McMurrich1914 Chapter 7
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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Chapter VII. The Development of the Connective Tissues and Skeleton
It has been seen that the cells of a very considerable portion of the somatic and splanchnic mesoderm, as well as of parts of the mesodermic somites, become converted into mesenchyme. A very considerable portion of this becomes converted into what are termed connective or supporting tissues, characterized by consisting of a non-cellular matrix in which more or less scattered cells are embedded. These tissues enter to a greater or less extent into the formation of all the organs of the body, with the exception of those forming the central nervous system, and constitute a network which holds together and supports the elements of which the organs are composed; in addition, they take the form of definite membranes (serous membranes, fasciae), cords (tendons, ligaments), or solid masses (cartilage), or form looser masses or layers of a somewhat spongy texture (areolar tissue). The intermediate substance is somewhat varied in character, being composed sometimes of white, non-branching, non-elastic fibers, sometimes of yellow, branching, elastic fibers, of white, branching, but inelastic fibers which form a reticulum, or of a soft gelatinous substance containing considerable quantities of mucin, as in the tissue which constitutes the Whartonian jelly of the umbilical cord. Again, in cartilage the matrix is compact and homogeneous, or, in other cases, more or less fibrous, passing over into ordinary fibrous tissue, and, finally, in bone the organic matrix is largely impregnated with salts of lime.
Two views exist as to the mode of formation of the matrix, some authors maintaining that in the fibrous tissues it is produced by the actual transformation of the mesenchyme cells into fibers, while others claim that it is manufactured by the cells but does not directly represent the cells themselves. Fibrils and material out of which fibrils could be formed have undoubtedly been observed in connective-tissue cells, but whether or not these are later passed to the exterior of the cell to form a connective-tissue fiber is not yet certain, and on this hangs mainly the difference between the theories. Recently it has been held (Mall) that the mesenchyme of the embryo is really a syncytium in and from the protoplasm of which the matrix forms; if this be correct, the distinction which the older views make between the intercellular and intracellular origin of the matrix becomes of little importance.
Fig. 91. - Portion of the Center of Ossification of the Parietal Bone of a Human Embryo.
Bone differs from the other varieties of connective tissue in that it is never a primary formation, but is always developed either in fibrous tissue or cartilage; and according as it is associated with the one or the other, it is spoken of as membrane bone or cartilage bone. In the development of membrane bone some of the connective-tissue cells, which in consequence become known as osteoblasts, deposit lime salts in the matrix in the form of bony spicules which increase in size and soon unite to form a network (Fig. 91). The trabecular of the network continue to thicken, while, at the same time, the formation of spicules extends further out into the connective-tissue membrane, radiating in all directions from the region in which it first developed. Later the connective tissue which lies upon either surface of the reticular plate of bone thus produced condenses to form a stout membrane, the periosteum, between which and the osseous plate osteoblasts arrange themselves in a more or less definite layer and deposit upon the surface of the plate a lamella of compact bone. A membrane bone, such as one of the flat bones of the skull, thus comes to be composed of two plates of compact bone, the inner and outer tables, enclosing and united to a middle plate of spongy bone which constitutes the diploe.
With bones formed from cartilage the process is somewhat different. In the center of the cartilage the intercellular matrix becomes increased so that the cells appear to be more scattered and a calcareous deposit forms in it. All around this region of calcification the cells arrange themselves in rows (Fig. 92) and the process of calcification extends into the trabecular of matrix which separate these rows. While these processes have been taking place the mesenchyme surrounding the cartilage has become converted into a periosteum (po), similar to that of membrane bone, and its osteoblasts deposit a layer of bone (p) upon the surface of the cartilage. The cartilage cells now disappear from the intervals between the trabeculae of calcified matrix, which form a fine network into which masses of mesenchyme (Fig. 93, pi), containing blood-vessels and osteoblasts, here and there penetrate from the periosteum, after having broken through the layer of periosteal bone. These masses absorb a portion of the fine calcified network and so transform it into a coarse network, the meshes of which they occupy to form the bone maigow (m), and the osteoblasts which they contain arrange themselves on the surface of the persisting trabeculse and deposit layers of bone upon their surfaces. In the meantime the calcification of the cartilage matrix has been extending, and as fast as the network of calcified trabeculse is formed it is invaded by the mesenchyme, until finally the cartilage becomes entirely converted into a mass of spongy bone enclosed within a layer of more compact periosteal bone.
Fig 92. - Longitudinal section of Phalanx of a Finger of an Embryo of 3 1/2 Months.
c, Cartilage trabeculae; p, periosteal bone; po, periosteum; x, ossification center. - (Szymonowicz.)
As a rule, each cartilage bone is developed from a single center of ossification, and when it is found that a bone of the skull, for instance, develops by several centers, it is to be regarded as formed by the fusion of several primarily distinct bones, a conclusion which may generally be confirmed by a comparison of the bone in question with its homologues in the lower vertebrates. Exceptions to this rule occur in bones situated in the median line of the body, these occasionally developing from two centers lying one on either side of the median line, but such centers are usually to be regarded as a double center rather than as two distinct centers, and are merely an expression of the fundamental bilaterality which exists even in median structures.
