1902), and Braus (1904), no attempt will be made here to review this work except so far as it deals directly with the development of the human limb.


During the third week of embryonic life the limb buds become filled with a vascular mesenchyme. The source of this tissue is uncertain. In part it may come from the primitive body-segments, but it seems probable that in the main it comes from the parietal layer of the unsegmented mesoblast. Toward the end of the fourth week a slight condensation of the mesenchyme can be seen at the centre of the arm bud, and early in the fifth week a similar condensation may be noted in the leg bud. This condensation represents the first rudiment of the skeleton of the limb. The tissue composing it may therefore be called * * scleroblastema. " From the scleroblastema there is developed a membranous skeleton. In this a cartilaginous skeleton is differentiated, and this in turn is replaced by the permanent osseous skeleton. We may thus distinguish three overlapping periods, a blastemal, a chondrogenous, and an osseogenous. We shall first consider in some detail the development of the skeleton of the inferior extremity and then more briefly that of the superior extremity.


Inferior Extremity

Blastemal Period.

At the time when the condensation takes place in the leg bud the latter has the general form shown in outline in Fig. 274. The bud projects considerably from the body, but shows no definite resemblance to the limb to which it is to give rise. The condensed tissue, scleroblastema, is not sharply outlined. It represents the region of the acetabulum and the proximal end of the femur.


Once begun, skeleton differentiation proceeds rapidly. In an embryo 11 mm. long (Fig. 275) it may be seen that from the original centre of skeletal formation the condensation of tissue has extended both distally and proximally, but much more freely in the distal direction. Distally the scleroblastema shows femur, tibia, fibula, and a foot-plate; proximally, an iliac, a pubic, and an ischial process. A series of sections through the skeletal mass shows that in the femur, tibia, and fibula chondrification has begun. At centres in the blastema of the ilium, ischium, and pubis a still earlier stage of chondrification has made its appearance. The leg of this embryo, therefore, represents a stage of transition from the blastemal to the chondrogenous stage of development.

Chondbogenous Period.

The further development of the skeleton of the limb during the second and third months of intra-uterine life may be followed in Figs. 276, 277, and 278. For the sake of convenience the development of the several parts of the skeleton will be taken up as follows: (a) the os coxae; (6) femur, hip-joint, tibia and fibula, and knee-joint; (c) ankle and foot. (a) The Os Coxm. — The pelvic scleroblastema of embryos of the stage illustrated in Fig. 275 undergoes a rapid development. Its iliac portion extends in a dorsal direction toward the vertebrae which are to give it support. The costal processes of the latter at the same time become fused into a dense mass of tissue which enters into close association with the iliac blastema (Fig. 276),

FlOB. 274-278.— (After Bardeen, Amer. Journ, of Anat.. 1905.) Lateral view of modda to illugtn the development of the dialnl pBrt of the spinal column and of the inferior extremity of embryos Bmm. loDK, In Firs. 27*. 275, and 276 the sderoblaatems i! «honn, and in this in KiEs. 275 and 276 I centres of chondrification. In FigP. 277 and 278 the Partilaginous afceleton ia shown, and in this in F 27Brentr«9 0fo99iliCBtion. Fig. 274, Length of embryo, mm. Fig. 275. I-ength of embryo. 11 mm. Fin. 270. Length of e bryo. 14 mm. Fig. 277. Length of emhryo. 20 mm. Fig. 278. Length of fetus. SO mm. Chd,. ehoi dorsalis; Co", first eoeeygeai vertebia; Cnifa /*, iwcHih rib: Fi, fibular F.o., foramen obturatum; . ilium; L.i., ligBmentum inguinale: M.id., membnns interdorsalin: P.. pubis; Pr.a.o.. proee.wus artii pmcessus tmnaverBUs ; Ti, tibia.


although for some time separated from this by a narrow band of tissue staining less densely than the blastema. Cranialwards the iliac blastema extends toward the abdominal musculature, to which it finally gives attachment.

While the blastemal ilium is thus becoming differentiated the pubic and ischial processes of the pelvic blastema extend rapidly forward. Ventral to the obturator nerve they become united by condensed tissue, which completes the boundary of the obturator foramen. Betwen the crest of the ilium and the ventral extremity of the pubis dense tissue is formed to give attachment to the oblique abdominal musculature. This represents the embryonic inguinal ligament and completes a femoral canal (Fig. 276).


While the blastemal pelvis is being completed the three centres of chondrification, barely visible in the 11 mm. embryo, give rise respectively to iliac, pubic, and ischial cartilages in which the adult form becomes gradually more distinct. (Compare Figs. 276, 277, and 278.) In embryos between 15 and 20 mm. long each of the three cartilages gives rise to a plate-like process over the head of the femur. These processes fuse with one another and give rise to a shallow acetabulum (Fig. 277), which during the third month gradually becomes deeper (Fig. 278). The iliac and ischial cartilages furnish a greater part of the floor of the acetabulum than the pubic cartilage and unite with one another before being joined by the pubic cartilage. Toward the end of the second month and the beginning of the third month of development the symphysis pubis is formed. This is at first composed of dense blastemal tissue. In this tissue first hyaline and then fibrocartilage become differentiated. At the centre of the joint a slight fissure may appear in adult life (Farabeuf, 1895).


(6) Femur and Hipjoint. Tibia, Fibula, and Knee-joint. — The rapid development of the blastemal skeleton of the lower limb has been briefly described above. Soon after the anlage of the femur makes its appearance condensation of tissue marks out the anlages of the tibia and fibula and the skeleton of the foot. This last seems to be at first a somewhat irregular continuous sheet of tissue. It is not clear whether or not the anlages of the tibia and fibula also begin as a continuous sheet which becomes divided, by ingrowth of blood-vessels, into tibial and fibular portions. The incomplete development of the interosseous fissure in an 11 mm. embryo suggests this (Fig. 275). The blastemal anlages of the tibia and fibula are here very incompletely separated.


Within the blastema of the femur, tibia, and fibula chondrification begins as soon as the outlines of the blastemal skeleton are fairly complete (Fig. 275). The embryonic cartilage appears slightly kneewards from the centre of the shaft of each bone and then eKtends toward the ends. The cartilage of the femur consists of a bar largest at the knee, whence it tapers off toward the hip. The cartilages of the lower leg lie nearly in a common plane. That of the tibia is larger than that of the fibula and toward the knee it broadens out considerably. At this stage the joints consist of a solid mass of mesenchyme (Figs. 279 and 280). The tissue uniting the femur and tibia has temporarily somewhat the appearance of precartilage (Fig. 283). From this period onwards the development of the individual bones and joints is rapid.


The cartilaginous femur expands at the expense of the surrounding blastemal perichondrium and at the same time acquires adult characteristics (Figs. 276, 277, and 278). the hip-joint is at first completely filled with a dense blastemal tissue (Fig. 280). While the embryo is growing from 20 to 30 mm. in length, cavity formation begins in the tissue lying between the cartilaginous floor of the acetabulum and the head of the femur. The first stage in the process is marked by a condensation of the capsular tissue immediately bordering upon the joint and of the perichondral tissue which at this stage covers the cartilages on tiieir articular surfaces as well as elsewhere. In the region of the ligamentum teres a fibrous band is likewise differentiated from the blastema of the joint. The rest of the tissue becomes looser in texture and ultimately is absorbed (Fig. 284). Henke and Eeyher (1874) gave a good account of the development of the hijKJoint. Moser (1893) has described that of the ligamentum teres. Schulin (1879) has given a good account of the later development of the joint cavity in its relations to the head and neck of the femur (see Figs. 286-288). It is to be noted that in the fetus 25 cm. long (Fig. 286) the joint cavity extends about the neck of the femur in a pocket lined on one side by perichondrium, on the other by the capsule of the joint, and that later the peridiondral lining becomes periosteum (Fig. 288).


Fig. .384.

Fig. 2S7. Fig. 288. Flos. 285 Ui 2B8.~(AfCaSchulin, Arohiv {. Asatoici*. 1879.) Fig. 285. Mcdiui SMtlon tlirouih th« knw-joiDt of a letut 13 on. lon(. a. p«MUb; b. a 286-288. Hip-joiat of s msle fetus '25 cm.'lonK, of it fenale cjiild six yean old, and of a male ndult. a. (wnfication ; d, epiphyseal aaxeoiis nudeiu : (, ligamentum teres.


The tibia and fibula at first lie nearly in the same plane (Fig. 275). As the head of the tibia enlarges toward the knee-joint it comes to lie ventral to the proximal extremity of the fibnla. This may be seen in Figs. 276 and 277.

The development of the knee-joint in man has been studied by a number of competent observers. Bernays (1878) gave a good review of the previous work of von Baer, Bruch, Henke, and Reyher, and an accurate description of the processes which take place. Of the more recent articles those of Schulin (1879), Kazzander (1894), and Lucien (1904) deserve mention. Until the embryo reaches a length of about 17 mm. the kneejoint is marked by a dense mass of tissue (Fig. 279). The medullary tissue at the knee, like that at the hip and other joints, is less dense than the surrounding cortical substance, so that when the cartilages of the femur, tibia, and fibula are first differentiated they seem to be connected by a tissue which, in some respects, resembles the cartilage of which they are composed (Fig. 283) ; but as the cartilages become more definite the apparent continuity disappears. As the musculature becomes differentiated a dense tendon for the quadriceps is formed in front of the knee-joint. At this period the joint is flexed at nearly a right angle.


In embryos of about 20 mm. the tissue immediately surrounding the cartilages becomes greatly condensed into a definite perichondrium. The peripheral blastemal tissue at the joint becomes transformed into a capsular ligament, strengthened in front by the tendon of the quadriceps. Within the joint most of the tissue begins to show signs of becoming less dense, but the menisci and the crucial ligaments, like the ligaments of the capsule, are differentiated directly from the blastema (Figs. 281 and 282). In the differentiation of the articular blastema the menisci first become distinct, then the capsule, then the crucial ligaments, the patella, and the lig. mucosum.


A knee-joint cavity first appears, in embryos about 30 mm. long, between the patella and the femur. according to Lucien, two other cavities somewhat later appear between the condyles of the femur and the menisci. These cavities secondarily communicate with the retropatellar cavity and with cavities formed between the menisci and the tibia. The cavity of the knee-joint is primitively partly divided into two parts by a median septum (lig. mucosum), which becomes greatly reduced in fetuses 10-12 cm. long (Fig. 285, C) and in the adult is replaced by a fat pad.


The shafts of the tibia and fibula are incompletely separated in the blastemal stage. The cartilages which arise in the scleroblastema are, on the other hand, separated by a distinct interval (Fig. 279). At first short and thick, the shafts become gradually more slender in proportion to their length. The fibula, at all times smaller, becomes increasingly more slender in comparison with the tibia. In fetases 50 mm. long (Fig. 278) both bones, and especially the fibula, are still relatively thick compared with the adult bones.


During a period of rapid development, in embryos of 15 to 20 mm., the tibia and fibala, like the femur, may extend so rapidly in length as to become temporarily distorted by resistance at the


Fia. 388.— (After R. Quoin. Qutin'a Anatomy, lOth «d„ vol. ii. Ft. I. Fign. 1S7 ud 158.) Osafi A. Bone st birth. B. Child under ax yean of ave. C. Child two to thres yean older thu B. D. PenoD of about twroty years. E. Acetabular resion of hip-bone at tourtaen yean of afe. 1. ililiro; 2, ixchium: 3, pubis; 4. oa aoeUbuli; S. bony nodules between ilium and ischiuni: 6 and 7, epiphyBeal lamins on ilium and ischiuni; S, 9, 10. 11, epiphyseg of aolerior inferior iliac spine, iliac crest isohial tuberosity, and gympbysia putni. ends. This is often especially marked in hardened specimens. Holl (1891), Sohomburg (1900), and others have called attention to this distortion.


(c) Ankle and Foot. — Of the papers dealing with the early development of the skeleton of the human foot the more important are those of Henke and Reyher (1874), Leboucq (1882), v. Bardeleben (188.3, 1885), Lazarus (1896), and Schomburg (1900). During the fifth week of embryonic development the free extremity of the limb bud becomes flattened and differentiated into a foot-plate (Fig. 275). Toward the end of the fifth week the anlages of the individual bones of the ankle and foot begin to become marked by specific condensation of the blastemal tissue. Within these anlages precartilage soon appears. The digital rays are marked at first by condensed bars of tissue, in which segmentation into metatarsals and phalanges appears during the period of chondrification (Fig. 276). The metatarsal cartilages become differentiated before the tarsal cartilages. The phalangeal cartilages appear relatively late.


Fig. 290.— (After R. Quain, Quain's Anatomy, 10th ed., vol. u, Pt. I, Fig. 169.) OssificaUon of the femur. A. Before the eighth month. B. At birth. C. About a year old. D. At about the fifth year. £. Near the age of puberty. 1, diaphysis; 2, distal epiphysis; 3, head; 4, great trochanter; 5, small trochanter.



Fig. 291.— (After R. Quain, Quain's Anatomy, 10th ed., vol. ii, Pt. I. Fig. 160.) Ossification of the tibia. A. Some weeks before birth. B. At birth. C. At the third year. D. Betweoi the eighteenth and twentieth years. E. Example of separate centre for tuberosity. 1, diaphysis ; 2, proximal epiphy> sis ; 2*, epiphysis of tuberosity; 3, distal epiphysis. The earliest appearance of the tarsal cartilages is found in an embryo about 14 mm. long (Fig. 276). Toward the end of the second month these cartilages become much more distinct (Fig. 277). By the middle of the third month the cartilages of the foot have a form distinctly corresponding to the adult. The similarity is still better marked at the end of the third month (Fig. 278).


The joint cavities begin to develop while the embryo is growing from 25 to 30 nun. in length. As in other cases, so here the blastemal tissue in which the cartilages are developed becomes condensed at their articulating ends and about the joint, while in the region of the joint the tissue becomes less dense and finally disappears, leaving a joint cavity. In embryos of about 30 mm. the joint cavities of the foot are filled with a loose fibrous tissue ; in fetuses of 50 nun. definite cavities are to be made out. During the progress of form differentiation above described, the shape of the foot is markedly altered. At the beginning of the development of the foot the tarsal and metatarsal bones lie nearly, though not quite, in the same plane as the bones of the leg. They are so arranged, however, that the foot is convex on its dorsal surface and concave on the plantar, and the projections of the calcaneus and talus serve to deepen the plantar fossa. The metacarpals spread widely apart. As differentiation proceeds, the metatarsals come to lie more nearly parallel to one another, and the tarsal elements become compacted in such a way as to give rise to the tarsal arch. The foot at the same time is dorsally flexed at the ankle and slightly everted. The toes are flexed. In the further development of the skeleton of the foot the various constituent structures are elaborated, and the foot gradually becomes more flexed dorsally and turned toward the fibular side.


Fig. 292.— (After R. Quain, Quain's Anatomy, 10th ed., vol. ii, Pt. I, Fig. 161.) Oaflification of the fibula. A. At birth. B. At about two yean. C. At about four years. D. At about twenty years. 1, diaphysis; 2, distal epiphysis; 3 proximal epiphysis.


Period of Ossification

The hip-bone is ossified from three primary centres, one for each of its constituent parts, the ilium, ischium, and pubis, and from several epiphyses. There is one primary centre of ossification for each of the other bones of the inferior extremity, and in addition most of the bones have one or more epiphyses. In the tarsus the calcaneus alone regularly has an epiphysis. The patella has no epiphysis. With the exception of those of the tarsal bones and of the various sesamoid bones the primary centres of ossification appear relatively early in intra-uterine life. At birth there are usually centres of ossification present in the calcaneus, talus, and cuboid, but not in the other tarsal bones. Ossification in these latter tarsal bones and in the sesamoid bones and the various epiphyses appears after birth. In the talus, according to Sewell (1906), dark-staining regions in the hyaline cartilage of which it is composed in the sixth fetal month indicate structural


Fig. 203.— (AfMr R. Quain, Quain's the bona of (he foot. A. Right foot or a digital phHJsnge< an owHlied. The Uraus i caneus. B. Fetus o( 7-8 monthe. Nurleit y«r. Nucleus in third cuDeiform. E. Ii G. About the age ol puijerty, calcaueua. Epiphyees of meta caneusi 1', in G, the epiphyMs of the caleaneu


nueloua o( the talun; 3, of the euboid : . . . . uLar; 7. of thosecond ouneiform; S, metAlanal bones; 8'. distal epiphysis of llie senmd metatarsal bone; S" p^o^i^lBl etiiphysis of the first metBtanal bone ; 9, first phalanx of Ihe second toe; 9', proximal epiphysifi of this phalanx; 9*, (hat of the first phalanx of th« ereiit U>9 ; 10. second phalanx; 10'. the ^pbysis of tliie phalanx; 10*, epiphysia of the terminal phalanx of the great toe : 11. terroiDal phalanx; 11 ', iu epiphysis. features characteristic of the adult bone. The following tables and the accompanying figures illustrate the process of ossification in the inferior extremity. Authors differ in the data which they give concerning the time of ossification of the various bones. When not otherwise indicated the tenth edition of Quain 's Anatomy is followed in the tables.

Table of Ossification of the Bones of the Inferior Extremity

(Days and weeks refer to the prenatal, years to the postnatal period.)


Bone.


Centres.


Time of appearance of centre.


Os COX2B Os ilium 56th day (Mall)


Femur


Os ischii


Os pubis


Os acetabuli.


Epiphyses: Those of the acetabulum


Crest of ilium Tuberosity of ischium


Patella Tibia . .