More striking exceptions are to be found in the long bones in which one or both extremities develop from special centers which give rise to the epiphyses (Fig. 94, ep, ep'), the shaft or diaphysis (d) being formed from the primary center. Similar secondary centers appear in marked prominences on bones to which powerful muscles
Fig. 93. - The Ossification Center of Fig. 92 More Highly Magnified. c, Ossifying trabeculse; cc, cavity of cartilage network; m, marrow cells; p, periosteal bone; pi, irruption of periosteal tissue; po, periosteum. - (Szymonowicz.)
are attached (Fig. 94, a and b), but these, as well as the epiphysial centers, can readily be recognized as secondary from the fact that they do not appear until much later than the primary centers of the bones to which they belong. These secondary centers give the necessary firmness required for articular surfaces and for the attachment of muscles and, at the same time, make provision for the growth in length of the bone, since a plate of cartilage always intervenes between the epiphyses and the diaphysis. This cartilage continues to be transformed into bone on both its surfaces by the extension of both the epiphysial and diaphysial ossification into it, and, at the same time, it grows in thickness with equal rapidity until the bone reaches its required length, whereupon the rapidity of the growth of the cartilage diminishes and it gradually becomes completely ossified, uniting together the epiphysis and diaphysis.
The growth in thickness of the long bones is, however, an entirely different process, and is due to the formation of new layers of periosteal bone on the outside of those already present. But in connection with this process' an absorption of bone also takes place. A section through the middle of the shaft of a humerus, for example, at an early stage of development would show a peripheral zone of compact bone surrounding a core of spongy bone, the meshes of the latter being occupied by the marrow tissue. A similar section of an adult bone, on the other hand, would show only the peripheral compact bone, much thicker than before and enclosing a large marrow cavity in which no trace of spongy bone might remain. The difference depends on the fact that as the periosteal bone increases in thickness, there is a gradual absorption of the spongy bone and also of the earlier layers of periosteal bone, this absorption being carried on by large multinucleated cells, termed osteoclasts, derived from the marrow mesenchyme. By their action the bone is enabled to reach its requisite diameter and strength, without becoming an almost solid and unwieldy mass of compact bone.
Fig. 94. - The Ossification Centers of the Femur.
a, and b, Secondary centers for the great and lesser trochanters; d, diaphysis; ep, upper and ep', lower epiphysis. - (Testut.)
During the ossification of the cartilaginous trabeculse osteoblasts become enclosed by the bony substance, the cavities in which they lie forming the lacuna and processes radiating out from them, the canaliculi, so characteristic of bone tissue. In the growth of periosteal bone not only do osteoblasts become enclosed, but bloodvessels also, the Haversian canals being formed in this way, and around these lamellae of bone are deposited by the enclosed osteoblasts to form Haversian systems.
Fig. 95. - A, Transverse Section of the Femur of a Pig Killed after Having Been fed with Madder for Four Weeks; B, the Same of a Pig Killed Two Months after the Cessation of the Madder Feeding. The heavy black line represents the portion of bone stained by the madder. - (After Flourens.)
That the absorption of periosteal bone takes place during growth can be demonstrated by taking advantage of the fact that the coloring substance madder, when consumed with food, tinges the bone being formed at the time a distinct red. In pigs fed with madder for a time and then killed a section of the femur shows a superficial band of red bone (Fig. 95, A), but if the animals be allowed to live for one or two months after the cessation of the madder feeding, the red band will be found to be covered by a layer of white bone varying in thickness according to the interval elapsed since the cessation of feeding (Fig. 95, B); and if this interval amount to four months, it will be found that the thickness of the uncolored bone between the red bone and the marrow cavity will have greatly diminished (Flourens).
The Development of the Skeleton
Embryologically considered, the skeleton is composed of two portions, the axial skeleton, consisting of the skull, the vertebrae, ribs, and sternum, developing from the sclerotomes of the mesodermal somites, and the appendicular skeleton, which includes the pectoral and pelvic girdles and the bones of the limbs, and which arises from the mesenchyme of the somatic mesoderm. It will be convenient to consider first the development of the axial skeleton, and of this the differentiation of the vertebral column and ribs may first be discussed.
Fig. 96. - Frontal Section through Mesodermic Somites of a Calf Embryo.
isa, Intersegmental artery; my, myotome; n, central nervous system; nc, notochord; sea and scp, anterior and posterior portions of sclerotomes.
The Development of the Vertebrae and Ribs
The mesenchyme formed from the sclerotome of each mesodermic somite grows inward toward the median line and forms a mass lying between the notochord and the myotome, separated from the similar mass in front and behind by some loose tissue in which lies an intersegmental artery. Toward the end of the third week of development the cells of the posterior portion of each sclerotome condense to a tissue more compact than that of the anterior portion (Fig. 96), and a little later the two portions become separated by a cleft. At about the same time the posterior portion sends a process medially, to enclose the notochord by uniting with a corresponding process from the sclerotome of the other side, and it also sends a prolongation dorsally between the myotome and the spinal cord to form the vertebral arch, and a third process laterally and ventrally along the distal border of the myotome to form a costal process (Fig. 97). The looser tissue of the anterior half of the sclerotome also grows medially to surround the notochord, filling up the intervals between successive denser portions, and it forms too a membrane extending between successive vertebral arches. Later the tissue surrounding the notochord, which is derived from the anterior half of the sclerotome, associates itself with the posterior portion of the preceding sclerotome to form what will later be a vertebra, the tissue occupying and adjacent to the line of division between the anterior and posterior portions of the sclerotomes condensing to form intervertebral fibrocartilages. Consequently each vertebra is formed by parts from two sclerotomes, the original intersegmental artery passes over the body of a vertebra, and the vertebrae themselves alternate with the myotomes. With this differentiation the first or blastemic stage of the development of the vertebras closes.