Ischial spine Ant. inf. spine of ilium Symphysis end of OS pubis (1 or 2 centres) Diaphysis Epiphyses: Distal end Head Great trochanter . . .


Small trochanter . .


FibuU.


Calcaneus


Diaphysis Epiphyses: Proximal end Distal end Tubercle (occas.)


Diaphysis Epiphyses: Distal end — Proximal end. Chief centre —


Epiphysis (distal end)


105th day (Mall)


4th to 5th fetal month


9th to 12th year


Soon after puberty


Soon after puberty. . . Soon after puberty. . . Soon after puberty. . . Soon after puberty. . .


18th to 20th year. (Sappey)


Time of fusion: general remarks.


The rami of the ischium and the pubis are united by bone in the 7th or 8th year (Ouain) ( 12-14 year^ Sappey). In the acetabulum the three hip bones are separated by a Y-shaped cartilage until after puberty. In this cartilage between the ilium and pubis the ' 'os acetabuli "appears betwe^i the ninth and twelfth years. This bone, variable in siae, forms a greater or less part of the pubic portion of the articular cavity. Leche (1884). Krause (1885), and many others consider it primarily an independent bone. About puberty between the ilium and ischium and over the acetabular surfaces of these bones small irregular epiphyseal centres appear. The OS acetabuli becomes imited to the pubic bone about puberty and soon afterwards the acetabular portions of the ilium and ischium and the ischium and pubis begin to become united by bone. The acetabular portions of the pubis and ilium are unit^ a little later. Osseous union takes place earlier on the pelvic than on the articular surface of the acetabulum. The imion of the several orimary centres and the epiphyses is usuaJly completed about the tw^itieth year. Fuses with main bone 20th to 25th year. Fusion begins in the 17th jrear and is completed between the 20th and 24th years (Sappey). 18th to 20th year (Poirier). 18th to 20th year (Poirier).


After the 20th year.


43d day (Mall)


Shortlybefore birth.»« , 20th to 24th year. Ist year 18th to 19th year. 3d to 4th year. (Osse18th year, ous granules soon afterbirth, Poirier) i 13th to 14th year ' 17th year (Quain). 8th year (Sappey) Proximal epiphysis 18th to 22d year I (Poirier). 3d to 5th year The osseous patella reaches its definitive form soon before puberty. 44th day (Mall). About birth | 19th to 24th year (Sappey). 2d year 16th to 19th year.


13th year 65th day (Mall). 2d year


Fuses with epiphysis of the proximal end and then with this to the diaphysis.


20th to 22d year.


3d to 5th year 22d to 24th year.


6th fetal month


10th year (Quain) 7th-8th year ( Sappey)


The chief nucleus is endochondral. A periosteal nucleus appears frequently in the 4-5 fetal month (Hassel wander). 15th-16th year (Quain). 16th-18th year (Poirier). (^ 17-21, average 20 years. 9 13-17, average 16 years (Hasselwander)


  • • Poirier, Traite d'Anatomie, vol. 1. page 227, gives a summary of the literature on the time of the appearance of this epiphysis. The epiphysis has some medico-legal importance, since its presence or absence has been utilized to determine whether a child is bom at term. Schwegel found it to appear between birth and the third year; Casper in the ninth fetal month. Hartmann found it lacking in 12 per cent, of cases at birth and in 7 per cent, of cases present as early as the eighth fetal month.

Bone.


TaluB.


Cuboid Cuneiform III. Cuneiform I... Cuneiform II.. Navicular


Metatarsals .


Phalanges: Terminal row


Middle row.


Proximal row


Centres.


Diaphjrsee Epiphyses


Sesamoid bones of the great toe


Time of appearance of coitre.


6th fetal month (Hassel wander).


About birth 1st year. 2d-3d year. 3d-4th year. 4th-5th year.


8th-10th week 3d-8th year....


Diaphyses Epiphyses (distal) Diaphyses Epiphyses Diaphyses Epiphyses


58th day (Mall). 4th year 4th-10th fetal month 3d year , 3d fetal month 3d year


cf 14th year 9 12th-13th year


Time of fusion: general remarks.


In the 7th-8th year the posterior part of the talus, the os trigonum, is frequently ossified from a special c«itre (v. Bardeleben). It fuses about the 18th year.


according to v. Bardeleben a second centre of ossincation appears much later than the primary in the navicular, and finally about the time of puberty a medial epiphyseal centre arises. The centre for the 2d metatarsal usually appears first, then come the 3rd, 4th, Ist and 5th. The epiphysis of the 1st metatarsal appears at the proximal end of the bone: the other epiphyses arise at the distal exida of the metatarsals. There may be a distal epiphysis in the first metatarsal also.17 In some instances a proximal epiphysis is formed cm the tuberosity of the fifth metatarsal (Gruber). The epiphyses unite with the shafts in the 17-21 year in males and in the 14-19 year in females. (Hassd wander) .


cf 13-23, average 16-21 year. 9 13-17, average 14-17 year (Hasselwander). (5^ 15-19 year. 9 13-16 year (Hassel wander). cf 15-17 year. 9 14-15 year (Hassel wander). The centres for the shafts of the phalanges often appear double, one for the dorsal and one for the plantar surface. The centres for the medial phalanges in each row usually appear before the more laterally placed centres. The ooEitre for the 5th terminal phalanx appears much later than the other centres in this row (Mall). according to Rambaud and Renault the epiphyses arise each from two centres which fuse together. In the terminal phalanx of the ^reat toe the ossification centre of the epiphysis often appears as early as the secona or even the first year. (Hassel wander) . Ossification may begin in the 8th year in females, in the 11th in males (Hasselwander).


>^ Mayet has described two centres of ossification for the proximal epiphysis of the first metatarsal, one of which represents the real metatarsal of the first digit.


Infantile Characteristics of the Skeleton of the Inferior Extremity. — In the infant the pelvis is small in proportion to the size of the body and contains a smaller proportion of the abdomino-pelvic viscera than in the adult. The cavity of the infantile pelvis is cone-shaped and diminishes in diameter from the entrance to the outlet (Fehling, 1876, Hennig, 1880). The blades of the ilium are relatively slightly developed. In the first half of fetal life the sacropelvic angle is similar to that of quadrupeds, but during the latter half and after birth the angle becomes greater, expanding from 55** to 90-110" in the adult (Le Damany)."

The acetabulum is relatively shallow in the new-bom as compared with the adult. The shafts of the long bones are relatively shorter and thicker. The neck of the femur is but slightly developed at birth. The infantile foot has certain ape-like characteristics and is strongly flexed and inverted. The head of the talus is directed more medialwards than in the adult, the first metatarsal is relatively short and inclined medialwards by the oblique articular surface of the first cuneiform (Leboucq, 1882).

Superior Extremity

Blastemal and Chondrogenous Periods.

In general the development of the superior extremity resembles that of the inferior extremity. The various stages of differentiation begin in the former a little earlier than in the latter. W. H. Lewis (1902) has described the earlier stages in the development of the arm. His description is closely followed here.


In an embryo at the end of the fourth week the scleroblastema of the limb bud is marked by a slight condensation of the tissue near the future head of the humerus. Early in the fifth week this condensation has extended to the distal part of the limb bud and the anlages of the scapula, humerus, radius, and ulna are distinguishable (see Fig. 294). The skeleton of the wrist and hand is marked by a plate of condensed tissue. There are no distinct centres of chondrification at this stage.

In an 11 mm. embryo marked alterations have taken place in the skeleton of the superior extremity (Fig. 295). Centres of chondrification appear.


The scapula is composed of precartilage surrounded by a dense blastema. It lies opposite the lower four cervical and the first one or two thoracic vertebrae. From the superior border there springs a large curved acromion process. On the medial (costal) surface, at the junction of the humerus with the scapula, arises a large hooked coraccid process. A slight ridge on the medial surface marks the future anterior border. The perichondrium is well marked onlv on the medial surface.


The clavicle is an ill-defined mass of condensed tissue which extends from the acromion about a third of the distance to the tip of the first rib. The coraccclavicular ligament is partially differentiated.


For recent accounts of the development of the pelvis, see Merkel (1902) and Falk (1908). Fehling recognized sexual differences in the pelvis early in fetal life.


The humerus is short and thick. The shaft is composed of embryonic cartilage surrounded by a dense layer of perichondrium. Towards each end of the shaft the central tissue is precartilaginous in character. The surrounding perichondrium is continued directly into the dense tissue of the neighboring skeletal parts.


There is more flexion at the elbow than during the preceding stage. The forearm is midway between supination and pronation. The core of the shaft of each bone is composed of hyaline cartilage. The hand-plate is composed of condensed mesenchyme. There are several centres of increased condensation which probably correspond to the carpal bones. The digits are marked by condensed tissue in which no segmentation into metacarpals and phalanges is visible.


In an embryo of 14 mm. the skeleton of the superior extremity is well advanced in development (Fig. 296). The form of the scapula is shown in this figure. It is composed mainly of cartilage, covered by a thick layer of perichondrium. It has migrated caudalwards so that less than one-half of it lies anterior to the level of the first rib. The clavicle is a rod composed of dense tissue. It extends from the acromion to the tip of the first rib, where it is continued into the sternal anlage. It contains a small core of a peculiar precartilaginous tissue. The acromioclavicular ligament is distinct. The humerus is larger and more slender than at the preceding stage and has expanded at each end. It is composed chiefly of cartilage surrounded by a thick perichondrium which is continuous with that of the lateral angle of the scapula. There are no signs of a joint cavity at the shoulder.


The ulna and radius are likewise composed of cartilage surrounded by a thick perichondrium continuous at one end with that of the humerus and at the other with that of the wrist. There are no joint cavities at the elbow.


The carpus is composed of a dense tissue in which are embedded cartilages which represent the bones of the wrist with the exception of the lunar and the pisiform. These are still composed of condensed tissue.


The metacarpals are represented by five slender cartilages surrounded by a dense perichondrium. The first metacarpal is only about half the length of the others. The phalanges of the first row, with the exception of that of the thumb, have cartilaginous cores. The basal phalanx of the thumb is composed of condensed tissue. At the tip of each digit is a mass of condensed tissue. There are no joint cavities present in the hand.

In an embryo 20 mm. long the cartilaginons anlages of various bones of the superior extremity are all well marked, except those of the distal row of the phalanges of the fingers. The clavicle extends from the acromion to the sternum. It is composed of a peculiar kind of precartitaginous tissue. The general shape of the other cartilages may be seen from Fig. 297. The spine of the scapula is not yet distinct. There are distinct coraccclavicular, costoclavicular, and interclavicular ligaments. There is no joint cavity at the shoulder, but a capsular and a coraeo-humeral ligament may be distinguished. The humerus has well-marked tuberosities and condyles. The ulna and radius are larger and longer than at the preceding stage. The olecranon, coraccid, and styloid processes are composed of cartilage and condensed tissue. The perichondrium about the ulna and radius is quite thick. The capsular and annular ligaments are present, but there are no joint cavities.


Magn. R : 1. Flu. 2Bn.— (AfU im. emiir.vo. Magn. about 13 :


All the bones of the carpus have cartilaginous centres. There are no joint cavities in the hand.

During the third month of development the cartilages of the superior extremity assume more and more the form characteristic of the adult bones ; in several ossification begins ; the joint cavities appear at this time.


The early development of the bones of the forearm and hand, and especially those of the wrist, has engaged the attention of several investigators. The following details are based upon the recent paper of Graefenberg (1906).

Forearm

The form and relations of the cartilaginous radius and ulna in the fifth, sixth, and the seventh weeks of embryonic life are shown in Figs. 298, 299, 300. The two cartilages are at first some distance from one another. The proc. styloideus of the ulna begins to develop during the latter part of the second month. It extends at first to the dorsal side of the triquetrum and becomes relatively large. Later the process becomes smaller, and is carried proximally and volarwards.

The discus articularis arises from a mass of tissue which lies between the radius and the styloid process of the ulna. This mass of tissue gives rise to a special centre of chondrification, which by some is supposed to represent the os intermedium antebrachii of the lower vertebrates.

The Carpus

Os Centrale. — Most of those who have studied the development of the human carpus have described a cartilage which is homologous with the os centrale of the carpus of lower vertebrates. The position of this element is shown in Figs. 298, 299, and 300. It later disappears. In the process of retrograde metamorphosis it may become divided into several parts. according to Graefenberg it does not fuse with any of the other carpal.


The Proximal Row or Bones. — The navicular arises from two centres of chondrification. It is homologous with the radiale of lower forms. The lunar is the last of the carpalia to be differentiated. according to Gegengaur and some other investigators, it is homologous with the os intermedium of the lower vertebrates. In man, however, there are no indications of its wandering from the forearm into the wrist. The triquetrum is relatively small when first differentiated, but grows rapidly in size (Figs. 298, 299 and 300). Perna has found it arising from two centres of chondrification. The pisiform is relatively large in embryonic stages. It is a canonic carpal element and not a sesamoid bone. It arises later than the triquetrum. During its development it wanders from the ulnar margin to the volar surface of the triquetrum.


The Distal Row. — The cartilages of the distal row are at first relatively large compared with those of the proximal row


J. AnBlomiMhe Hefte, IBOO, Fi«. 1.) Dorml view of ■ model of th» le forearm and hand of * Gve-weeks human onbryo. Magn. TO : I. jf ibe skeleton of the forcann and hand of a


liniu; the cmtmla betwe


atum; »un.. humerus; M.mo)'., multaii(ulun ., pare radialit, ubt., i»r« ulnaris : P., pisiforme ; The oafwtatum Um betweeo tha bunatum and the muKanpi


Fig. 300.— A. (After Gr^fenberc, 1906. Pig. G.) ekelelOD of the right hand of a ten-weeks human fetua.


(Figs. 298 and 300). The capitatum is the largest, next comes the hamatum. The multangalar carpalia are small; the M. majus is for a time considerably smaller than the M. minus. The capitatum and hamatum are the first elements of the carpus to undergo chondrification. The hamulus ossis hamati is differentiated from a special centre of chondrification.


urn tuid hsmBtum. B. Elbow-joint of a fetui c. centnl udLdq: d, perichoadr*! put of Juia anoa. D, £, F. Should er-joiat o[ a fetua 13 m of mpiiulei t. intracepsuUr eonnwtive tiMi


Anat. Abt., 1879.) Fir diocarpflj ioiut in thrc« 


Metocarpalia. — These are at first relatively large. The first metacarpal, according to some investigators, represents a basal phalanx. Galen was the first to express the view that the metacarpal of the thumb is not present. Others have thought that a phalanx is missing from the thumb, but Graefenberg accepts the view of Galen. The other four cartilaginous metacarpals arise at some distance from one another (Fig. 298). They spread apart distally. The bases are first brought into contact with one another, and later the distal ends. The fifth metacarpal articulates at first with the triquetrum (Fig. 298) and later with the hamatum (Fig. 299).


The phalanges are differentiated in serial order, the basal row appearing first, the terminal row last. according to Graefenberg, the terminal phalanges show evidence of being composed of two elements, a proximal and a^ distal. The latter is composed of cartilage, the cells of which rapidly enlarge. It may represent a fourth phalanx. Primitively the digits were probably composed of many phalanges. The terminal phalanges are at first smaller than those of the middle row, but then develop faster so as to exceed them in length. After a time retrograde metamorphosis overtakes the distal ends of the terminal phalanges, so that the middle row once more exceeds the terminal in length. The tuberositas unguicularis is composed of fibrous tissue which becomes transformed directly into bone. The sesamoid bones, according to Thilenius, are more numerous in the fetus than in the adult. The following table, after Pfitzner and cited by Dwight (1907) illustrates this. This table is based on a study by Thilenius of 30 hands of fetuses of the fourth month, and of 1440 hands by Pfitzner of individuals from fourteen to eighty-nine years of age. The Roman numerals indicate the metacarpophalangeal joints opposite which sesamoid bones were found, and the Arabic numerals represent the percentage of frequency with which the bones were found.


Fig. 302. (After R. Qiiain, Qtiain's Anatomy, 10th ed., vol. ii, Pt. 1, Fig. 1 15.) Ossification of the clavicle. A. Clavicle at birth. B. At about the twenty-third year. 1, shaft; 2, epiphyma.

Joints

Schulin (1879) has given some account of the development of the joints of the upper extremity. At the shoulderjoint (Fig. 301, D, E, F) a joint fissure arises in the periphery of the intermediate zone and thence extends inwards between the head of the humerus and the glenoid fossa (Fig. 301, D). The joint cavity extends into the perichondrium for some distance on each side of the head of the humerus, so that there is from a very early period a well-developed layer of intracapsular connective tissue (6). The labrum glenoidale is differentiated at an early period. After the joint cavity appears the head of the humerus undergoes considerable development. (Compare D, E, F, Fig. 301.) During fetal development the tendon of the long head of the biceps sinks in through the capsule of the joint. For a time it is covered by a layer of synovial membrane which attaches it to the capsule, but in the third or fourth month it becomes free in the joint cavity (Welcker, 1878). The elbowjoint (Fig. 301, B and C) develops in a position of flexion at about 90°. The perichondral part of the joint cavity (Fig. 301, B, d) develops before the intercartilaginous part. The distal end of the humerus undergoes marked alterations in form during the development of the joint (Fig. 301, C). In the wris1>joiiit cavities appear during the third month (Fig. 301, A). At the radiocarpal joint the joint cavity arises from three separate fissures (&).


Fig. 303.—


Fig. 306. Fia8. 305 and 306. — (After R. Quain, Quain's Anatomy, 10th ed., vol. ii, Pt. I, Figs. 119 and 120.) Ossification of the radius and uhia. Fig. 305. Radius. A. At term. B. At 2 years. C. At 5 years. D. At 18 years. Fig. 306. Ulna. A. At birth. B. At the end of the fourth year. C. At 12 years. D. At 19 to 20 years. Ep., epiphysis.