Fig. 97. - Transverse Section through the intervertebral plate of the First Cervical Vertebra of a Calf Embryo of 8.8 mm.
be 1 , Intervertebral plate; m i , fourth myotome; s, hypochordal bar; XI, spinal accessory nerve. - (Froriep.)
In the second or cartilaginous stage, portions of the sclerotomic mesenchyme become transformed into cartilage. In the posterior portion of each vertebral body, that is to say in the portion formed from the anterior halves of the more posterior of the two pairs of sclerotomes entering into its formation, two centers of chondrification appear, one on each side of the median line, and these eventually unite to form a single cartilaginous body, the chondrification probably also extending to some extent into the denser anterior portion of the body. A center also appears in each half of the vertebral arch and in each costal process, the cartilages formed in the costal processes of the anterior cervical region uniting across the median line below the notochord, to form what has been termed a hypochordal bar (Figs. 97 and 98). These bars are for the most part but transitory, recalling structures occurring in the lower vertebrates; in the mammalia they degenerate before the close of the cartilaginous stage of development, except in the case of the atlas, whose development will be described later. As development proceeds the cartilages of the vertebral arches and costal processes increase in length and come into contact with the cartilaginous bodies, with which they eventually fuse, and from the vertebral arches processes grow out which represent the future transverse and articular processes.
The fusion of the cartilage of the costal process with the body of the vertebra does not, however, persist. Later a solution of the junction occurs and the process becomes a rib cartilage, the mesenchyme surrounding the area of solution forming the costo-vertebral ligaments. At first the rib cartilage is separated by a distinct interval from the transverse process of the vertebral arch, but later it develops a process, the tubercle, which bridges the gap and forms an articulation with the transverse process.
The mesenchyme which extends between successive vertebral arches does not chondrify, but later becomes transformed into the interspinous ligaments and the ligamenta ftava, while the anterior and posterior longitudinal ligaments are formed from unchondrified portions of the tissue surrounding the vertebral bodies.
As was pointed out, the mesenchyme in the region of the cleft separating the anterior and posterior portions of a sclerotome becomes an intervertebral fibrocartilage, and, as the cartilaginous bodies develop, the portions of the notochord enclosed by them become constricted, while at the same time the portions in the intervertebral regions increase in size. Finally the notochord disappears from the vertebral regions, although a canal, representing its former position, traverses each body for a considerable time, but in the intervertebral regions it persists as relatively large flat disks forming the pulpy nuclei of the fibrocartilages.
The mode of development described above applies to the great majority of the vertebrae, but some departures from it occur, and these may be conveniently considered before passing on to an account of the ossification of the cartilages. The variations affect principally the extremes of the series. Thus the posterior vertebrae present a reduction of the vertebral arches, those of the last sacral vertebrae being but feebly developed, while in the coccygeal vertebrae they are indicated only in the first. In the first cervical vertebra, the atlas, the reverse is the case, for the entire adult vertebra is formed from the posterior portion of a sclerotome, its lateral masses and posterior arch being the vertebral arches, while its anterior arch is the hypochordal bar, which persists in this vertebra only. A welldeveloped centrum is also formed, however (Fig. 98), but it does not unite with the parts derived from the preceding sclerotome, but during its ossification unites with the centrum of the epistropheus (axis), forming the odontoid process of that vertebra. The epistropheus consequently is formed by one and a half sclerotomes, while but half a one constitutes the atlas.
The extent to which the ribs are developed in connection with the various vertebrae also varies considerably. Throughout the cervical region they are short, the upper five or six being no longer than the transverse processes, with the tips of which their extremities unite at an early stage. In the upper five or six vertebrae a relatively large interval persists between the rib and the transverse process, forming the costo-transverse foramen, through which the vertebral vessels pass, but in the seventh vertebra the fusion is more extensive and the foramen is very small and hardly noticeable. In the thoracic region the ribs reach their greatest development, the upper eight or
Fig. 9,8. - Longitudinal Section through the Occipital Region and Upper Cervical Vertebrae of a Calf Embryo of 18.5 mm.
has, Basilar artery; ch, notochord; Kc l ~ 4 , vertebral centra; lc 2 ~ 4 , intervertebral disks; occ, basioccipital; Sc x ~ 4 , hypochordal bars. - (Froriep.)
nine extending almost to the mid-ventral line, where their extremities unite to form a longitudinal cartilaginous bar from which the sternum develops (see p. 166). The lower three or four thoracic ribs are successively shorter, however, and lead to the condition found in the lumbar vertebras, where they are again greatly reduced and firmly united with the transverse processes, the union being so close that only the tips of the latter can be distinguished, forming what are known as the accessory tubercles. In the sacral region the ribs are reduced to short flat plates, which unite together to form the lateral masses of the sacrum, and, finally, in the coccygeal region the blastemic costal processes of the first vertebra unite with the transverse processes to form the transverse processes of the adult vertebra, but no indications of them are to be found in the other vertebrae beyond the blastemic stage.
The third stage in the development of the axial skeleton begins with the ossification of the cartilages, and in each vertebra there are typically as many primary centers of ossification as there were originally cartilages, except that but a single center appears in the body. Thus, to take a thoracic vertebra as a type, a center appears in each half of each vertebral arch at the base of the transverse process and gradually extends to form the bony lamina, pedicle, and the greater portion of the transverse and spinous processes; a single center gives rise to the body of the vertebra; and each rib ossifies from a single center. These various centers appear early in embryonic life, but the complete transformation of the cartilages into bone does not occur until some time after birth, each vertebra at that period consisting of three parts, a body and two halves of an arch, separated by unossified cartilage (Fig. 99, A). At about puberty secondary centers make their appearance; one appears in the cartilage which still covers the anterior and posterior surfaces of the vertebral body, producing disks of bone in these situations (Fig. 99, B, en and el), another appears at the tip of each spinous and transverse process (Fig. 99, B), and in the lumbar vertebrae others appear at the tips of the articulating processes. The epiphyses so formed remain separate until growth is completed and between the sixteenth and twenty-fifth years unite with the bones formed from the primary centers, which have fused by this time, to form a single vertebra.
Fig. 99. - A, A Vertebra at Birth; B, Lumbar Vertebra showing Secondary Centers of Ossification. a, Center for the articular process; c, body; el, lower epiphysial plate; en, upper epiphysial plate; na, vertebral arch; s, center for spinous process; t, center for transverse process. - (Sappey.)
Each rib ossifies from a single primary center situated near the angle, secondary centers appearing for the capitulum and tuberosity.
In some of the vertebras modifications of this typical mode of ossification occur. Thus, in the upper five cervical vertebrae the centers for the rudimentary ribs are suppressed, ossification extending into them from the vertebral arch centers, and a similar suppression of the costal centers occurs in the lower lumbar vertebrae, the first only developing a separate rib center. Furthermore, in the atlas a double center appears in the persisting hypochordal bar, and the body which corresponds to the atlas, after developing the terminal epiphysial disks, fuses with the body of the epistropheus (axis) to form its odontoid process, this vertebra consequently possessing, in addition to the typical centers, one (double) other primary and two secondary centers. In the sacral region the typical centers appear in all five vertebrae, with the exception of rib centers for the last one or two (Fig. ioo) and two additional secondary centers give rise to plate-like epiphyses on each side, the upper plates forming the articular surface for the ilium. At about the twenty-fifth year all the sacral vertebrae unite to form a single bone, and a similar fusion occurs also in the rudimentary vertebrae of the coccyx.
Fig. 100. - A, Upper Surface of the First Sacral Vertebra, and B, Ventral View of the Sacrum showing Primary Centers of Ossification.c, Body; na, vertebral arch; r, rib center. - (Sappey.)
The majority of the anomalies seen in the vertebral column are due to the imperfect development of one or more cartilages or of the centers of ossification. Thus, a failure of an arch to unite with the body or even the complete absence of an arch or half an arch may occur, and in cases of spina bifida the two halves of the arches fail to unite dorsally. Occasionally the two parts of the double cartilaginous center for the body fail to unite, a double body resulting; or one of the two parts may entirely fail, the result being the formation of only one-half of the body of the vertebra. Other anomalies result from the excessive development of parts. Thus, the rib of the seventh cervical vertebra may sometimes remain distinct and be long enough to reach the sternum, and the first lumbar rib may also fail to unite with Its vertebra. On the other hand, the first thoracic rib is occasionally found to be imperfect.
The Development of the Sternum
Longitudinal bars, which are formed by the fusion of the ventral ends of the anterior eight or nine cartilaginous thoracic ribs, represent the future sternum. At an early period the two bars come into contact anteriorly and fuse together (Fig. 101), and at this anterior end two usually indistinctly separated masses of cartilage are to be observed at the vicinity of the points where the ventral ends of the cartilaginous clavicles articulate. These are the episternal cartilages (em), which later normally unite with the longitudinal bars and form part of the manubrium sterni, though occasionally they persist and ossify to form the ossa suprasternal. The fusion of the longitudinal bars gradually extends backward until a single elongated plate of cartilage results, with which the seven anterior ribs are united, one or two of the more posterior ribs which originally took part in the formation of each bar having separated. The portions of the bars formed by these posterior ribs constitute the xiphoid process.