Fig, 307.— <Aftar R. Quiin, Quwn'n Anatomy. 10th ed., vol, ii. Pt. I. Tie. 121.) OniGcatioD of the bODB at Ihe liMid. A. At birth. The carpus is cBrti1««inous. The thiLla of the metacarpals and phalanflee are oeaified. B. At the ead of the first /ear. C. About the third year, D. At the iifth year. E. At the ninth year. 1, csapitaCum; 2, hamatum; 3, triquetrum; 4, lunatuin; 5. multangulum loajus; 6, navicular; 7. multanAulum minus; 8, metacarpal shafts; 8*. four maCacarpalepiphyBce; 8', tHatof the thumbs 0. boasl phalanges; 9*, tbeii epiphyses; 9'. that of the thumb; 10. [Diddle pholaosea; 10', epiphysisof (erminal phalaUTt of thumb; It, terminal phalanflee of the fingers; 11*, their epiphysca. The development of the digital joints has been previously described (Figs. 226-228).


Period of Ossification.


With the exception of the clavicle the bones of the superior extremity pass through a stage of embryonic hyaline cartilage before becoming ossified. The shaft of the clavicle, which is the first bone in the body to exhibit a centre of ossification, is ossified in a peculiar kind of cartilage (Mall). The ends of tbis bone exhibit the more usual type of ossific cartilage. The following table gives the approximate periods when the various centres of ossification appear and the time of fusion of the various centres which unite to form the individual bones. Authors differ considerably concerning these data. When not otherwise indicated, the data included in this table are based upon those given in Quain's Anatomy, 10th edition, vol. 2, p. 106. according to Pry or (1906), the epiphyses of the hand appear earlier and unite to the shaft earlier in females than in males a|>d in the first-bom children earlier than in those bom later. **The fully developed hand of the female is at least two years in advance of the male.^^ Similar conditions have been found by Hasselwander in the skeleton of the foot.

Table of Ossification of the Bones of the Superior Extremity

(DtLYB and we^s refer to the prenatal, years to the postnatal period.)


Bone.


Clavicle


Scapula


Centres.


Diaphysis Sternal epiphysis.


Time of appearance of centre.


Primary centres: 1. That of the body, the spine, and the base of the glenoid cavity. 2. Goraooid process 3. Subcoraooid


Himierus .


Epiphyses: Acromial epiphyses Epiphysis of the inferior angle. Epiphyses of the vertebral border. Epiphjrses of upper surface of coraccid. Epiphysis of surface of glenoid fossa.


Diaphysis Epiphyses: Head Tuberculum majus . . . Tuberculum minus Capitulum Epioondylus med Lateral margin of trochlea. E|Hcondylus lat


6th week 18th to 20th year.


Union of primary and secondary centres; remarks.


8th week (Mall).>* 1st year. 10th to 12th year. 15th to 18th year. 16 to 18th year. 18th to 20th year. 16th to 18th year. 16th to 18th year.


6th to 7th week (Mall) 1st to 2d year. 2d to 3d year. 3d to 5th year. 2d to 3d year. 5th to 8th year. 11th to 12th year. 12th to 14th year.


There are two centres in the shaft, a medial and a lateral. These blend on the 45th day (Mall). Shaft and epiohysis unite between the 20tn and 25th years. The chief centre appears near the lateral angle. The subcoraccid centre appears at the base of the coraccid process and also gives rise to a part of the superior mar|^ of the glenoid fossa. The coraccid process joins the body about the age of puberty. The acromial epiphjrsMtl centres (two or three in number) fuse with one another so(m after their u>pearance and with the spine between the 22d and 25th years ((jjuain); 20th 3rear (Wilms). The subcoraccid and the epiphjrses of the coraccid process, the i^enoid fossa, the inferior anjB^e, and the vertebral margin join between the 18th and 24th years in the order mentioned (Sappey).


The epiphyses of the head, the tuberculum majus Bnd the tuberculum minus (the last is inconstant) imite with one another in 4th-6th srear and with the shaft in 20th-25th year. The epiphyses of the capitulum, lateral epicondyle, and trochlea unite with one another snd then in the 16th~ 17th year join the shaft. The epiphysis of the medial epicondyfe joins the shaft in the 18th year.


1* according to Poirier, Traitd d'Anatomie, p. 138, two centres appear in the eighth week, and unite in the third month to form a centre of ossification for the body of the scapula.


Bone.


Radius


Ulna


Carpus


Metacarpals ,


Centres.


Phalanges First row —


Middle row..


Diapbysis , Epiphyses: Carpal end Humeral end Diaphysis Epiphyses: Carpal end Humeral end Os capitatum Os hamatum Os triquetrum Oslunatum Os naviculare Os mult, maj Osmult. min Os pisiforme Diaphsrses Proximal epiphysis of the first metacarpal Distal epiphyses of the metacarpals.


Time of appearance of centre.


7th week (Mall)


9 8th month, cT 1 6th month (Pryor), 6th-7th year.


7th week.


9 6th-7th year. (^ 7th-«th year ( Pryor loth year.


Diaphyses Proximal epiphyses . .


93d-6th month (^ 4th-10th month. V5th-10th month. ^ 6th-12th month. 9 2d-3d year. cT about 3 yean, 93rd-4thyear. (^ about 4 years. Vat 4 years, or early in 5th year. d about 5 years. 94th-5thyear. (5'6th-6thyear. 94th-5thyear (f 6tn-6th year. 99th-10thyear. cT 12th-l3th year. 9th week (Mall)


3d year. 2d year.


9th week (Mall) l8t-3d year (Pryor).


Terminal row


Diaphyses Proximal epiphyses . . Diaphyses Proximal epiphyses .


Sesamoid bones


llth-12thweek(Mall) 2d-3d year.


7th-8th week. 2d-3d year.


Union of primary and secondary centres; remarks.


The superior epiphysis and shaft unite between the 17th and 20th years. The inferior epiphysis and shaft about the 21st year (Pryor); 921st year, cf2l8t-25th year (Sappey). Sometimes an epiphysis IS found m the tuberosity (R. and K.) and in the styloid process (Sappey). The centre for the shaft of the ulna arises a few days later than that for the radius. The proximal epiphysis is united to the shaft about the 17th year; the inferior epiphysis between the 18th and 20th years; 9 20th 7 21st years, cf 21st -24th years (Sappey) . There is sometimes an epiphysis in the styloid process (Sohwegel) and in the tip of the olecranon process (Sappey). The navicular sometimes has two centres of o.osification (Serres. Rambaud and Renault). Serres uid Pryor have described two centres of ossification in the lunatum. Debierre has described two centres in the pisiform, one in a girl of eleven, the other in a boy of twelve. The OS hamatum may have a special centre for the hamular process. Pryor has found two centres in the triquetrum. Pryor (1908), describes the centres of ossification of the carpal bones as assuming shapes characteristic of each bcme at an early period.


The centres for the shafts of the seComd and third metacarpals are the first to appear. There may be a distal epiphysis for the first metacarpal and a proximal epiphysis for the second. Pryor (1906). found the distal epiphysis of the first metacarpal in about 6 per cent, of cases. It is a family characteristic. It arises before the 4th year and unites later. Pryor found the proximal epiphysis of the second metacarpal in six out of two hundred families. It unites with the shaft between the 4th and 6th-7th year; sometimes, however, not until the 14th year. In the seal and some other animals all the metacarpals have proximal and distal epiphyses (Quain). The epiphyses join the shafts between the 15th and 20th years. There may bean independentepiphysis for the styloid process of the 5th metacarpcd. The epiphysis of the metacarpal of the index finger appears first. This is followed by those of the 3d, 4th, 5th, and 1st digits. The shafts of the phalanges of the second and third fingers are the first to show centres of ossification. The phalanges of the little finf^er are the last, ^he epiphysis in the middle finger is the first to appear. This is followed by those of the 4th, 2d, 5th, and 1st digits. The centres in the shafts of this row are the last to appear. The epiphysis of the phalanx of the middle finger is the first to appear. This is followed by thof«e of the ring, index, and little finger (Pryor). The terminal phalanx of the thumb is the first to show a centre of ossification in the shaft. This is the first centre of ossification in the hand. It is developed in connecti ve tissue while the centres of the other phalanges are developed in cartilage (Mall). The epiphysis of the ungual phalanx of the thumb is followed by those of the middle, ring, index, uid littlenngers. The fusion of the epiphyses of the phalanges with the diaphyses takes place in the 18th-20th year. Ossification begins generally in the 13th14th years, and may not take place until after middle life (Thilenius). For table of relative frequency in the embryo and adult see p. 385.


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Hassenstein, W. : Zur Reifebestimmung des Fetus aus dem Knochenkern der Oberschenkelepiphyse. Zeitsehrift fiir Medizinalbeamte. 1892.

Heimann, Alfr. und Potpeschnigg, K. : Uber die Ossifikation der kindlichen Hand. Jahrbuch fur Kinderheilkunde. Bd. 65, S. 437-456. 1907.

Heine, Otto : Uber den angeborenen Mangel der Kniescheibe. Berl. klin. Wochen schrift. Bd. 41. 1904.

Henke und Reyher : Studien iiber die Entwicklung der Extremitaten des Menschen insbes. der G^lenkflachen. Sitzungsb. d. K. Akad. der Wiss., Wien. Math.-naturw. Klasse. Bd. 70, 3. T., S. 217. Wien 1874. Hennig, C: Das kindliche Becken. Arch, fiir Anat. und Physiol. Anat. Abt» 1880.

Hepburn, D.: The Development of Diarthrodal Joints in Birds and Mammals. Journ. of Anat. and Physiol. Vol. 23, p. 507. 1889. Hissbach, Friedr.: Uber Polydactylie, deren Wesen und Bedeutung. Med. Diss. Leipzig 1902.

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HuETER, C: VirchoVs Arch. Bd. 25, 26, 28, 36. See also Klinik der Gelenk krankheiten. S. 9. 1870.

HuLKRANTz, J. W. : Das Ellenbogengelenk und seine Mechanik. Jena 1897.

Humphrey: The Angle of the Neck with the Shaft of the Femur at Different Periods of Life and under Different Circumstances. Joum. of Anat. and Physiol. Vol. 23, p. 273, 387. 1889.

Jenkins : The Morphology of the Hip Joint. Brit. Med. Joum. No. 2393, p. 1702. 1906.

JoACHiMSTHAL, G. : Uber Brachydactylie und Hyperphalangie. Virchow's Arch. Bd. 151. 1898. Die angeborenen Verbildungen der oberen Extremitaten. Rontgenbilder. Fortschritte auf dem Gebiet der Rontgenstrahlen. Erganzungsh. 1900. Die angeborenen Verbildungen der unteren Extremitaten. Rontgenbilder. Fortschritte auf dem Gebiet der Rontgenstrahlen. Erganzungsh. Bd. 8. 1902. Uber angeborene Defektbildungen am Oberschenkel. Beitrag zur Gynakol. und Geburtshilfe. Festschrift f. Leopold Landau. Berlin 1902. Verschiedene Formen angeborener Fussdeformitaten. Verb. d. Deutsch. Ge seUschaft fur Chir. Bd. 1, S. 66-67. 1906.

Johnston, H. M. : Epilunar and Hypolunar Ossicles, Division of the Scaphoid and Other Abnormalities in the Carpal Region. Joum. of Anat. and Physiol. Vol. 41, p. 59-65. 1906.

JuLiEN, A. : Loi de ^apparition du premier point epiphysaire des os longs. Compt. Rend. Acad. Sc. T. 114, p. 926-929. Paris 1892. Kastschenko, N. : Uber die Entwicklung der Finger beim menschlichen Embryo. Charkow 1884. (Russian.)

EIazzander, G. : Beitrag zur Lehre iiber die Entwicklungsgeschichte der Patella. Med. Jahrbuch. Bd. 1, S. 59-78. Wien 1886. Sullo svilluppo deir articolazione del ginocchio. Monitore Zool. Ital. Vol. 5, p. 220. 1894. Uber die Entwicklung des Kniegelenkes. Archiv f . Anat. und Physiol. Anat. Abt. S. 161. 1894.

Keskineff, G. : Contribution a Fetude des hypertrophies congenitales des membres. Th^se. Nancy 3900. KiNDL, J.: Fiinf Falle von angeborenen Defektbildungen an den Extremitaten. Zeitscht. f iir Heilkunde. Bd. 38, S. 110-138. 1907.

KiRCHNER, A. : Die Epiphyse am proximalen Ende des os metatarsale V. nebst Bemerkungen zur Calcaneus-Epiphyse. Anat. Hefte. Bd. 33, p. 513-551. 1907.

Klaatsch, H. : Die wichtigsten Variationen am Skelett der freien unteren Extremitat des Menschen und ihre Bedeutung fiir das Abstammungsproblem. Ergebnisse der Anatomic und Entwicklungsg. Bd. 10, S. 599-719. 1900.

Klaussner, Ferd. : Uber Missbildungen der menschl. Gliedmassen und ihre Entstehungsweise. Wiesbaden 1900.

KoLLMANN, J.: Handskelett und Hyperdactylie. Verhdl. Anat. Ges. (Wiirzburg). 1888. (Diskussion zu diesem Vortrag: A. Froriep, M. Fiirbringer.) Anat. Anz. Bd. 3. 1888.

XoNiKOW, M. : Zur Lehre von der Entwicklung des Beckens und seiner geschlecht lichen Differenzierung. Arch, fiir Gynakologie. Bd. 45, S. 19. 1894.

Krause, W.: Os acetabuli. Internationale Monatsschrift fiir Anatomie und Histologie. S. 150. 1885.

Lambertz : Die Entwicklung des mensehlichen Knochengeriistes wahrend des f etalen Lebens, dargestellt an Rontgenbildern. Fortschr. auf d. Gebiet d. Rontgenstrahlen. 1900.

Lazarus: Zur Morphologic des Fussskeletts. MorphoL Jahrbuch. Bd. 24. ' 1896. Leboucq, H. : Le developpement du premier metatarsien et de son articulation tarsienne chez I'homme. Archives de Biologie. T. 3, p. 335. 1882. Uber die Entwicklung der Fingerphalangen. Verhdl. Anat. Ges. (Tubingen) 1899. Anat. Anz. Bd. 16. Erghft. 1899. Recherches sur le developpement des phalanges terminales des doigts chez Fhomme et les mammifferes. Ann. soc. med. Gand. T. 89. 1904. Legke^ W. : Das Vorkommen und die morphologisehe Bedeutung des Pfannen knochens (os acetabuli). Intern. Monatschr. f. Anat. u. Physiol. Bd. 1, p. 363. 1884. Lewis, W. H. : Development of the Arm in Man. Amer. Joum. of Anat. Vol. 2, page 145. 1902. Lubsen, J.: Zur Morphologic des Ilium bei Saugem. Petrus Camper, Dl. II. 3 Aufl. 1903. LuciEN, M. : Developpement de Particulation du genou et formation du ligament adipeux. Bibl. Anat. Suppl. T. 13. 1904. Ludlopp: Uber Wachstum und Architektur der unteren Femurepiphyse und oberen Tibiaepiphyse. Ein Beitrag zur Rontgendiagnostik. Beitrage klin. Chir. Bd. 38, S. 64-75. 1903. Die Entwicklung der unteren Femurepiphyse und oberen Tibiaspitze im Rontgenbild. Ver. wissensch. Heilk. Konigsberg 1903.

Mall, F. P. : On Ossification Centers in Human Embryos Less than One Hundred Days Old. Amer. Joum. of Anat. Vol. 5, p. 433. 1906. Matet: Developpement de Fextremite du premier metatarsien. Bull, de la Societe Anat. de Paris. Annee 70, p. 384-388. 1895.

Mehnert, E. : Untersuchungen iiber die Entwicklung des Beckengiirtels bei einigen Saugetieren. Morphol. Jahrbuch. Bd. 16; S. 97-112. 1889.

Merkel: Beckenwachstum. Anat. Hefte. Bd. 1, S. 121-150. 1902.

MoLLiER : Die paarigen Extremitaten der Wirbeltiere. Anat. Hefte. Bd. 8, 16 und 24. 1893-97. Moser: Uber das ligamentum teres des Huftgelenkes. Schwalbe's Morphol. Arbeiten. Bd. 2, S. 36. 1893. Neuhauser, H. : Die Beckendrehung. Zeitschrif t fiir Morphol. u. Anthropol. Bd. 3. Stuttgart 1901.

NoLTE, Ad.: Ein Fall von kongenitalem totalem Tibiadefekt. Diss. Leipzig 1903. Pagenstecher, E.: Beitrage zu den Extremitatenmissbildungen. Deutsche Zeitschrift fur Chir. Bd. 60. 1901.

Parker and Shattuck: Pathology and Etiology of Club-Foot. Trans. Path. Soc. of London. Vol. 35. 1884.

Paterson, a. M. : Development of the Sternum and Shoulder Girdle in Mammals. Brit. Med. Joum. 1902.

Perna, G. : L'os trigonum ed il suo omologo nel carpo. Arch. Ital. Anat. e Embriol. Vol. 2, p. 237-254. 1903.

Petersen: Untersuchungen zur Entwicklung des mensehlichen Beckens. Arch. fiir Anat. und Physiol. Anat. Ab. S. 67-96. 1893.