The ossification of the sternum (Fig. 102) partakes to a certain extent of the original bilateral segmental origin of the cartilage, but there is a marked condensation of the centers of ossification and considerable variation in their number also occurs. In the portion of the cartilage which lies below the junction of the third costal cartilages a series of pairs of centers appears just about birth, each center probably representing an epiphysial center of a corresponding rib. Later the centers of each pair fuse and the single centers so formed, extending through the cartilage, eventually unite to form the greater part of the body of the bone. In each of the two uppermost segments, however, but a single center appears, that of the second segment uniting with the more posterior centers and forming the upper part of the body, while the uppermost center gives rise to the manubrium, which frequently persists as a distinct bone united to the body by a hinge-joint.
Fig. 101. - Formation of the Sternum in an Embryo of About 3 cm. el, Clavicle; em, episternal cartilage. - (Ruge.)
A failure of the cartilaginous bars to fuse produces the condition known as cleft sternum, or if the failure to fuse affects only a portion of the bars there results a perforated sternum. A perforation or notching of the xiphoid cartilage is of frequent occurrence owing to this being the region where the fusion of the bars takes place last.
Fig. 102. - Sternum of New-born Child, showing Centers of Ossification. I to VII, Costal cartilages. - (Gegenbaur.)
Fig. 103. - Reconstruction of the Chondrocranium of an embryo of 14 mm. as, Alisphenoid; bo, basioccipital; bs, basisphenoid; eo, exoccipital; m, Meckel's cartilage; os, orbitosphenoid; p, periotic; ps, presphenoid; so, sella turcica; s, supraoccipital. - (Levi.)
The suprasternal bones are the rudiments of a bone or cartilage, the omosternum, situated in front of the manubrium in many of the lower mammalia. It furnishes the articular surfaces for the clavicles and is possibly formed by a fusion of the ventral ends of the cartilages which represent those bones; hence its appearance as a pair of bones in the rudimentary condition.
The Development of the Skull
In its earliest stages the human skull is represented by a continuous mass of mesenchyme which invests the anterior portion of the notochord and extends forward beyond its extremity into the nasal region, forming a core for the nasal process (see p. 99). From each side of this basal mass a wing projects dorsally to enclose the anterior portion of the medullary canal which will later become the cerebral part of the central nervous system. No indications of a segmental origin are to be found in this mesenchyme; as stated, it is a continuous mass, and this is likewise true of the cartilage which later develops in it.
The chondrihcation occurs first along the median line in what will be the occipital and sphenoidal regions of the skull (Fig. 103) and thence gradually extends forward into the ethmoidal region and to a certain extent dorsally at the sides and behind into the regions later occupied by the wings of the sphenoid (as and os) and the squamous portion of the occipital (s). No cartilage develops, however, in the rest of the sides or in the roof of the skull, but the mesenchyme of these regions becomes converted into a dense membrane of connective tissue. While the chondrification is proceeding in the regions mentioned, the mesenchyme which encloses the internal ear becomes converted into cartilage, forming a mass, the periotic capsule (Fig. 103, p), wedged in on either side between the occipital and sphenoidal regions, with which it eventually unites to form a continuous chondro cranium, perforated by foramina for the passage of nerves and vessels.
The posterior part of the basilar portion of the occipital cartilage presents certain peculiarities of development. In calf embryos there are in this region, in very early stages, four separate condensations of mesoderm corresponding to as many mesodermic somites and to the three roots of the hypoglossal nerve together with the first cervical or suboccipital nerve (Froriep) (Fig. 104). These mesenchymal masses in their general characters and relations resemble vertebral bodies, and there are good reasons for believing that they represent four vertebrae which, in later stages, are taken up into the skull region and fuse with the primitive chondrocranium. In the human embfyo they are less distinct than in lower mammals, but since a three-rooted hypoglossal and a suboccipital nerve also occur in man it is probable that the corresponding vertebrae are also represented. Indeed, confirmation of their existence may be found in the fact that during the cartilaginous stage of the skull the hypoglossal foramina are divided into three portions by two cartilaginous partitions which separate the three roots of the hypoglossal nerve. It seems certain from the evidence derived from embryology and comparative anatomy that the human skull is composed of a primitive unsegmental chondrocranium plus four vertebrae, the latter being added to and incorporated with the occipital portion of the chondrocranium. Emphasis must be laid upon the fact that the cartilaginous portion of the skull forms only the base and lower portions of the sides of the cranium, its entire roof, as well as the face region, showing no indication of cartilage, the mesenchyme in these regions being converted into fibrous connective tissue, which, especially in the cranial region, assumes the form of a dense membrane.
But in addition to the chondrocranium and the vertebras incorporated with it, other cartilaginous elements enter into the composition of the skull. The mesenchyme which occupies the axis of each branchial arch undergoes more or less complete chondrification, cartilaginous bars being so formed, certain of which enter into very close relations with the skull. It has been seen (p. 92) that each half of the first arch gives rise to a maxillary process which grows forward and ventrally to form the anterior boundary of the mouth, while the remaining portion of the arch forms the mandibular process. The whole of the axis of the mandibular process becomes chondrified, forming a rod known as Meckel's cartilage, and this, at its dorsal end, comes into relation with the periotic capsule, as does also the dorsal end of the cartilage of the second arch. In the remaining three arches cartilage forms only in the ventral portions, so that their rods do not come into relation with the skull, though it will be convenient to consider their further history together with that of the other branchial arch cartilages. The arrangement of these cartilages is shown diagrammatically in Fig. 105.
Fig. 104. - Frontal Section through the occipital and upper Cervical Regions of a Calf Embryo of 8.7 MM.
ai and ai 1 , Intervertebral arteries; be 1 , first cervical intervertebral plate; bo, suboccipital intervertebral plate; c 1 - 2 , cervical nerves; eh, notochord; K, vertebral centrum; m l - 3 , occipital myotomes; m 4 - 5 , cervical myotomes; 1 - 3 , roots of hypoglossal nerve; vj, jugular vein; x and xi, vagus and spinal accessory nerves. - (Froriep.)
By the ossification of these various parts three categories of bones are formed: (1) cartilage bones formed in the chondrocranium, (2) membrane bones, and (3) cartilage bones developing from the cartilages of the branchial arches. The bones belonging to each of these categories are primarily quite distinct from one another and from those of the other groups, but in the human skull a very considerable amount of fusion of the primary bones takes place, and elements belonging to two or even to all three categories may unite to form a single bone of the adult skull. In a certain region of the chondrocranium also and in one of the branchial arches the original cartilage bone becomes ensheathed by membrane bone and eventually disappears completely, so that the adult bone, although represented by a cartilage, is really a membrane bone. And, indeed, this process has proceeded so far in certain portions of the branchial arch skeleton that the original cartilaginous representatives are no longer developed, but the bones are deposited directly in connective tissue. These various modifications interfere greatly with the precise application to the human skull of the classification of bones into the three categories given above, and indeed the true significance of certain of the skull bones can only be perceived by comparative studies. Nevertheless it seems advisable to retain the classification, indicating, where necessary, the confusion of bones of the various categories.
Fig. 105. - Diagram showing the Five Branchial Cartilages, I to'F.
At, Atlas; Ax, epistropheus; 3, third cervical vertebra.
The Ossification of the Chondrocranium
The ossification of the cartilage of the occipital region results in the formation of four distinct bones which even at birth are separated from one another by bands of cartilage. The portion of cartilage lying in front of the foramen magnum ossifies to form a basioccipital bone (Fig. 106, bo), the portions on either side of this give rise to the two exoccipitals (eo), which bear the condyles, and the portion above the foramen produces a supraoccipital (so), which represents the part of the squamous portion of the adult bone lying below the superior nuchal line. All that portion of the bone which lies above that line is composed of membrane bone which owes its origin to the fusion of two or sometimes four centers of ossification, appearing in the membranous roof of the embryonic skull. The bone so formed (ip) represents the interparietal of lower vertebrates and, at an early stage, unites with the supraoccipital, although even at birth an indication of the line of union of the two parts is to be seen in two deep incisions at the sides of the bone. The union of the exoccipitals and supraoccipital takes place in the course of the first or second year after birth, but the basioccipital does not fuse with the rest of the bone until the sixth or eighth year. It will be noticed that no special centers occur for the four occipital vertebrae, these structures having become completely incorporated in the chondrocranium, and even the cartilaginous partitions which divide the hypoglossal foramina usually disappear during the process of ossification.
Fig. 106. - Occipital Bone of a Fetus at Term. bo, Basioccipital; eo, exoccipital; ip, interparietal; so, supraoccipital.
Two pairs of centers have been described for the interparietal bone and it has been claimed that the deep lateral incisions divide the lower pair, so that when the incisions meet and persist as the sutura mendosa, separating the so-called inca bone from the rest of the occipital, the division does not correspond to the line between the supraoccipital and the interparietal, but a portion of the latter bone remains in connection with the supraoccipital. Mall, however, in twenty preparations, found but a single pair of centers for the interparietal.
Occasionally an additional pair of small centers appear for the uppermost angle of the interparietal, and the bones formed from them may remain distinct as what have been termed fontanelle bones.
Fig. 107. - Sphenoid Bone from Embryo of 3^ to 4 Months. The parts which are still cartilaginous are represented in black, as, Alisphenoid ; b, basisphenoid; /, lingula; os, orbitosphenoid ; p, internal pterygoid plate. - (Sappey.)
In the sphenoidal region the number of distinct bones which develop is much greater than in the occipital region. At the beginning of the second month a center appears in each of the cartilages which represent the alisphenoids (great wings). These cartilages do not, however, represent the entire extent of the great wings and their ossification gives rise only to those portions of the bone in the neighborhood of the foramina ovale and rotundum and to the lateral pterygoid plates. The remaining portions of the wings, the orbital and temporal portions, develop as membrane bone (Fawcett) and early unite with the portions formed from the cartilage. At the end of the second month a center appears in each orbito sphenoid (lesser wing) cartilage (Fig. 107, os), and a little later a pair of centers (b), placed side by side, are developed in the cartilage representing the posterior portion of the body; together these form what is known as the basisphenoid. Still later a center appears on either side of the basisphenoids to form the UngulcB (I), and another pair appears in the anterior part of the cartilage, between the orbitosphenoids, and represent the presphenoid.