Pfitzner, W. : Beitrage zur Kenntnis der Missbildungen und Variationen des mensehlichen Extremitatenskeletts. Morphol. Arb. Bd. 1-8. 1892-1898. Uber Brachyphalangie und Verwandtes. Verb. Anat. Ges. Anat. Anz. Bd. 14, Erghf. 1898.

Die morphol. Elemente des menschlichen Handskeletts. Zeitschrift fiir Mor phol. und Anthropol. Bd. 2. 1900. Bd. 4. 1901. PiCQu6: Formule de Fossification des phalanges, des metaearpiens de la clavieule et des cotes. Compt. rend. soe. biol. Paris. Vol. 4, p. 247-248. 1892.

Preleitner, K. : Zwei Falle von angeborenem partiellem Claviculardef ekt. Wiener klin. Wochenschrift. Bd. 16. 1903.

Pbyor, J. W. : The X-ray in the Study of Congenital Malformations. Medical Record. Nov. 1906. Ossification of the Epiphyses of the Hand. X-ray Method. Bulletin of the State College of Kentucky. Series 3, No. 4. 1906. The Chronology and Order of Ossification of the Bones of the Human Carpus. Bulletin of the State University, Lexington, Ky. April 1908.

Pye, W. : On the Growth Rate of the Bones of the Lower Extremities with Especial Reference to Rickety Curvatures. Joum. of Anat. and Physiol. Vol. 23, p. 116. 1889.

Rabl: Gedanken und Studien iiber den Ursprung der Extremitaten. 2Jeitschrift fiir wiss. Zool. Bd. 70, S. 474-558. 1901.

Rabl : Tiber einige Probleme der Morphologie. Verb. anat. Ges. Anat. Anz. Bd. 23. Erganz.-Heft. 1903.

Ranke, W. : Dia Ossification der Hand unter Rontgenbeleuchtung. MUnch. med. Wochenschr. S. 1365. 1898.

Reiner, M. : Uber die kongenitalen Femurdefekte. Zeitschrift fiir orthopad. Chirurgie. Bd. 9. 1901.

Rieder, H. : Uber gleichzeitige? Vorkommen von Brachyund Hyperphalangie an der Hand. Deutsch. Arch. klin. Med. Bd. 66, S. 330-348. 1899.

Retterer: De Tossification du pisiforme de Phonmie, du chien et du lapin. C. R. Soc. Biol. Paris. T. 5. 1898. Ebauche squelettog^ne des membres et developpement des articulations. Journal de I'Anat et de la Physiol. Annee 38, p. 473-^509, 580-623. 1902. Rbtzius, G. : Uber die Auf richtung des fetal retrovertierten Kopf es der Tibia bei Menschen. Zeitschrift Morphol. und Anthrop. Bd. 2. 1900. Robertson, W. G.: A Case of Supernumerary and Webbed Fingers. Edinburgh Medical Journal. Vol. 14, p. 535-^6. 1900.

RoMiTi, G. : Sui caratteri sessuali nel bacino del neonato. Atti della Societli Toscana di Science Natural!. Vol. 8. Pisa 1892.

Rosenberg, E. : Uber die Entwicklung der Wirbelsaule und das Centrale Carpi des Menschen. Morphol. Jahrbuch. Bd. 1. Leipzig 1876.

RiiCKERT: Ossifikation des menschlichen Fussskeletts. Sitzungsb. d. Konig. Bay. Akad. Math.-nat. Kl. S. 65-72. Munchen 1901. RuGE, Ernst: Die Entwicklung des Skeletts der vordenen Extremitat von Spinaxniger. Morphol. Jahrbuch. Bd. 30, S. 1-27. 1902. Sachs, Adalb. : Uberangeborene Def ekte der Schliisselbeine. Diss. Leipzig 1902.

ScHiCKELE, G. : Beitrag zur Lehre des normalen und gespaltenen Beckens. Beitrage Geburtsh. und Gynakol. Bd. 4, S. 243-272. 1901.

ScHOMBURG, H. : Untersuchungen der Entwicklung der Muskeln und Knochen des menschlichen Fusses. Dissertation. GWttingen 1900.

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E. THE SKULL, HYOID BONE, AND LARYNX

General. Features.


We may distinguish in the skull, considered purely topographically, a neural and a visceral region. The neural region serves to protect and support the brain and sense organs; the visceral, for the alimentary and respiratory tracts. A sharp demarcation between the two regions is not possible. Thus the base of the skull, especially its axial portion, has relations to both regions, and through change of function changes in the two regions may be brought about (ear ossicles). We shall begin with a description of the axial part of the skull, which generally is counted a part of the neural region. The axial region is that portion which is continued forward from the vertebral axis. It includes the basal portion of the occipital bone and the body of the sphenoid. In the embryo the chorda dorsalis extends anteriorly to the hypophysis. The axial region of the skull is thus divisible into chordal and prechordal portions, the former lying posterior, the latter anterior to the hypophysis. The chordal portion, is further divisible into an otic part, which corresponds roughly with that portion of the base of the skull which articulates with the temporal bone, and a postotic part, which extends to the otic part from the spinal column. The prechordal region supports the orbitotemporal and ethmoidal portions of the skull.


The neural region lies dorsal, lateral, and apical from the axial region with which it is intimately associated. It serves to encapsulate the brain (cranial cavity) and the organs for hearing, smell, and vision (petrous portion of the temporal bone, orbital and nasal cavities).


The visceral region lies chiefly ventral and ventrolateral to the axial region. In part it is closely associated with the neural region. It includes the pterygoid processes of the sphenoid, the hard palate, the bones of the upper and lower jaws, and the hyoid bone. From the primitive visceral skeleton of the head are also derived the bones of the middle ear and the cartilages of the larynx. In the development of the complex skeletal apparatus of the head, overlapping blastemal or membranous, chondrogenous, and osseogenous stages may be distinguished. The origin of the mesenchyme of the head has already been described (p. 297). It is at first rather loose in structure, but soon becomes condensed in various regions. This condensation usually marks the beginning of the differentiation of the mesenchjine into muscles and into various connective-tissue structures of more or less definite form, tendons, fascias, dermis, submucous coats, membranes of the brain, and portions of the organs of special sense and the anlages of the skull and the larynx. The membranous anlage of the skeleton of the head is gradually developed from several centres of condensation. In part it is transformed into cartilage, forming the chondrocranium.


The chondrocranium arises through the fusion of a considerable number of cartilages which originate from independent centres of chondrification. Some of these centres of chondrification arise in mesenchymatous tissue which shows no well-marked condensation preceding the formation of cartilage. The transformation of membranous tissue into cartilage in some instances takes place very rapidly, in other instances slowly. The chondrocranium reaches its highest relative development in the third month of intra-uterine life. At this period it comprises the axial region of the skull, the auditory and olfactory capsules, the orbital wings and the bases of the temporal wings of the sphenoid, the occipital condyles, and the tectum posterius which lies dorsolateral to the occipital and temporal regions (Figs. 312 and 313). In the first and second branchial arches well-marked cartilaginous skeletal structures are formed ; in the first the malleus and incus; in the second, the stapes and the styloid process of the temporal bone. Ventrally the second, third, fourth, and fifth branchial arches give origin to a cartilaginous hyoid bone and to some of the cartilages of the larynx. Ossification begins during the second month in man. The skeleton of the head at this period, with the exception of the chondrocranium described above, is composed of membranous tissue. Ossification takes place in part directly in the membranous tissue of the skull, in part in the chondrocranium. Most of the individual bones of the human skull arise from two or more centres of ossification, and many of them are partly membranous, partly cartilaginous in origin. Neither the centres of ossification nor the bones developed from them correspond very perfectly with the centres of chondrification from which the chondrocranium arises. The chondrocranium is mainly, but not completely, replaced by bone. The cartilages of the septum and alse of the nose, and the fibrocartilago basalis, for instance, represent remnants of the chondrocranium. Parts of the primitive cartilaginous skeleton are converted into fibrous tissue instead of into bone. The stylohyoid ligament is an example of this. Gaupp has shown that the cavum cranii of mammals is not quite homologous with that of reptiles. On each side there lies a space, the cavum epitericum, above the ala temporalis, which in reptiles is outside of and in mammals forms a part of the cranial cavity (Mead, 1909, Voit, 1909).


We may now consider the more important stages in the development of the skull in somewhat greater detail.

Blastemaxi Period.

At the end of the second week of intra-uterine development the chorda dorsalis extends to the dorsal margin of the buCoopharyngeal membrane (Fig. 229). On each side of it mesenchyme fills in the space between the brain, pharynx, and ectoderm. As the head develops the mesenchyme increases in amount. It extends dorsally and apicalward so as to surround completely the brain and its appendages. When the flexures of the brain appear, mesenchyme extends into the fissures between the various segments of the neural tube. An especially large fold of mesenchyme Mittelhirnpolster) is formed beneath the midbrain flexure (Fig. 266). The chorda dorsalis for a time remains attached to the ectoderm of the caudal wall of the hypophyseal pocket, then loses this connection and terminates free in the tissue immediately behind the hypophysis beneath the midbrain flexure.


Toward iiie end of the fourth week the post-otic portion of the axial region of the skull becomes marked by a condensation of mesenchyme. This condensed tissue or *' occipital plate is not sharply outlined. It consists of a blastemal central portion with two lateral processes on each side, a caudal rod-like ** neural" process and a flat apical process (see Fig. 231). Between these two processes run the roots of the hypoglossal nerve. The chorda dorsalis, surrounded by a perichordal sheath, lies in the sagittal axis of the plate. At this period it may still be united to the epithelium of the pharyngeal vault. It has been previously pointed out that the post-otic axial region of the manamalian head may be considered to be composed of at least three segments comparable to the spinal segments. This segmentation is best marked by the myotomes which develop in the lateral portions of these segments. That part of the occipital plate which lies in the most distal of the segments resembles in some respects a spinal sclerotome tsee p. 334).


The apical end of the occipital plate extends into a thin layer of dense tissue which surrounds the dorsal portion of the pharynx. The chorda dorsalis extends forward in this tissue nearly to the hypophysis. The tissue in which the chorda runs becomes much thicker near the hypophysis than where it lies opposite the otic labyrinth. The latter is surrounded by a layer of condensed tissue connected for a short distance with the retropharyngeal tissue.


The mesenchyme in the visceral region of the head is much condensed, but as yet no skeletal structures are definitely outlined.


The chorda dorealis is composed of densely packed cells surrounded by a very faint sheath. Outside of the chordal sheath there is a weU-marked layer of mesenchyme cells or perichordal membrane. In the region of the spine and of the occipital plate a space is seen between the chordal sheath and the perichordal membrane. This space is not seen apical to the occipital plate. During the early part of the second month the membranous anlage of the skull becomes extensively developed.


Fig. 308.— (After Q. Levi, Arch. f. mikr. Anst. u. EDdrickliuwefchinble, 1900, vol. Iv, Fir. t.) Mem The anterior and posterior lateral processes of the occipital plate become united lateral to the hypoglossal nerve, so that the hypoglossal foramen is completed and the membranous pars lateralis of the occipital is formed. This pars lateralis is continued into the membranous vault of the skull, the origin of which is described below. The condensed tissue of the post-hypophyseal region increases in amount and extends about the hypophyseal pocket into the region apical from this, thus completing the anlage of the body of the sphenoid {Fig. 308). This gives rise to orbitotemporal and ethmoidal processes. The orbitotemporal process is first marked by a mass of dense mesenchyme which extends ventrolaterally toward the ectoderm


caudal to the optic cup. It is connected with dense tissue which surrounds the anlage of the orbit and with the anlages of the membranous fioor and vault of the skull. In it are developed the orbital and temporal wings of the sphenoid, the origin of which will be described in connection with the chondroeranium. The ethmoidal process extends anteriorly in the median line from the anlage of the body of the sphenoid into the region between the nasal fosste. It forms the anlage of the nasal septum and gives rise to parts of the membranous floor of the cranial cavity and the roof of the mouth (Fig. 310).


Fia. aOS.— {After C. Levi, Arch. t. mikr. Anat. u. EDt1rickluii(>B.. IBOO. vol. Iv, fl«. 2.) Skull of u embryo 14 mm, long. The tissue of the capsules of the lahjTinth increases in amount as the labyrinth becomes differentiated. The tissue which encloses the region of the semicircular canals and the vestibule forms an oval mass the outlines of which do not conform to that of the enclosed canals (Fig. 309). This tissue is less dense than most I)art9 of the membranous skeleton of the head and at an early period becomes transformed into embryonic cartilage (see p. 407). The cochlear portion of the labyrinth (Fig. 310) is enclosed by a dense mesenchyme which becomes converted into cartilage at a later period. lateral from the nasal fossa the tissue becomes generally somewhat condensed, though less so than the tissue in the septum. In the perinasal tissue condensation gradually marks out the lateral and ventral portions of the nasal capsule and the membranous floor of the ethmoidal and orbital portions of the cranial cavity (Fig. 310). From the lateral wall membranous processes project into the nasal fossa. These are the anlages of the concha?.


The floor of the cranial cavity at this period is formed posteriorly by the occipital plate with its lateral processes and by the capsule of the labyrinth. Between the two is a fissure for the passage of the glossopharjTigeus, vagus, and spinal accessory nerves and the jugular vein (Fig. 309). Apically the floor is formed by a thin sheet of condensed tissue, which is slightly marked over the ethmoidal region where the olfactory nerve passes through it and is better marked on the anterior medial portion of the roof of the orbit. This portion is connected caudally with the orbital wing of the sphenoid (Fig. 310). Between the orbital region and the capsule of the labyrinth, in the vicinity of the Gasserian ganglion, the floor of the cranial cavity is incomplete. More medially the floor of the cranial cavity is formed by two membranes, one of which arises from the anterior margin of the auditory capsule and the neighboring part of the body of the sphenoid, and the other from the posterior margin of the orbital wing of the sphenoid. These two membranes extend upwards into the midbrain fold, fuse, and furnish a short central skeletal support for the mesenchyme in this fold (Fig. 266). They enclose the lateral process of Rathke's pocket. They form no part of the definitive skeleton.


The roof of the cranial cavity is formed by a dense membranous layer which first becomes marked at the side of the head in embryos 9-11 mm. in length. At this stage there is a plate of dense tissue formed between the caudolateral margin of the orbit and the caudal lateral process of the occipital plate. It lies lateral to the Gasserian ganglion and the capsule of the labyrinth, with the latter of which it comes in contact. Below it is connected with the orbitotemporal process and the dense tissue of the region of the branchial clefts. This membrane gradually extends so that it forms a complete membranous vault. Ventrally it is continuous with the ventrolateral margin of the membranous covering of the ethmoidal and orbital portions of the floor of the cranial cavity. Laterally it becomes connected with the temporal wing of the sphenoid, the auditory capsule, and the lateral part of the occipital. Caudally it is continued into the much thinner membrana reuniens dorsalis of the spinal canal.


During the period under consideration the brain only partially fills the cranial cavity. A large amount of loose mesenchyme intervenes between the brain and the floor and vault of the cranial cavity. This tissue is especially abundant in the region of the flexures of the brain and about the hemispheres (Fig. 266). In it the falx cerebri and other membranous supports of the brain are developed. During the latter part of the second month an extensive plexus of vessels develops on the cerebral side of the membranous vault. The anlages of the alveolar borders of the upper and lower jaws become marked by condensation of tissue along the upper and lower margins of the entrance into the oral cavity. This condensed tissue at first forms a flat plate, but later sends processes in an aboral direction.


Chondrogenous Period.


A large amount of study has been devoted to the development of the chondrocranium or primordial cranium in the different vertebrates. An excellent summary of the chief literature on the subject is given by Gaupp (1906). The chief work on the development of the human chondrocranium has been done bv Dursv, Spoendli, Hannover, Froriep, v. Noorden, Jaccby, O. Hertwig, and Levi. The development of the chondrocranium in man begins early in the second month. Its relatively most complete differentiation is reached toward the end of the third month, although some parts of it undergo a still greater elaboration before conversion into bone.


At the end of the third month (see Figs. 312 and 313) the caudal half of the chondrocranium forms a ring of cartilage about the posterior portion of the brain. The thick ventral portion of this ring comprises medially the basilar portion of the occipital and laterally the capsule of the labyrinth and the partes laterales of the occipital. The dorsal portion of the ring is composed of a thin plate of cartilage, the tectum posterius, the only part of the cranial vault which becomes cartilaginous in man. In the partes laterales of the occipital the hypoglossal foramina may be seen. The processes which bound them anteriorly serve as the posterior boundaries of the jugular foramina.


The caudal portion of the chondrocranium is united to the apical portion by the relatively slender body of the sphenoid. At the junction between the two is a large dorsum sellsB. The apical portion from above appears somewhat quadrangular. The caudal angle of the quadrangle forms the body of the sphenoid ; the apical angle, the ventral end of the nasal capsule ; and the lateral angles, the tips of the alae orbitales of the sphenoid. In the mid-line a well-developed nasal septum extends forward from the body of the sphenoid. Seen from the side (Fig. 312) the dorsal surface of the body of the sphenoid and the dorsal and anterior margins of the nasal septum form three sides of a hemihexagon. At the junction of the dorsal and anterior margins of the nasal septum there is a prominent crista galli.

From the body of the sphenoid the temporal and orbital wings project laterally. On each side of the dorsal margin of the nasal septum there may be seen a quadrangular cribriform plate, the lateral margins of which are united to the ala orbitalis by plates of cartilage (cartilagines spheno-ethmoidales) which extend over the orbit. There is also a plate of cartilage which extends to the ala orbitalis from the dorsal surface of the axial region of the chondrocranium near the junction of the sphenoidal and ethmoidal regions.