In addition to these ten centers in cartilage and the membrane portion of the alisphenoid, two other membrane bones are included in the adult sphenoid. Thus, a little before the appearance of the center for the alisphenoids an ossification is formed in the mesenchyme of each lateral wall of the posterior part of the nasal cavity and gives rise to the medial lamina of the pterygoid process, the mesenchyme at the tip of the ossification condensing to form a cartilaginous hook-like structure over which the tendon of the tensor veli palatini plays. This cartilage later ossifies to form the pterygoid hamulus, the medial pterygoid lamina being thus a combination of membrane and cartilage, the latter, however, being a secondary development and quite independent of the chondrocranium.
By the sixth month the lingular have fused with the basisphenoid and the orbitosphenoids with the presphenoid, and a little later the basisphenoid and presphenoid unite. The alisphenoids and medial pterygoid laminae remain separate, however, until after birth, fusing with the remaining portions of the adult bone during the first year.
The cartilage of the ethmoidal region of the chondrocranium forms somewhat later than the other portions and consists at first of a stout median mass projecting downward and forward into the nasal process (Fig. 108, Ip), and two lateral masses (lm), situated one on either side in the mesenchyme on the outer side of each olfactory pit. Ossification of the lateral masses or ectethmoids begins relatively early, but it appears in the upper part of the median cartilage only after birth, producing the crista galli and the perpendicular plate, which together form what is termed the mesethmoid. When first formed, the three cartilages are quite separate from one another, the olfactory and nasal nerves passing down between them to the olfactory pit, but later trabecular begin to extend across from the mesethmoid to the upper part of the ectethmoids and eventually form a fenestrated horizontal lamella which ossifies to form the cribriform plate.
The lower part of the median cartilage does not ossify, but a center appears on each side of the median line in the mesenchyme behind and below its posterior or lower border. From these centers two vertical bony plates develop which unite by their median surfaces below, and above invest the lower border of the cartilage and form the vomer. The portion of the cartilage which is thus invested undergoes resorption, but the more anterior portions persist to form the cartilaginous septum of the nose. The vomer, consequently, is not really a portion of the chondrocranium, but is a membrane bone; its intimate relations with the median ethmoidal cartilage, however, make it convenient to consider it in this place.
When first formed, the ectethmoids are masses of spongy bone and show no indication of the honeycombed appearance which they present in the adult skull. This condition is produced by the absorption of the bone of each mass by evaginations into it of the mucous membrane lining the nasal cavity. This same process also brings about the formation of the curved plates of bone which project from the inner surfaces of the lateral masses and are known as the superior and middle conchse (turbinated bones). The inferior and sphenoidal conchae are developed from special centers, but belong to the same category as the others, being formed from portions of the lateral ethmoidal cartilages which become almost separated at an early stage before the ossification has made much progress. Absorption of the body of the sphenoid bone to form the sphenoidal cells, of the frontal to form the frontal sinuses, and of the maxillaries to form the maxillary antra is also produced by outgrowths of the nasal mucous membrane, all these cavities, as well as the ethmoidal cells, being continuous with the nasal cavities and lined with an epithelium which is continuous with the mucous membrane of the nose.
Fig. 108. - Anterior Portion of the Base of the Skull of a 6 to 7 Months' Embryo.
The shaded parts represent cartilage. cp, Cribriform plate; hn, lateral mass of the ethmoid; Ip, perpendicular plate; of optic foramen; os, orbitosphenoid. - (After von Spec.)
In the lower mammalia the erosion of the mesial surface of the ectethmoidal cartilages results, as a rule, in the formation of five conchae, while in man but three are usually recognized. Not infrequently, however, the human middle concha shows indications, more or less marked, of a division into an upper and a lower portion, which correspond to the third and fourth bones of the typical mammalian arrangement. Furthermore, at the upper portion of the nasal wall, in front of the superior concha, a slight elevation, termed the agger nasi, is always observable, its lower edge being prolonged downward to form what is termed the uncinate process of the ethmoid. This process and the agger together represent the uppermost concha of the typical arrangement, to which, therefore, the human arrangement may be reduced.
A number of centers of ossification - the exact number is yet uncertain - appear in the periotic capsule during the later portions of the fifth month, and during the sixth month these unite together to form a single center from which the complete ossification of the cartilage proceeds to form the petrous and mastoid portions of the temporal bone (Fig. 109, p). The mastoid process does not really form until several years after birth, being produced by the hollowing and bulging out of a portion of the petrous bone by out-growths from the lining membrane of the middle ear. The cavities so formed are the mastoid cells, and their relations to the middle-ear cavity are in all respects similar to those of the ethmoidal and sphenoidal cells to the nasal cavities. The remaining portions of the temporal bone are partly formed by membrane bone and partly from the branchial arch skeleton. An ossification appears at the close of the eighth week in the membrane which forms the side of the skull in the temporal region and gives rise to a squamosal bone (s), which later unites with the petrous to form the squamosal portion of the adult temporal, and another membrane bone, the tympanic (/), develops from a center appearing in the mesenchyme surrounding the external auditory meatus, and later also fuses with the petrous to form the floor and sides of the external meatus, giving attachment at its inner edge to the tympanic membrane. Finally, the styloid process is developed from the upper part of the second branchial arch, whose history will be considered later.