The nasal fossae are bounded laterally by a plate of cartilage which is united posteriorly to the anterior extremity of the body of the sphenoid, dorsally to the lateral edge of the cribriform plate, and anteriorly to the nasal septum. The inferior margin of this lateral plate curves inwards, but does not extend to the nasal septum. The inferior surface of the nasal fossa thus is not closed off by cartilage. Anteriorly, however, the inferior aperture is rendered very narrow by a paraseptal cartilage (see p. 413). From the lateral nasal cartilage there arises a short process which encircles a part of the nasolachrymal duct (processus paranasalis).


The orbit is bounded above by the orbital wing of the sphenoid and the processes attached to this; posteriorly by the lateral extremity of the ala temporalis, much of which has already become ossified; and medially by the lateral nasal cartilage. The floor and the lateral part of the roof of the orbit are formed of membrane bone. At this period the parietal, frontal, nasal, and lachrymal bones, the maxilla, the ej^gomaticum and the squama temporalis, the tympanicum, the laminae mediales of the pterygoid process of the sphenoid, the vomer, and the palatine bones are beginning to become ossified as membrane bones (see Fig. 321).


Those portions of the skeleton of the head derived from the visceral arches are shown in Figs. 311, 312, and 314. From the mandibular arch are derived MeckePs cartilage, the malleus, and the incus. The malleus and incus have nearly their definitive form, although relatively far greater in size than in the adult skull. Meckel's cartilage, which is continued from the capitulum of the malleus into the mandible, is a temporary structure which disappears at a later period. It is at this time flanked by a mandible formed of membrane bone. The stapes, which at this period has its characteristic form, and the styloid process of the temporal bone are derived from the second branchial arch. The cartilaginous hyoid bone and the chief laryngeal cartilages are clearly outlined, although the hyoid bone is not thus represented in the model. These cartilages are derived from the second, third, fourth, and fifth branchial arches.


The skeleton of the rudimentary head of amphioxus is composed of the chorda dorsalis, membranous tissue, and a few scattered structures, cartilaginous in character. The cyclostomes have a rather complicated chondrocranium, the roof of which is formed of membrane except for a slender tectum synoticum. The occipital region is missing" and the cranium terminates caudally in the labyrinth region. Li selachians the cranial cavity of the cartilaginous skull has a complete roof, side walls, and floor, but is open in front and behind (f. magnum). In the vertebrates above the selachians a chondrocranium is formed during embryonic development, but the degree of its elaboration and the extent to which it is retained in the adult skull vary greatly in the different classes of vertebrates. In the higher vertebrates the chondrocranium is largely replaced by bone, partly of the investment (membranous), partly of the substitution (cartilaginous) type. In man the chondrocranium is relatively slightly developed and the investment bones are relatively extensive.

Having considered briefly the cartilaginous skeleton of the head at the height of its development, we may now take up in more detail the development of its component parts.


Occipital Region, the Capsule of the Labyrinth, and the Tectum Posterius

Base and Partes Laterales of the occipital. — ^A brief description of the development of the posterior part of the occipital has already been given in connection with the description of the cervical vertebrae. Early in the second month a centre of chondrification appears in the posterior part of the blastemal anlage of the occipital on each side of the median line (Fig. 308). Apparently a separate centre arises in the caudolateral (neural) process, but this very quickly fuses with the main parachordal centre, and it is possible that it is^. not always present. Each parachordal cartilage ext.end^ loijward at the side of the chorda dorsalis until the region is reached where the chorda dorsalis enters the dense retropharyngeal mesenchyme. Here the two parachordal plates fuse dorsal to the chorda into a single median plate which extends forward to the sphenoidal region (Fig. 266). At first the parachordal cartilages are separated from one another posteriorly by dense tissue (Fig. 266) and the median plate is similarly separated from the sphenoidal cartilage. Before the end of the second month, however, the posterior extremities of the parachordal cartilages become united ventral to the chorda and the median occipital plate becomes fused to the sphenoidal cartilage, at first laterally and then in the sagittal plane. The chorda dorsalis at this period runs in dense tissue in a dorsal groove in the occipital cartilage, then through this cartilage into the retropharyngeal tissue, thence dorsalwards in the line of suture between the occipital plate and the sphenoidal cartilage and terminates dorsal to the sphenoid cartilage (Fig. 266).


Laterally a cartilaginous process extends out in the blastema on each side of the hypoglossal nerve. The process caudal to this nerve, as above mentioned, apparently, at times at least, has a separate centre of chondrification, like the neural process of a spinal vertebra, but this becomes much more quickly fused with the body than does the latter. The apical lateral cartilaginous process is formed considerably later than the caudal.


The hypoglossal foramen is at first completed by blastemal tissue. In this tissue ventrolateral to the foramen there appears a separate centre of chondrification. The cartilage arising here soon becomes fused to the processes extending out from the median plate on each side of the hypoglossal nerve, thus completing the cartilaginous boundary of the foramen and the pars lateralis of the occipital cartilage. From it there extends in an apico-Tfentral direction, lateral to the jugular foramen, a prominent jugular process. The condyloid process is developed on the caudal side of the posterior lateral process of the occipital (Fig. 264, p. 344).


Chorda Dor salts. — The suboccipital portion of the chorda dorsalis becomes more and more irregular in form during the third month. Small processes are given off, some of which become separated from the chorda. In this region the chorda remains longest united to the pharyngeal epithelium. Some processes of the chorda are found, even in the second month, connected with processes of the pharyngeal epithelium. The connections are probably partly primary and partly secondary (Fig. 266). In the fourth month the chorda usually becomes discontinuous in places. After this it is gradually absorbed. During the chondrification of the basioccipital the chorda tissue is pressed back between the dorsal side of the occipital plate and the tip of the dens epistrophei. Chordomata may arise here. The Labyrinth. — ^While the semicircular canals are being differentiated the mass of tissue in which they are embedded becomes somewhat loose in texture and gradually from without medialwards becomes transformed into a peculiar kind of precartilage, the cells of which long remain nearer to one another than in most cartilage (Fig. 31D). The cochlear portion of the capsule becomes chondrified much later than the capsule of the canalicular part. The semicircular canals are lined by epithelium which abuts directly against the surrounding cartilage. The fibrous coat of the labyrinth is gradually differentiated from the cartilage. The oval and round foramens become distinct during the period of chondrification, because the tissue which covers them remains membranous while the surrounding tissue is converted into cartilage.


The capsule of the labyrinth is at first incomplete (Fig. 310). At the end of the second month the geniculate ganglion and the facial nerve lie in a slight groove on the vestibular portion of the capsule (Fig. 310), while the cochlear and vestibular ganglia extend into the large dorsal fissure between the canalicular and cochlear portions of the capsule. As development proceeds the anterolateral extremity of the dorsal edge of the cochlear portion of the capsule extends in a dorsolateral direction so as to cover the two auditory ganglia. At the same time the groove containing the geniculate ganglion and the neighboring portion of the facial nerve becomes converted into a canal (Fig. 313). The saccus endolymphaticus is not included in the otic capsule (Fig. 310). Lateral from the foramen endolymphaticum, in which the ductus endoljTiiphaticus is enclosed, lies the fossa subarcuata (Pig. 313).


Fig. 310.— Modd


In the human ehondroeranium it is not deep. In the petrosa of children of from 2 to 10 years of age it is much deeper ; in adults it again becomes slialLow (Mead).


Fig. 311.—


From the capsule of the labyrinth above the ossicles a process grows forward (P. perioticus superior Gradenigo) {Figs. 311, 312, 313). Ventrally this extends into a piate composed of fibrous connective tissue. This plate is connected with the pars cochlearis.

The tegmen tympani is formed from the cartilaginous process and the accompanying fibrous plate. The cartilaginous process is well shown in Fig. 313. Into the finer details of the development of the skeleton of the internal ear we cannot here enter. The cochlear portion of the capsule of the labyrinth is long connected by a fairly dense mesenchyme with the median plate of the occipital. After the chondrification of this portion of the capsule it becomes fused to the cartilage of the median plate, forming with it a continuous cartilaginous structure (Fig. 313). Across the jugular foramen somewhat irregular bars of cartilage may be formed (Fig. 313, right side).


In vertebrates below birds and nuumnals the auditory capsules lie in the lateral wall rather than in the floor of the cranial cavity. In man the basal position of the auditory capsules is more marked than in any of the lower mammals.


Tectum Postering. — The cranial vault, as previously pointed out, is formed at first by a thin dense layer of membranous tissue, which is closely applied to the lateral side of the capsule of the labyrinth and extends ventrally into the dense tissue of the branchial region. Posteriorlj^ and inferiorly it is attached to the pars lateralis of the occipital. This membrane at first completes the f. jugulare. In the sixth week cartilage begins to extend into it from the posterior lateral (neural) process of the occipital. This cartilage extends as a flat band rapidly in an anterior direction in the membranous vault. In a 14 mm. embryo it has extended anteriorly above the otic region, but lies at some distance from the dorsolateral margin of the otic capsule. Soon after this it extends in a ventral direction so as to be closely applied to the otic capsule posteriorly and dorsally (Fig. 311), but even toward the end of the second month it is still distinctly separated from this by a narrow band of membranous tissue. Later the two become fused (Fig. 313). Between the capsule and the margin of the cartilaginous vault there are several apertures for the passage of bloodvessels. During the third month the vault cartilages of each side extend dorsally and become united so as to complete a flat bridge of cartilage between the right and left occipitotemporal regions. This bridge of cartilage is called the tectum posterius or synoticum.


The description here given of the development of the tectum posterius differs in several respects from those of Levi, Bolk, and some other investigators. It is based on a study of several embryos between 11 and 20 mm. in length which the writer has had at his disposal. Possibly there are individual variations in the mode of the development of the tectum.


Levi describes a squama occipitalis which arises from a separate centre of chondrification, fuses with the pars lateralis of the occipital, and extends in an anterodorsal direction in the membranous vault; and a squama temporalis, which arises from a separate centre, fuses with the auditory capsule, and extends dorsally into the membranous vault (Fig. 309). The squama occipitalis and squama temporalis become fused and the temporal squama greatly reduced at the expense of the occipital squama. The occipital squamsB fuse to form the tectum posterius. according to Bolk, there is first formed a cartilaginous band, anterior interotic band, between the auditory capsules or the parietal plates applied to these. The posterior margin of this band extends into the membrana spinoso-occipitalis, which is attached laterally to the ear capsules and to the partes laterales of the occipital and posteriorly extends into the membrana reuniens dorsalis. Posterior to the interotic band of cartilage a second band is formed by outgrowth of cartilage from the partes laterales of the occipital and the caudal part of the otic capsule. This latter cartilaginous band is separated from the former by a membranous interval in which temporarily a pair of cartilages appear. There also appears in front of the anterior interotic band a temporary centre of chondrification. In the lower mammals there has frequently been described a cartilaginous lamina parietalis lying above the auditory capsule and united to the commissura orbitoparietalis.

Orbitotemporal Region

In man the cartilage of the orbitotemporal region, forms the basis for the ossification of the body, of the orbital and temporal wings, and of the laminae laterales of the processus pterygoidei of the sphenoid. These parts have special centres of chondrification which at first are separate but which fuse later. The chondrification of the body of the sphenoid begins in the median line anterior and ventral to the apical end of the chorda dorsalis in embryos between 12 and 13 mm. long. The position of this cartilage in a 14 mm. embryo is shown in Fig. 266. Prom this centre an arm of cartilage (Rathke's Schadelbalken) extends forward on each side of the hypophyseal pocket. In front of this the two processes unite to form the anterior part of the body of the sphenoid. In the lower vertebrates a pair of cartilages, trabeculae, are formed, one on each side of the hypophysis. These cartilages usually unite with one another apically and with the occipital parachordal cartilages caudally. It is a question whether or not these trabecul© are homologous with the sphenoidal cartilage above described (see Gaupp, 1906, p. 826). The caudal part of the body of the sphenoid becomes fused with the apical end of the median occipital plate and sends a process, the dorsum sellae, upward toward the midbrain fold. The apical end of the chorda comes to lie in the cartilage at the base of the dorsum sellae or between the cartilage and the perichondrium of the sella turcica or of the dorsum sellae. In the cartilage the chorda soon disappears; under the perichondrium it persists longer than elsewhere in the cranium and may give rise to chordomata (Williams). The cartilaginous body of the sphenoid gradually assumes the shape characteristic of the adult bone. During the third month the fossa hypophyseos, the tuberculum sellae, and the sulcus chiasmatis become fairly distinct (Fig. 313).


The hypophyseal canal is at first relatively large and is much broader than it is long. The tissue immediately about is very slowly converted into cartilage during the third month. occasionally a patent canal is found in the adult bone.


The cartilaginous ala temporalis (see Fig. 310) arises in the orbitotemporal blastema some distance below the membrane which forms the floor of the cranial cavity. It is only at a much later period that the temporal wing helps to bound the cranial cavity. During the latter half of the second month two portions may be distinguished in the ala temporalis, a medial and a lateral (Figs. 309 and 310). The medial portion (processus alar is, Hannover) lies in the plane of the body of the sphenoid. It consists at first of blastemal tissue which extends from the body of the sphenoid opposite the hypophysis laterally and then posteriorly so as partially to enclose the internal carotid artery. It has a special centre of chondrification. It approaches closely but does not fuse with the otic capsule (30 mm. fetus). A closed foramen caroticum is found in several mammals, but is transitory when present, and probably is not constant in the human embryo (Levi). The lateral part of the ala temporalis arises in a plane ventral to the medial part. The condensed blastema of which it is at first formed becomes fused to the ventral surface of the medial t part near where this turns posteriorly about the internal carotid artery. The lateral part of the ala temporalis is small where it joins the medial part, but expands rapidly as it extends laterally, anterior to the otic capsule and ventral to the cranial border of the trigeminus ganglion. It has a separate centre of chondrification. The lateral part becomes cartilaginous later than the medial but becomes ossified much sooner (Fig. 313). From the ventral surface of the medial end of the lateral portion of the ala temporalis a short process extends ventralwards. This represents the anlage of the lateral lamella of the pterygoid process.


The ganglion of the trigeminus lies at first caudal to the lateral part of the ala temporalis, and the first and second branches of this nerve as well as the motor nerves of the eye pass forward medial to this process. During the period of chondrification the second branch of the trigeminus becomes enclosed in the foramen rotundum. The third division of the trigeminus at first passes down between the ala temporalis and the otic capsule. It later becomes embedded in a groove on the posterior margin of the ala temporalis. This groove is converted into the foramen ovale before or during the period of ossification. The foramen spinosum is similarly formed about the middle meningeal artery.


The ala orbitalis is differentiated from the orbitotemporal blastema first by condensation of tissue and then by chondrification. It is larger at first than the ala temporalis. In a 14 mm. embryo a blastemai process, the taenia metoptica of Gaupp, arises from the side of the body of the sphenoid, extends up behind the optic nerve and then over this into a plate of membranous tissue which forms the roof of the orbit and the floor of the cranial cavity. A second blastemai process, taenia preoptica, extendts from the side of the anterior extremity of the body of the sphenoid in front of the optic nerve laterally into the orbital plate. Chondrification (Fig. 310) appears first in the taenia metoptica in the region posterior to the optic nerve, and from here extends medialwards to fuse with the anterior part of the body of the sphenoid and lateralwards into the orbital plate (Fig. 313). The orbital plate has a separate centre of chondrification. The taenia preoptica apparently becomes chondrified through extension of cartilage into it from the body of the sphenoid. Chondrification begins later in this than in the taenia metoptica and the orbital plate. During the third month the ala orbitalis becomes fused into a single piece of cartilage and at the same time joined by bands of cartilage (cartilago spheno-ethmoidalis) to the lateral edge of the cribriform plate of the ethmoid (Fig. 313). In many mammals the outer end of the ala orbitalis is connected with the cartilage of the cranial vault dorsal to the otic capsule (parietal plate) by a bridge of cartilage, the commissura orbitoparietalis (Gaupp). This bridge, which is lacking in man, encloses a large (sphenoparietal) foramen.


Ethmoroal Region and the Nasal Capsule

The ethmoidal region and the nasal capsule are the last portions of the chondrocranium to become cartilaginous. In an embryo 20 mm. long and in the eighth week of development (Figs. 310 and 311) the tissue is still membranous, although both the nasal septum and the lateral wall of the nasal capsule are evidently in a precartilaginous stage. In the third month the cartilaginous capsule is extensively developed (Figs. 312 and 313).


The chondrification of the septum apparently takes place by anterior extension from the cartilage of the ventral part of the body of the sphenoid. The septum is at first relatively thick, especially on the ventral margin. From the anterior part of this thickened ventral margin of the septum a **paraseptal" cartilage becomes isolated on each side (third month).


In many of the lower mammals the anterior part of the ventral margin of the septum becomes joined to the lateral wall by a band of cartilage, the lamina transversalis anterior, thus separating the " fossa narina " from the " fenestra basilaris." The paraseptal cartilage in these mammals extends from the posterior margin of the lamina transversalis anterior into the fenestra basilaris. In man the lamina is not developed, so that a long fissura rostroventralis is present in the nasal capsule. The paraseptal cartilage primitively in mammals, but not in the repliles, furms a sheath for Jaccbson's organ, but in mao it has lost this function. It, however, persists until after birlb (E. Schmidt). according to Mihalkovics, several isolated pieces of cartilage found in the third month lateral to the inferior margin of tbe nasal septum may indicate rudiments of the L, transversalis anterior.


CsnilBga Meckali


Posteriorlj' the cartilage of the nasal septum is much narrower than it is anteriorly. It does not extend into the blastemal septum between the nasopharyngeal passages. The chondrtfication of the lateral walls (C. paranasalis) of the Dasal fossa; seems to take place independently, but the lateral cartilage is soon joined to the nasal septum, anteriorly forming the cartilaginous roof and sides of the nose, tectum nasi, and paries nasi, and somewhat later it is posteriorly united to the region where the sphenoidal cartilage passes over into the cartilage of the septum. Through infolding of its inferior margin the lateral wall of the nasal fossa posterior to the narina nasi furnishes the anlage of the maxillary turbinate, concha inferior. This is at first simple in form though later more complicated. Late in the third month it develops an accessory process curved upwards, and in the fifth month exhibits extensive folds (Mihalkovics). It becomes separated from the lateral wall when the latter undergoes retrograde metamorphosis (seventh month, Killian).



During the blastemal period folds in the surrounding mesenchyme project into the nasal fossa. On the posterior dorsal part of the lateral wall there is formed a fold of tissue which, according to Peter (1902), may be looked upon as having been derived from the caudodorsal part of the median wall. This fold gives rise to the anlage of the middle turbinate, concha media. The anlage of the superior turbinate arises in a manner similar to the middle. Following this there are formed much later the anlages of three more turbinate processes. Thus there are five chief ethmoidoturbinate processes in addition to the maxilloturbinate already described. Apiealwards, between the concha media and the concha inferior, there appears a rudimentary nasoturbinate which gives rise to the agger nasi and the uncinate process. (See Killian, 1895, 1896; Peter, 1902.)


Besides the chief turbinates there are numerous accessory turbinates. The bulla ethmoidalis arises from accessory processes in the meatus beneath the middle turbinate. The complicated changes which take place in the nasal turbinates cannot be entered upon in detail in this section.^


Chondrification of the ethmoidal turbinates, of the uncinate process, and of the bulla ethmoidalis begins in the fourth month. The cartilaginous capsule of the nose at first is open toward the olfactory bulb, but during the third month the cribriform plate is formed by chondrification of tissue between various nerve bundles (Fig. 313). The lamina cribrosa is characteristic of mammals, but is not present in all.


In most of the lower mammals the caudal margin and the caudal part of the inferior margin of the lateral wall of the nasal capsule bend towards the nasal septum and then forwards so as to bound a cupola-shaped recess (the sinus terminalis) at the caudodorsal extremity of the nasal fossa. In man this recess, the anlage of the sinus sphenoidalis of the osseous cranium, is not much developed and has no ventral cartilaginous wall. A membranous septum is, however, formed between the meatus nasopharyngeus and the cupola-shaped recess. The septum becomes ossified, forming the floor of the sphenoidal sinus. The paranasal cartilage bounds the recess laterally, but does not bound the meatus nasopharyngeus. The latter becomes bounded laterally by a membrane bone (os pterygoideum). In the third to fourth month a short cartilaginous process (proc. paranasalis) arises from the lateral wall of the nasal capsule and encircles the lachrymal duct.


The fate of the cartilaginous nasal capsule is varied. Parts become ossified, parts are converted into connective tissue or disappear, and parts pass over into the cartilaginous portion of the skeleton of the adult nose. The greater part of the posterior portion of the capsule becomes ossified as the ethmoid bone. The dome-shaped wall of the sinus terminalis gives the basis for the concha sphenoidale (ossiculum Bertini). The maxilloturbinate (concha inferior) and a part of the nasal septum likewise become ossified. Parts of the septum and of the inferior portion of the lateral wall above the maxilloturbinate, however, disappear and are replaced by parts of the neighboring membrane bones. The cart, paraseptalis remains till after birth. A large part of the septum and parts of the roof of the nose remain cartilaginous throughout life. The C. alares majores become separated by development of connective tissue from the rest of the nasal capsule during the fourth to fifth month of intra-uterine life. The C. alares minores and the C. sesamoidia; are differentiated from the C. atares majores. The cartilago spheno-ethmoidalis, the orbital wing of the cartilaginous ethmoid, which during the third month extends as a broad plate between the lateral margin of the lamina crlbrosa of the ethmoid and the ala orbitalis of the sphenoid (Fig. 313), in the fourth to fifth month is broken up into several pieces and absorbed.


  • See the description of the development of the nose in the section on the organs of special sense.

u, 1907. Fi(. 366.J VisMnl skeleton of th» tuB 8 cm. lon(.

Derivatives of the Visceral Arches

From the visceral arches are derived the bones of the middle ear, the styloid process of the temporal bone, the stylohyoid ligament, the hyoid bone, and the cartilago thyroidea. In the human embryo the formation of the blastemal ossicles and of the hyoid bone is a fairly direct process, but their relations to the embrj'onic skeleton of the mandibular and hyoid arches (Meckel's and Eeichert's cartilages) are more or less clearly marked. The relations of the laryngeal cartilages to the visceral arches are not so definite.


Toward Uie end of the first month the tissne in the branchial arches, in the lateral region of the head immediately dorsal to these, and about the larynx becomes much condensed. according to I. Broman, the tissue in the dorsal part of the mandibular arch region is divided by the third division of the fifth nerve into lateral and medial portions, while that in the hyoid arch region is similarly divided by the seventh nerve. The relations of these divisions of the blastema of the first two arch regions to the auditory ossicles are described as follows: The proximal portion of the lateral part of the blastema of the mandibular arch region gives rise to the anlage of the incus. From this in a 14 mm. embryo a process of condensed tissue may


Fia. 315.— (After Bronuo. AnmtomiKhc Hefte. 1897. vol.


be followed anteriorly, lateral to the fifth nerve, into the anlage of the maxilla. This later disappears. The anlage of the incus soon fuses with the blastema of the otic capsule {Fig. 315, A), but becomes separated again at the time of chondrification. The proximal part of the lateral division of the blastema of the hyoid arch region gives rise to the anlage of the tympanohyale (laterohyale). This in turn becomes fused to the capsula auditiva and to the styloid process (Fig. 311).


The cartilage of the external ear is differentiated from the blastema of the dorsolateral region of both the mandibular and the hyoid arches. The proximal end of the medial part of the blastema of the mandibular arch region is checked in development by the vena jugularis primitiva. The portion beyond this gives rise to the anlage of the malleus (Fig. 311), and this is continued into a condensed band of tissue that may be followed in the mandibular arch to the mid-ventral line. This band is the anlage of Meckel's cartilage and appears in an embryo 11 mm. long as a rod of dense tissue.


The proximal end of the medial part of the blastema of the hyoid arch region gives rise to the anlage of the stapes (Fig. 311). 21 This anlage is from the first connected by a band of blastemal tissue with the anlage of the incus. The band of tissue develops into the crus longum incudis (Fig. 315, C).^^ Immediately ventral to the anlage of the stapes there is formed a small band of tissue (interhyale, Broman; lig. hyostapediale, Fuchs), which connects this anlage with the main hyoid arch. It lies beneath the facial nerve (Fig. 315, A and B). It forms a partial sheath for this nerve. In the second month it disappears, so that the stapes anlage is no longer connected with the main hyoid scleroblastema. The latter is a rod-like process which extends from the tympanohyale (laterohyale) medialwards to the anlage of the body of the hyoid bone. It is visible in an 11 mm. embryo.^^ It gives rise to the styloid process, the stylohyoid ligament, and the lesser comu of the hyoid bone.

It is convenient to consider the development of the ossicles and of Meckel's cartilage separately from the development of the hyoid bone, the styloid process, and the laryngeal cartilages.

The Ossicles and Meckel's Cartilage

During the latter half of the second month Meckel's cartilage becomes chondrified. Its position at this period is shown in Fig. 311. It does not reach quite to the midventral line. Later it sends a process upwards parallel to the medial line (see Figs. 312-324).

Dorsally Meckel's cartilage is continued into the capitulum of the malleus (Figs. 311, 314, 315, B and C).^* Toward the end of the second month the malleus is fairly well differentiated (Figs. 311 and 315, B). The manubrium extends medialwards in a dense mass of tissue which intervenes between latter arises, according to Fuchs, from a centre which lies in the region where the temporo-mandibular joint is later dif


'"according to Hugo Fuchs (1905), in the rabbit the anlage of the stapes lies dorsal and anterior to the hyoid arch region and arises not in connection with the hyoid arch but rather in connection with the otic capsule. There is later formed a temporary connection between the anlage of the stapes and that of the skeleton of the hyoid arch, the " ligamentum hyo-stapediale." ""according to Fuchs (1905), in the rabbit the anlage of the erus longum of the incus arises apparently independently of the main anlage of the incus. "according to Fuchs (1905), in the rabbit it first appears in the region of the hyoid bone and thence extends dorsalward.

    • Acx?ording to Fuchs (1905), there is a common malleus-incus anlage in the rabbit, which arises independently, chondrifies from a separate centre, and becomes secondarily fused to Meckel's cartilage. The

the lateral extremity of the tubotympanic cavity and the medial end of the external auditory meatus.^^


In Fig. 311 the most medial part of this tissue and the medial extremity of the external auditory meatus are shown. From the manubrium a ** lateral" process is at first directed downwards. As development proceeds the manubrium comes to be directed downwards and the lateral process is turned outwards. The crista mallei arises during the fourth month. It is not due to the outgrowth of a process, but rather to absorption of the underlying cartilage. The joint surfaces between the malleus and incus have from the first two chief facets, as in the adult. The greater facet is at first directed laterally, the smaller dorsally. When rotation takes place the greater facet faces dorsally, the smaller medially. At the beginning of the third month the accessory facets of the joint surface and the **Sperrzahn" of Helmholtz appear. The cartilaginous malleus is at first joined to the cartilaginous incus by dense tissue, in which later a joint cavity arises.


The incus (Figs. 311, 314, and 315, B and C) becomes chondrified during the latter half of the second month. It has a special centre of chondrification, which first appears in the head and then extends to the processes. The head at this period is embedded in dense membranous tissue (Fig. 310).


Cartilage extends into the cms longum as far as the joint between it and the stapes. This joint is at first composed of dense tissue but is later differentiated into a true joint. The cms brevis is formed when chondrification starts in the anlage of the incus. At this p)eriod the head of the incus becomes somewhat separated from the capsule of the labyrinth, with which it has been temporarily fused. A short blastemal process is left which extends dorsally from the incus to the capsule. Into this process the cartilaginous cms breve extends. In Fig. 311 the space between the cms breve and the auditory capsule is shown slightly too wide in order to reveal the deeper structures. The processus lenticularis is not formed until the crus longum has begun to ossify. At the beginning of the third month the crus breve, cms longum, and the manubrium of the malleus lie nearly in a plane, a condition noted by Helmholtz in the adult. The malleus and Meckel's cartilage are homologous with the skeleton of the lower jaw in the inferior vertebrates. The incus is homologous with the quadrate portion of the palato-quadrate. The palate portion is not represented.


The first definite differentiation of the stapes is seen when the cells of the anlage form a ring of tissue concentrically arranged about the stapedius artery. This is at first separated from the capsule of the labyrinth by loose tissue, but later becomes fused to it, although still distinguishable by the arrangement of the cells. When chondrification sets in, it becomes still more clearly marked oflF. From the first it has an oblique position (about 45^ to the horizon). Chondrification begins during the latter part of the second month. At the end of the third month the hitherto circular stapes begins to take its definite form. The artery persists to the end of the third month. As the foot-plate of liie stapes becomes differentiated the lamina fenestris ovalis becomes thin.


ferentiated, and from which the articular part of the squamosum also arises. according to Fuchs, the mandibular joint of mammals is homologous with the quadrato-articular joint of the lower forms. according to most investigators, the quadra to-articular joint is homologous with the malleus-incus joint in mammals, a view originally advanced by Reichert. See Gaupp (1906), Van Kampen (1905), Mead (1909). "according to Fuchs (1905), in the rabbit the manubrium arises separately from the anlage of the head of the malleus, to which it extends from the hyoid arch region.

Tympanohyale, Reichert's Cartilage, the Hyoid Bone, and the Laryngeal Cartilages

The tympanohyale (laterohyale) arises from the proximal part of the lateral blastema of the hyoid arch region. It becomes chondrified from a separate centre and then proximally fuses to the cartilaginous otic capsule, while distally it becomes fused with the chief cartilage of the hyoid arch. The part of the otic capsule with which the tympanohyale fuses is a process that lies on the ventrolateral ^nrface of the promontory of the lateral semicircular canal, the crista parotica. The processus perioticus superior is developed at the apical end of this crest. The proximal end of the tympanohyale is enclosed in the tympanic cavity and utilized in the formation of the wall of a canal containing the nervus facialis, the musculus stapedius, and a few blood-vessels (foramen stylomastoideum primitivum, Broman).


The chief blastemal skeletal element of the hyoid arch is a rod of tissue which is proximally connected both with the anlage of the stapes and with that of the tympanohyale. Distally it extends to the lateral margin of the anlage of the body of the hyoid bone. It loses its proximal connection with the stapes, becomes chondrified from a separate centre and finally fused with the distal end of the cartilaginous tympanohyale (Figs. 311, 314). It is now known as Reichert's cartilage. Subsequently it becomes transformed into the lesser cornu of the hyoid, the stylohyoid ligament, and the styloid process. It has been pointed out above that the mesenchyme of the visceral arches toward the end of the first month becomes verv dense and that a dense mass of tissue surrounds the anlage of the larynx. This mass of tissue is especially developed ventral and lateral to the larynx and is connected with the dense blastema of the hyoid and of the more posterior visceral arches. During the second month there are developed in this tissue the anlages of the body and of the greater cornua of the hyoid bone and of the laminsB and the cornua of the thyroid cartilage of the larynx. The appearance of the structures mentioned above toward the end of the second month is shown in Fig. 311. Their appearance about the middle of the second month is shown in Fig. 316. The body of the hyoid is developed from the ventral part of this dense tissue in front of the proximal end of the larynx. It may be barely distinguished in an 11 mm. embryo. Pl-ecartilage appears in it in a 14 mm. embryo. At about this time it has the form shown in Fig. 316, A and B. The form is essentially similar


See, howe\'er, note 24, p. 419, and note 43, p. 141.


Fig. 316.— (After Kallius, Anatomische Hefte. 1897, vol. ix, Taf. XXVI, Figs. 20, 21, and 22.) Three figures to illustrate the development of the hyoid bone and the laryngeal cartilages. A and B. hyoid, hyothyreoid, and thyroid cartilages, from an embr3ro 39-40 days old: A, ventral view; B. lateral view. C. Medial view of the cricoid cartilage of an embryo 40-42 days old. The ceotra of chondrification are outlined by heavy lines. C. m., comu minus; C. hyothyr., cartilago byothyreoidea; Thyr^ cartilago thyreoidea. at the end of the second month (Fig. 311), but it is still composed largely of dense tissue and precartilage. During the third month it becomes more highly differentiated. The body of the hyoid bone probably represents a copula of a visceral arch or the fusion of two such copulaB. Kalliiis found in the cow an anterior and a posterior anlage, the former of which may represent a hyoid, the latter a third visceral arch copula. No such double anlage has been found in man.


The anlage of Reichert 's cartilage in the 11 mm. embryo above mentioned is more highly developed than the body of the hyoid. It is composed of a very dense tissue, which is connected with the blastema of the body. When chondrification takes place Reichert 's cartilage long remains separated from the cartilage of the body of the hyoid by a narrow band of dense tissue which forms a kind of primitive joint. Finally the two cartilages become fused.


Between the body of the hyoid bone and the laminae of the thyroid cartilage in the dense tissue lateral to the larynx there is developed a curved cartilaginous bar, which we may call the hyothyroid cartilage (Figs. 311, 316, A and B). Ventrally this bar is joined at first by dense tissue, later by cartilage, to the back of the body of the hyoid. Dorsally it becomes fused to the cartilage of the lamina of the thyroid. It is invisible in an embryo of 11 mm. and becomes chondrified apparently from a single centre at about the time of the chondrification of Reichert's cartilage. It represents the skeleton of the third and a part of the fourth visceral arches. Its ventral portion becomes the greater cornu of the hyoid bone and its dorsal inferior portion the superior cornu of the thyroid cartilage. The two portions become discontinuous at the end of the third month, so that a small cartilago triticea is separated on the one side from the great cornu of the hyoid bone and on the other from the superior cornu of the thyroid cartilage. Connective tissue serves at the same time to form connecting ligaments, but the definite lig. hyothyroideum is not well developed until after birth, when the hyoid bone becomes further separated from the thyroid cartilage.


The blastemal laminae of the thyroid cartilage appear about the middle of the second month. One appears on each side at the p)eriphery of the dense tissue surrounding the ventral part of the larynx. This anlage has the form of a slightly curved quadrilateral plate in which a foramen may be seen (Fig. 316, B). There are two centres of chondrification, one cranial and the other caudal to the foramen. The cranial centre is continuous with the hvothjToid cartilage and later becomes united on each side of the foramen to the cartilage of the caudal centre. The foramen is usually closed by cartilage, but occasionally remains patent throughout life. The inferior cornu is developed from the dorsal part of the caudal margin of the lamina. Ventrally the laminae of each side become united by the membranous tissue into which the cartilages of the cranial and caudal margins of the laminae extend, and finalljr unite in the mid-ventral line. Between the two margins there is an orifice closed merely by membrane. In this a special centre of chondrification appears. This medial cartilage eventually becomes united to the cartilage of the laminae, so that the central orifice is closed in the tenth to thirteenth week (according to Kallius). The cranial margin of the thyroid cartilage is at first nearly level. The incisura arises through the rapid development of the laminae lateral to the median line. The cornu inferius, the tuberculum superius and inferius, and the linea obliqua are developed during the latter part of the fourth month. The thyroid cartilage is supposed to be derived from the fourth and fifth visceral arches. The central cartilage probably represents copulae.

The cricoid cartilage is the first of the cartilages of the larynx to show definite hyaline tissue. About the lower part of the larynx there is formed a dense band of tissue. In this tissue a semicircular cartilaginous process appears. (In Fig. 316, C, the cartilage is surrounded by dark lines.) It is bilaterally better developed than in the mid-line, but if there are two bilaterally placed centres these quickly fuse ventrally. The cartilage slowly develops in the dorsal direction. Fig. 311 shows it at the end of the second month. In the third month the ring is completed and the posterior lamina is developed. The arjiiSBnoid cartilage develops from the blastema continued cranialward from the cricoid cartilage (Fig. 316, C). A special centre of chondrification appears in the seventh week. The first part of the cartilage developed represents the posterior portion, chiefly the proc. muscularis. From this the processus vocalis grows ventrally. This process, however, is long blastemal and does not reach its definitive form till the end of the fourth month (Kallius). The apex of the cartilage grows cranialwards, so that the definitive form of the cartilage, with the exception of the proc. vocalis, is approached by the end of the third month. There is regularly present in later fetal life on the arytaenoid cartilage ventral to the cart, comiculata a process which disappears after birth. Its place is marked by the origin of the ligament which extends to the cartilago cuneiformis. It is probable that it represents the cartilaginous process which connects the arytaenoid and cuneiform cartilages in some animals (Kallius). The blastemal anlage of the cart, corniculata is continuous with that of the cart, arytaenoidea. Toward the end of the third month it chondrifies from a special centre. The cartilage of the epiglottis does not appear until the end of the fifth month. It has a single median centre.

The cuneiform cartilages develop in the blastema of the plicaB aryepiglotticsB. They appear toward the end of the seventh or early in the eighth month.


Period of Ossification.

In the human skull the membrane bones are extensively developed, compared with those in lower forms. Some of the centres of ossification in the membranous tissue arise before any of the centres in cartilage. Thus Mall (1906) has found centres of ossification for the mandible (39th day), maxilla (39th day) and premaxilla (42d day), before any centre of ossification has appeared in the chondrocranium. The first centres of ossification to appear in the chondrocranium are those in the occipitale laterale (56th day), the basioccipital (65th day), orbitosphenoid (83d day), and basisphenoid (83d day).


The complexity of tiie ossification of the bones of the skull makes it advisable to discuss briefly the development of each of the iudividnal bones recognized in the hnman skull rather than to treat of the bones in classified groups. Most of the bones of the human skull arise from two or more centres of ossification, some of which represent individual bones in the lower vertebrates.

Occipitale

In the occipital bone five elementary parts may be distinguished, a basal (basioccipital), two condylar (oecipitalia lateralia, exoccipitals), an occipitale superius (squama, inferior part), and an interparietal (squama, superior part). The interparietal arises in membranous tissue, the other parts in cartilage.


Fig. 317.—


The basioccipital and the two oecipitalia lateralia arise each from a separate centre of ossification in the ehondrocranium, and at birth are still separated from one another by cartilage (iPig. 317, A, B, C). The centres for the oecipitalia lateralia appear on the 56th day and that for the basioccipital on the 65th (Mall).


The occipitale superius and the interparietal are at birth fused into a single plate of bone.^ The occipitale superius arises from four, the interparietal from two centres of ossification (Mall). according to Mall, the first centres of ossification to appear are two bilaterally placed centres for the occipitale superius which arise in the cartilage immediately dorsal to the foramen magnum (55th-56th day). These two centres soon imite across the midline,^® and are joined by two secondary centres, one of which arises on each side. occasionally an additional unpaired median centre appears on the dorsal margin of the foramen magnum.^ More often, however, there arises a small process, on the inferior margin of the squama in the medial line (Fig. 317, C, 4). This process, later enclosed by bone, gives origin to the crista occipitalis interna (Lengnick, 1898).


The two bilaterally placed interparietal centres appear on the 57th day in the membranous tissue which extends anteriorly from the occipitale superius. They are rectangular in form and unite on the 58th day to form the interparietal bone. The interparietal unites with the occipitale superius in the first half of the third month of intra-uterine life to form the squama of the occipital. Fusion takes place in the median line before it does laterally ; at birth the lateral fusion is usually incomplete. The interparietal may remain partially or wholly separated from the occipitale superius throughout life. In many of the lower manmials the interparietal normally remains distinct from the supraoccipital. according to Ranke, the squama occipitalis arises from four pairs of centres of ossification. Various investigators diifer considerably in the number of centres which they ascribe to this part of the occipital. Anterior to the interparietals a pair of pre-interparietal bones (Fig. 317, B) are apparently not infrequent. The osseous union of the occipitale superius and the occipitalia lateralia begins in the first or second year and is completed in the second to fourth year; that of the basioccipital and the occipitalia lateralia begins in the third or fourth but is not completed until the fifth or sixth year or later. The basioccipital forms the anterior fourth or fifth of the condyles. Some authors describe condylar epiphyses. The basioccipital is united to the basisphenoid by cartilage up to about the twentieth year (16th to 20th, Toldt). Ossific union is completed one or two years later (Quain's Anatomy, 10th ed.). Epiphyseal discs like those which complete the bodies of the vertebrae are described as arising and fusing with the contiguous surfaces of the basisphenoid and basioccipital before the two bones become united by synostosis. At the centre of the synostosis a mass of fibrocartilage frequently long persists. Remnants of the chorda dorsalis may likewise persist here and give rise to tumors. (See Poirier, Virchow, Welcker, Luschka, Steiner.)


" By some authors the bone here called occipitale superius is designated infraoccipital; and the bone here called interparietal is called supra-occipital. {See Poirier, Traite d'Anatomie, vol. i, p. 408-109,)

" according to Toldt, instead of two there may be a single medially placed centre. according to Bolk (1903), the ossification arises in the membranous part of the tectum synoticum. "* Ossiculum Kerckringii, Kerckring, 1670, Manubrium ossis occipitalis, R. Virchow. Ranke showed that it arises in cartilage and membranous tissue. Bolk found in one instance an independent cartilaginous nucleus in this region.

Considerable variation is found at the base of the occipital bone. {See Swjetscbnikow, 1906, p. 155.) Variations of this kind are associated with variations of the atlas.

Os Sphenoidale=

In man the sphenoid bone arises chiefly from ossification centres which appear in the orbitotemporal region of the chondrocranium. To it several hones of membranous origin become fused. In the sphenoid one may distinguish fourteen centres of ossification: four in the basisphenoid, two presphenoid, two alisphenoid, two orbitosphenoid, two pterygoid, and two intertemporal. In


•t Auitomy. lOUi ed^ vol. il. Pt. I. Tig. TS.> Ostification of sphenoid ID early period, seen from above: I, the als temporalea oeeified; 2, the alie ion hu rncireled Ibo optic Fommen. ukd a miall luture is diitiDguishable at 3, nuclei of basisphenoid. B. Back part of the boae shown in A: *. mediaJ . C. ICopied from Heckd, Archiv. vol. i, lab. vi, Fig. 23i, and stated to be 4. Duclai ot piesphenoid; 5, separate lateral proceeaea of the body (UngulKl: ined to th« baaiapbeaoid and the medial plerygoid plalca (not ecen in the addition to these centres there are several in the ossicula Bertini which in part fuse with the presphenoid after birth. (See below.) In each of the greater icings, alisphenoids, a centre of ossifica^ tion appears toward the end of the second month in the chondrocranium between the maxillary and the mandibular nerves. From this centre ossification extends into the lateral lamina of the pterygoid and into the lateral portion of the greater wing (Figs. 322 and 318, A and C). From the main centre a lamella of bone is usually formed about the mandibular branch of the trigeminal ner\'e, thus separating a foramen ovale from the foramen lacerum. according to some authors, there are two centres of ossification in the alisphenoid and external pterygoid which fuse together at an early period (Sappey).


Tn the latter part of the third month (Mall) a bilaterally placed pair of centres appears in the basisphenoid (Fig. 322 and Fig. 318, A). The two centres unite in the fourth month. After this union two other centres arise (sphenotics, Sutton, 1885), give origin to the lingulae, and fuse with the body (Fig. 318, C). The superior margin of the alisphenoid is strengthened by a membranous bone (Hannover, 1880). This bone, called the os intertemporale by Ranke, occasionally persists as an independent structure or may be fused to the squama temporalis or to the frontalis.


The nuclei for the medial pterygoid plates appear in the second month (57th day. Mall) (Fig. 318, B). They fuse with the nuclei of the greater wings in the fourth month. They are said to arise in cartilage, which develops in membranous tissue independent of the chondrocranium (Hannover, Graf v. Spee). according to Fawcett (1905), however, the main part of the medial pterygoid plate is ossified in membrane, although the hamulus is transformed into cartilage before ossification. according to Gaupp, there is questionable propriety in applying the term os pterygoideum to the lamina medialis if thereby one would imply homology with the os pterygoideum of reptiles. One should probably homologize it with the lateral part of the parasphenoid. Each of the lesser wings, orbitosphenoids, is ossified from a centre which appears in the ninth week lateral to the optic foramen (Fig. 318, C). On the medial side of each optic foramen a centre of ossification, presphenoid, appears early in the third month (Fig. j 318, C). The centre for the orbital wing fuses with the corresponding presphenoid centre in the fourth month. The two presphenoid centres fuse with one another in the eighth month. acccording to Hannover (Gaupp, 1906), there are four presphenoid centres. Toldt and Sutton describe but two.


The presphenoidal centres become partially united to the ba sisphenoids in the seventh or eighth month. At birth, however, there is a ventral wedge of cartilage between the two portions of the bone. This does not disappear till late in childhood.^^ The greater wings become joined to the body of the sphenoid during the first year after birth. The base of the great wing spreads out over the side of the body of the sphenoid. Between it and the presphenoid there may be formed a small canalis craniopharyngeus lateralis (Sternberg, 1890). occasionally the hypophyseal canal persists as a canalis craniopharjTigeus medius.


The posterior end of the nasal septum (crista sphenoidalis and rostrum sphenoidale) is ossified by extension of bone from the presphenoid. The concha sphenoidalis (ossiculum Bertini) arises through ossification of the posterior cupola (Kuppel) of the cartilaginous uasal capsule (see p. 416). Ossification begins in the fifth intrauterine month in the medial (paraseptal) wall of the cupola, and in the seventh to eighth month a secondary centre arises in the lateral wall. In the membranous floor of the cupola toward the end of intra-uterine life further centres of ossification arise and fuse with the bone originating in the two primary centres. By the third year each terminal nasal sinus is surrounded by bone except toward the nasal fossa, where an opening called the "sphenoidal foramen" persists. Each bone lies on the inferior surface of the presphenoid, lateral to the crista aphenoidalis and the rostrum sphenoidale, to which it is united by connective tissue. About the fourth year the superior and medial parts of the capsule begin to be absorbed, so that the presphenoid comes to bound the sinus terminalis. Laterally absorption of the bony capsule also takes place while the inferior portion and that surrounding the sphenoidal foramen become fused with the ethmoid. In the ninth to twelfth year, however, this portion fuses with the sphenoid and the sinus terminalis extends into the body of the latter (Gaupp, 1906; Cleland, 1863; Toldt, 1882).


" In many mammals the sphenoid remains permanently divided into two parts, a presphenoid, which comprises the apical end of the body and the lesser wings, and a postsphenoid, which comprises the sella turcica, the great wings, and the pterygoid processes.

Os Ethmoidale

The ethmoid bone arises from one medial and two lateral primary and from several secondary centres in the cartilaginous nasal capsule. The ossification of the posterior cupola of the cartilaginous nasal capsule in man as the ossiculum Bertini has been described in connection with the sphenoid bone. In the quadrupeds this portion of the nasal capsule is ossified in conjunction with the ethmoid (Gaupp, 1906). ub^iUS In each lateral wall of the nasal capsule a centre appears in the fifth to sixth fetal month. It gives rise to the lamina papyracea, and in the eartiiaginoui.i seventh and eighth months ossification „ ^'°^^'>— ('^f"^ ""^'?i ^^ , . . .? , J IV 1 Renault. f torn Poiri«f.Poiner«.dCharpy, extends into the conchse and the lamT™wd'AnBunnie. voi.i. Fig.402.) o»iina eribrosa. The ethmoidal cells are ^('r ."'.■»« of "^^^ '™"' '~^'"' closed off by folds of mucous membrane which arise in the latter half of fetal life and extend between the lamina cribro.sa and the upper concha and between the concha' (Fig. 319). Into these folds of mucous membrane ossification extends from the concbie so as to give rise to osseous walls for the ethmoidal cells.


Ossification begins late in the first year^^ independently in the superior portion of the nasal septum (lamina perpendicularis). It extends into the crista galli, the cribriform plate, and the lamina perpendicularis. Sappey and Poirier, following Rambaud and Renault, describe several centres on each side of the upper margin of the lamina perpendicularis at the base of the crista galli. From these centres ossification extends successively to the crista galli, the lamina cribrosa, and the lamina perpendicularis. In the crista galli in the second year a secondary nucleus arises. Ossification of the process is not completed before the fourth year. In the second year two accessory nuclei appear in the anterior part of the lamina cribrosa. By the sixth year the lateral parts of the ethmoid become united to the medial part (v. Spee and most authors). ^^ Ossification of the ethmoid is not completed until the sixteenth year. Synchondrosis exists between the lamina cribrosa and the sphenoid until toward puberty. About the fortieth to forty-fifth year the lamina perpendicularis becomes united to the vomer.

CONCHA INFERIOR

This arises in cartilage from a separate centre of ossification which appears in the latter half of fetal life (seventh month, Toldt; fifth month, Quain, Graf v. Spee).^*

VOMER

The vomer arises from a bilaterally placed pair of nuclei which appear during the eighth week (Quain, Mali), near the back of the inferior margin of the cartilaginous nasal septum. These centres unite beneath the inferior margin of the septum, but superiorly they extend on each side of the nasal septum so as to enclose the cartilaginous septum between two thin plates of bone. The two plates of bone gradually become coalesced from behind forwards. Union is completed about the age of puberty (Quain). On the anterior and superior margins a permanent groove remains for the attachment to the lamina perpendicularis ossis ethmoidalis and the cartilago septi nasi. Although the vomer develops on each side of the cartilaginous nasal septum and at its expense, it is regarded as a true membrane bone.

according to v. Spee, before birth. The lamina cribrosa ossifies in part through extension of ossification from the crista gnWi and from the lateral ossific centres and in part from accessory centres. according to Sappey, Poirier, Toldt, and some other authors, the central part of the ethmoid becomes united to the lateral parts through ossification in the lamina cribrosa at the end of the first or in the second year. "Third or fourth month after birth (Sappey, Testut, Poirier).


OS PAIiATINUM

The OS palatinum (a membrane bone) arises from a single centre of ossification which appears in the eighth week (Kolliker, Le Double, Mall, Fawcett) in a region corresponding with the angle between the horizontal and vertical parts, or, according to Fawcett, in the region of the vertical plate. Bambaud and Renault, Sappey, Cruveilhier, and others describe two or more centres. The vertical part extends upwards on the medial surface of the lateral wall of the cartilaginous nasal capsule and by this is separated for some time from the maxilla. It extends between the cartilaginous inferior and middle conchae and the cartilaginous lateral wall of the nasal capsule, thus separating the posterior extremities of the former from the latter. The pars horizontalis appears much earlier than the processus orbitalis and the processus sphenoidalis.

OS NASALE

This is a membranous bone which develops on the surface of the cartilaginous nasal capsule. The underlying cartilage is still present at birth, but subsequently becomes absorbed. It is usually stated that there is a single centre of ossification which appears at the end of the second month. Zuckerkandl (1895) has suggested that the anomalies of development shown by the nasal bone indicate that it may arise at times from two or even three centres of ossification. according to Perna (1906), the nasal bone arises from two anlages, a lateral membranous and a small medial cartilaginous. Remnants of the suture between the two may exist as an incisura nasalis.

OS LACRYMALE

This is a membrane bone which arises on the lateral wall of the posterior part of the cartilaginous capsule of the external nose. The centre of ossification appears in the third month (83d day. Mall). The facies lacrjinalis ossifies first, then the crista and hamulus, and lastly the facies orbitalis. In the adult the bone varies greatly in form (see MacAUister, v. Spee, Zabel, Le Double, etc.). occasionally the bone in the adult is bipartite, indicating an accessory centre of ossification.

OS TEMPORAL.E

The human temporal bone is the result of the fusion of several distinct elements, petrosal (periotic), squamosal, tympanic, tympanohyale, and stylohyale. In addition it encloses the three bones of the middle ear. At birth it consists of three pieces, a squamosum, a petrosmn and a tympanieum. These become fused together during the first year. The squamosum (Figs. 320, 321) is ossified from a single centre of ossification which appears in the membranous tissue in the region near the base of the zygomatic process of the squama {Mall, 1906).** From the posterior part of the squamosum a post-auditory process grows downwards beneath the region of the linea temporalis between the tympanic and petrosal portions of the bone. It forms the superior anterior part of the mastoid. The squamosum encloses laterally the cavnm tympani and the antrum mastoideum, from which the mastoid cells subseqaentljdevelop."


+++++++++++++++++++++++++++++ Fig. 3^0.— <AIUr Ssppey. Tni


+++++++++++++++++++++++++++++

The tympanieum (Figs, 320, 321, 324) is a membrane bone. Its centre of ossification appears toward the end of the third month in the anterolateral part of the external membranous wall of the cavum tympani, near the angle between the capitulum of the malleus and Meckel's cartilage. It has a concave surface turned toward the latter. From this centre a band of bone grows first downwards, medialwards, and backwards and then upwards and lateralwards so as to form a semicircular bone surrounding the tj-mpanic membrane. In the tenth fetal month first the free ends of the bone fuse with the squamosum and then the under part fuses with the petrosum. By addition of osseus tissue to the lateral " according to Rambaud and Renault, Toldt, Poirier, and others, tbe squamosimi arises from three centres of ossification, one at the base of the zygomatic process, one in the squamosa, above this, and one behind the tympanieum. AmODg the variations found in the squamosal portion of the temporal bone are a division iiito a superior and an inferior part or into an anterior and a. posterior part.


"Fiichs (li)05-l£)07), chiefly from the study of rabbit embryos, has come to the conclusion that the squamosum of mammals is composed of three parts, a I and a zygomatic (quadrato-jupale) part, each ossitied in membrane, articular part prefomjed in cartilajre (pars artieularis quadrati).


and medial margins of the bone, the primary narrow band becomes converted by the third year into a broad rolled-up plate of bone, which medially forms the ventral wall of the cavnm tympani and laterally the ventral wall of the meatus acusticus estemus. At this period the inferior surface of the tympanicum presents an aperture of some size which usually but not always becomes closed. according to Rambaud and Renault, Hammar, and others, the bone arises from several centres of ossification.


ahown. The pi


Tympanohyale (laterohyale) and stylohyale. See p. 439. The periotic portion, os petrosum {Fig. 322), arises from the ossification of the cartilaginous otic capsule. There are several centres of ossification, these centres arise during the fifth month and become fused with one another in the sixth. The descriptions of the centres of ossification in the labyrinth given by various authors differ considerably. That of Vrolik, as adopted by Gaupp (1906), is here chiefly followed. The first centre to appear is one in the region of the promontory between the fenestra vestibuli and the fenestra cochleae in a fetus 17 cm. long. Ossification extends Vol. I.— 28

from here around the fenestra vestibuU and forms that part of the petrous bone which lies below the poms aousticus iDtemus and the fenestra cochlearis. A second centre appears on the dorsolateral surface of the central paf t of the capsule over the superior semicircular canal. Ossification extends from this into the region of the processus perioticus superior. It forms most of the cranial surface of the petrous bone, gives rise to the superior


boundaries of the internal auditory meatus and the fenestra cochlearis, and also to a portion of the superior medial part of the mastoid. A third centre arises at the anterior proximal part of the cochlea near the incisura pro-otica. More caudally, medial to the fossa subarcuata, there arises a fourth centre of ossification. The fifUi centre appears on the outer surface of the posterior part of the capsule in the region of the posterior semicircular canal ; the sixth centre arises slightly in front of the fifth. From the last two centres ossification extends into the parietal plate and the pars mastoidea of the tectum posterius. At the end of the sixth month the labyrinth is completely enclosed by bone.

The tegmen tympani is ossified partly in membrane, partly in the cartilage of the processus perioticus superior by extension from the periotic capsule. There is occasionally found in man a separate bone in the anterior part of the tegmen tympani. This perhaps represents the ossiculum accessorium malleoli, which in some mammals has an independent centre of origin on the upper side of the proximal end of Meckel's cartilage (van Kampen, 1905). The embryonic epithelial labyrinth at first lies in a cavity surrounded by cartilage. The inner surface of this cartilage becomes transformed into membranous tissue, and this in turn in part furnishes a membranous covering for the labyrinth, and in part becomes ossified, forming the inner lining of the bony labyrinth, including the modiolus, the lamina modioli, and the lamina spiralis ossea. (See Kolliker, 1879; Bottcher, 1869.) The canalis caroticus is represented by a slight groove in. the cartilaginous skull. It becomes converted into a canal during the period of ossification. At birth the central part only is roofed over. Between the apex pyramidis and the sphenoid a part of the chondrocranium persists as the fibrocartilago basalis, which lies in the foramen lacerum. Formation of mastoid cells does not begin until after birth, but in the second year they extend from the antrum into the mastoid process. The Canalis Facialis. — The short facial canal in the chondrocranium is equivalent merely to the first part of the facial canal in the adult (part from the porus acusticus int. to the region of the geniculate ganglion) . Before the chondrocranium is replaced by bone the nerve passes out from the canal above mentioned, then beneath the crista parotica and over the stapes, and thence backwards and downwards beneath the tympanohyale, and then outwards and ventralwards toward the surface of the body (Fig. 311). The nervus petrosus sup. major leaves the main trunk near the lateral orifice of the cartilaginous canaP® and runs forward on the lateral wall of the capsule. The chorda tympani separates from the main trunk behind the stylohyale, runs forwards lateral to this cartilage and then between the malleus and incus. It is not enclosed in a canal in the chondrocranium. When the lateral wall of the auditory capsule becomes ossified, the facial nerve is enclosed at first in a groove and later in a canal. While the facial nerve is being enclosed bony lamellae likewise enclose the stapedius muscle and the chorda tympani. The facial nerve and Eeichert's cartilage may be looked upon as caught between the tympanicum and the mastoid part of the petrosum, the chorda tympani and Meckel's cartilage as caught between the tympanicum and the squamosum, the tuba auditiva (Eustachii) as enclosed between the tympanicum and the pars cochlearis of the petrosum (van Kampen).


" In the human fetus and in Talpa through a special opening in the external orifice of the facial canal (E. Fischer).


OS PAKIETAXiE

The parietal arises as a membrane bone from two centres (Toldt), a superior and an inferior, which soon fuse into a single centre which lies in the region of the tuber parietale. The centres appear toward the end of the second month and apparently sometimes arise as a single centre. The ossification radiates outwards from the combined centre of ossification toward the sphenoidal and occipital angles, so as to give rise for a time to an hourglassshaped plate of bone (Mall). At a later period a notch is left in the sagittal margin of the bone in front of the occipital angle. The notches of the bones of the opposite sides together form the sagittal fontanelle, which toward the end of fetal life usually becomes closed, but may sometimes be recognized after birth.

OS FRONTALE

This has two centres of ossification, one on each side of the body in the region of the tuber frontale (Fig. 321). These centres arise toward the end of the second month (56th day. Mall). From each a lateral half of the bone is formed. The orbital part of the bone appears in the ninth week. Toldt found no secondary centres, but these have been described by Rambaud and Renault, Serres, Jhering, v. Spee, and others. These accessory centres include, on each side of the bone, one for the spina trochlearis, one for the processus zygomaticus, one for the posterior part of the orbital region, and one which arises late in the spina frontalis lateral to the foramen caecum. The two lateral halves of the bone are separated at birth, but during the first year become approximated in the midsagittal line. The middle part of the frontal suture becomes ossified in the second year. By the eighth year the suture is usually obliterated except inferiorly. Near the root of the nose the frontal suture is sometimes widened to form a fontanella metopica, in which an os metopicum may be formed. Ossicles may appear also in other parts of the frontal suture (Schwalbe, 1901; Fischer, 1901). The frontal sinuses begin to be developed early (first year, Toldt), but develop very slowly until toward puberty. They increase in size until late in life.

FONTANEU^S

The chief fontanelles are spaces covered by membranes which lie between the incomplete angles of the parietal and the neighboring bones.^^ The anterior fontanelle (fonticulus frontalis) is situated between the frontal angles of the parietal and the posterosuperior angles of the two parts of the frontal bone. It remains open until the third year. The posterior fontanelle (fonticulus occipitalis) is situated between the occipital angles of the parietal bones and the superior angle of the occipital. This fontanelle at birth is nearly obliterated, but the bones which bound it are still separated by membrane and are movable. It becomes closed between the third and sixth month. The lateral fontanelles (fonticulus mastoideus, fonticulus sphenoidalis) are situated between the sphenoidal and mastoidal angles of the parietal and the neighboring bones. The fonticulus mastoideus closes during the first half of the second year, the fonticulus sphenoidalis in the third year. The flat bones of the skull are united by definitive sutures by the end of the fourth year.^^ Special small bones (Wormian bones) may develop in the fontanelles during the process of ossification.

MAXILLA

The human maxilla consists of two distinct parts, a medial, the incisivum (premaxilla) of lower forms, and a lateral, or maxilla proper. according to Mall (1906), each of these parts of the bone ossifies from a single centre. The two centres appear at the end of the sixth week and become united at the end of the second or early in the third month. Each of the centres gives rise to a part of the frontal process. The alveolar borders of the two bones unite before the frontal processes do. Authors differ greatly in the number of centres which they ascribe to the bone. The number given varies from two to six. Hertwig's model and Schultze's illustration of the bones of the skull of an embryo of the third month are cited by Mall in support of his own observations.

At first the maxilla lies lateral to the cartilaginous nasal capsule. Aiter this cartilaginous capsule is in large part absorbed the maxilla helps to bound the nasal cavity, the maxilloturbinale becomes joined to it, some of the ethmoid cells are closed off by it, and the sinus maxillaris is formed. The formation of the alveolar process begins in the fourth fetal month and is completed after the twentieth year. according to Mihalkovics (1899), the proc. paranasalis of the nasal capsule is caught up in the ossifying maxilla. This is said to account for the islands of cartilage sometimes described in the ossifying maxilla.'*® The infraorbital nerve and vessels lie at first in a groove on the orbital surface of the maxilla, but later become enclosed by a lamina of bone which extends upwards on the lateral side, and then bends medialwards (see Fig. 321). The lateral part of the floor of the orbit and the infraorbital nerve lie at first very near the alveolar process of the maxilla. The development of the sinus maxillaris gradually serves to separate them. Compare the maxilla shown in Fig. 321 with that of an adult skull. The sinus maxillaris at birth is represented by a slight depression on the medial surface of the maxilla opposite the second molar. After birth this depression extends laterally into the maxilla beneath the groove of the infraorbital nerve and bloodvessels. After the second dentition the sinus becomes greatly enlarged.


+++++++++++++++++++++++++++++ Fig. 323.— (After Toldt, Anat. Atlas, 1900, Heft 1, Figs. 176. 177.) The left maxilla of a fetus 30 cm. long. A. Medial surface. B. Inferior surface.

+++++++++++++++++++++++++++++


"For an a<?count of the transitory sagittal fontanelle between the parietal bones see above under Os parietale. For the metopic fontanelle see above under Os frontale. " For the time and order of closure of the chief fontanelles see Adachi (1900).


OS ZYGOMATICUM

This appears on the 56th day as a small, three-cornered centre in the membranous tissue beneath and lateral to the eye (Fig. 321). On the 58th day it is four-cornered. Two of the comers give rise to processes partially encircling the orbit, one of the others extends to the maxilla, and one toward the temporal bone (Mall, 1906). From this period on, the adult form of the bone is steadily approached.** "according to Le Double (1906), the number of centres described by various authors for the maxilla, including: the incisivum, is as follows : One, Camper, Rousseau, Cleland; two, Jamain ; three, Serres, Meckel, Cruveilhier; five, B6clard, Sappey, Leidy, Poirier, W. Krause; five or six, Portal; six, Rambaud and Renault; seven, Weber.

    • In fetuses of the fourth to fifth month there may arise in the alveolar part small cartilaginous islands which have no connection with the nasal capsule and which disappear during the ossification of the upper jaw (Gaupp, 1906).
  • ^In the adult partially or completely bipartite and tripartite malar bones are not very infrequent. In the Japanese and Ainos there is found a considerable percentage of skulls in which the malar bone is partially or completely divided by a horizontal suture (os Japanicum, os Ainoicum). Various writers differ greatly


AUDITORY OSSICLES

These begin to ossify during the last half of the fifth month. The malleus has a centre of ossification from which all parts of the bone arise except the processus anterior. This centre arises in the upper part of the coUum and from here ossification spreads to the other parts. The manubrium, the last part of the bone to become ossified, reaches its definitive form before birth. The processus anterior (Folii) arises at the end of the second month as a slender membrane bone on the medial side of Meckel's cartilage. It reaches its definite length in the middle of the sixth month. The proximal end fuses with the collum mallei toward the end of the fifth month (Broman), at the time of ossification of the latter. When the malleus is ossified it becomes clearly demarcated from the rest of Meckel's cartilage. The latter slowly atrophies, and is replaced by connective tissue, which first appears at its periphery. The incus is ossified from a single centre which appears in the upper part of the crus longum. Ossification extends from here into the other parts of the bone, including the processus lenticularis. The ossification begins in fetuses 19-20 cm. long and by the time of birth has reached its definitive extension (Broman). In the stapes, a centre of ossification usually appears in the basal portion in fetuses 21 cm. long (Broman). From this centre the bone is ossified. The capitulum is usually ossified by the end of the sixth month.

TYMPANOHYALE

The tympanohyale is probably derived from the cartilaginous tympanohyale described in connection with the chondrocranium, although tympanohyale and stylohyale are fused before ossification appears. Late in fetal life it is ossified from a special centre and becomes included between and fused to the petrosum and the tympanicum. It helps to bound the tympanic cavity medial to the OS tympanicum. in the number of centres of ossification which they ascribe to the bone. Le Double (1906) classified these writers as follows: Those describing one centre of ossification: Meckel, Beclard, Hyrtl, Sappey, Cruveilhier, Jamain, Leidy, Baraldi, Lachi, Romiti, Langer-Toldt, Stieda, Merkel, Graf V. Spee, Hartmann. Those describing one or two centres: Parker and Bettany. Those describing two centres : Kerckring, Lieutaud, Garbiglietti, Macalister. Those describing two or three centres : Morris and Rauber. Those describing one, occasionally two, rarely three : Breschet, Gruber. Those describing one, occasionally three : Virchow, Albrecht, Testut, Thane. Those describing three centres : 0. Schultze, Kollmann, Frassetto, Minot, Spix, Calori, Quain, Rambaud and Renault, Schrenck, Poirier, Kolliker, C. Toldt, Le Double.

STYLOHYALE

The cartilaginous stylohyale gives rise to the styloid process. This ossifies after birth and is usually united to the tympanohyale by cartilage until middle age when it may become united to the latter by bone (Flower, 1870). KERATOHYALE.

This is derived from the proximal part of Reichert's cartilage. It gives rise to the stylohyoid ligament. occasionally it may become ossified and fused to the stylohyale and the lesser comu of the hyoid. OS HYOIDEUM.

This has five primary centres of ossification, — one for the body and one for each of the greater and each of the lesser comua. Ossification begins in the body and greater comua late in fetal life ; in the lesser comua some time after birth. The greater cornua and body unite in. middle life. The lesser comua are united by bone to the body of the hyoid only rarely. Usually a cartilaginous union remains throughout life. according to Eambaud and Renault, the centre in the body arises by fusion of two bilaterally placed centres. After puberty secondary centres for the tips of the greater comua are described (Poirier, Traite d'Anatomie).

MANDIBLE

This is ossified in the membranous tissue lateral to Meckel's cartilage. The centre of ossification may appear as early as the 39th day. By the 42d day the ramus and alveolar process may be distinguished. In a fetus 55 days old the beginnings of the coronoid process and condyle are visible. Before the end of the second month sockets for the teeth may be distinguished. By the middle of the third month the mandible has reached its characteristic shape (Mall, 1906). ^^ During the development of the mandible cartilage is produced in the membranous tissue of the tip of the condyle, in the angulus, the proc. coronoideus (see Fawcett, 1904; Low, 1905), and, according to Henneberg, in traces also on the superior lateral and the medial alveolar margins and the inferior lateral margin of the jaw. This cartilage* has nothing to do with Meckel's cartilage. From Meckel's cartilage, however, there may be derived the cartilage in the symphysis of the jaw in which small bones, ossicula mentalia, develop in the eighth month. There are usually two pairs, or one unpaired bone and one pair of bones. They lie in the lower portion of the symphysis and after birth become fused to the mandible and help form the protuberance of the chin; see v. Mies (1893), Toldt (1905-6), V. Bardeleben (1905). according to Low (1905), Meckel's cartilage near the incisors becomes ossified and fused to the mandible. according to Fawcett (1904), Meckel's cartilage becomes ossified from the foramen mentale to the median line. The ossified cartilage becomes enclosed in membrane bone. At birth the lower jaw usually consists of lateral parts united at the symphysis by fibrous tissue. Osseous union takes place in the first or second year after birth.


" Several investigrators, among whom may be mentioned Rambaud and Renault and Wolff (1888), describe several centres of ossification for the mandible.


Hutubrium dullei Fici, 324,— (A(t«r Koll


TEMPOBOMANDIBULAB JOINT

This joint is developed between the membrane which covers the condyle of the mandible and the periosteum of the squamosum. In the loose tissue between the two a condensation marks the beginning of the differentiation of the discus articularis. On each side of this discus a joint cavity develops. Each joint cavity is throughout life lined by fibrous tissue. Beneath the joint periosteum of the mandible and of the temporal bone a thin layer of cartilage is produced (see Kjellberg, 1904). according to Walliseh (1906), in the new-bom the tubereulum articulare is still undeveloped and the condyle is flatter than in the adult. The condyle reaches its definitive form and the tubereulum is developed after the teeth appear.*^ "according to Fuebs (1905), tbe temparomandibular joint in rabbits, and hence by inference in other mammalB, is homolt^^us witb the quadra to-articular joint of reptiles. As mentioned above, following Reiehert, most investigators have eome to the conclusion that the reptilian quadra to-articular joint is represented in mammals by tbe joint between the malleus and incus, while the temporomandibular joint of mammals is phylogenetieally a new structure, a squamosodental joint. (See Gaupp, 1906.)


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