Fig. 109. - The Temporal Bone at Birth. The Styloid Process and Auditory Ossicles are not Represented. p, Petrous bone; s, squamosal; t, tympanic. - (Poirier.)
The various ossifications which form in the chondrocranium and the portions of the adult skull which represent them are shown in the following table:
Region of Chondrocranium.
IBasioccipital Exoccipitals Supraoccipital
Basisphenoid Presphenoid Lingulae Alisphenoids Orbitosphenoids Mesethmoid
Parts of Adult Skull; Basilar process. Condyles.
Squamous portion below superior nuchal line.
Greater wings (in part) . Lesser wings. Lamina perpendicularis. Crista galli. Nasal septum. Lateral masses. Superior concha. Middle concha.
The Membrane Bones of the Skull
In the membrane forming the sides and roof of the skull in the second stage of its development ossifications appear, which give rise, in addition to the interparietal and squamosal bones already mentioned in connection with the occipital and temporal, to the parietals and frontal. Each of the former bones develops from a single center which appears at the end of the eighth week, while the frontal is formed at about the same time from two centers situated symmetrically on each side of the median line and eventually fusing completely to form a single bone, although more or less distinct indications of a median suture, the metopic, are not infrequently present.
Furthermore, ossifications appear in the mesenchyme of the facial region to form the nasal, lachrymal, and zygomatic bones, all of which arise from single centers of ossification. In the case of each zygomatic bone, however, three osseous thickenings appear on the inner surface of the original ossification, which then disappears and the thickenings unite to form the adult bone, though occasionally one or more of their lines of union may persist, producing a bipartite or tripartite zygomatic.
The vomer, which has already been described, belongs also strictly to the category of membrane bones, as do also the maxillae and the palatines; these latter, however, primarily belonging to the branchial arch skeleton, with which they will be considered.
The purely membrane bones in the skull, are, then, the following: Interparietals Part of squamous portion of occipital.
Pterygoids Medial pterygoid plates.
Squamosals Squamous portions of temporals.
Tympanies Tympanic plates of temporals.
The Ossification of the Branchial Arch Skeleton
It has been seen (p. 171) that a cartilaginous bar develops only in the mandibular process of the first branchial arch. In the maxillary process no cartilaginous skeleton forms, but two membrane bones,
Fig. 110. - Diagram of the Ossifications of which the Maxilla is Composed, as seen from the Outer Surface. The Arrow Passes through the Infraorbital Canal. - (From von Spee, after Sappey.)
the palatine and maxilla, are developed in it, their cartilaginous representatives, which are to be found in lower vertebrates, having been suppressed by a condensation of the development. The palatine bone develops from a single center of ossification, but for each maxilla no less than five centers have been described (Fig. no). One of these gives rise to so much of the alveolar border of the bone as contains the bicuspid and molar teeth; a second forms the nasal process and the part of the alveolar border which contains the canine tooth; a third the portion which contains the incisor teeth; while the fourth and fifth centers lie above the first and give rise to the inner and outer portions of the orbital plate and the body of the bone. The first, second, fourth, and fifth portions early unite together, but the third center, which really lies in the ventral part of the nasal process, remains separate for some time, forming what is termed the premaxilla, a bone which remains permanently distinct in the majority of the lower mammals.
The above is the generally accepted view as to the development of the maxilla. Mall, however, maintains that it has but two centers of ossification, one giving rise to the premaxilla and the other to the rest of the bone. The maxillary center makes its appearance about the middle of the sixth week.
Since the condition known as hare-lip results from a failure of the maxillary process to unite completely with the frontonasal process (see p. 100), and since the premaxilla develops in the latter and the maxilla in the former, the cleft may pass between these two bones and prevent their union (see also p. 284).
The upper end of Meckel's cartilage passes between the tympanic bone and the outer surface of the periotic capsule and thus comes to lie apparently within the tympanic cavity of the ear; this portion of the cartilage divides into two parts which ossify to form two of the bones of the middle ear, the malleus and incus, a description of whose further development may be postponed until a later chapter. At about the middle of the sixth week of development a plate of membrane bone appears to the outer side of the lower portion of the cartilage and gradually extends to form the body and ramus of the mandible.
In the region of the body the bone develops so as to enclose the cartilage, together with the inferior alveolar (dental) nerve which lies to the outer side of the cartilage, but in the region of the ramus the bone remains entirely to the outer side of the cartilage and nerve, whence the position of the mandibular foramen on the inner surface of the adult bone. The anterior portion of Meckel's cartilage becomes ossified by the extension of ossification from the membrane bone into it, the portion corresponding to the body of the bone behind the mental foramen disappears and the portion above the mandibular foramen is said to become transformed into fibrous connective tissue and to persist as the spheno-mandibular ligament. At the upper extremity of the ramus two nodules of cartilage develop, quite independently, however, of Meckel's cartilage (Fig. in, Cr and Cy), and ossification extends into these from the ramus to form the coronoid and condyloid processes. And, finally, two other independent cartilages appear toward the anterior extremity of each half of the bone, one at the alveolar (C t ) and the other at the lower border (C 2 ), and these, also are later incorporated into the bone without developing special centers of ossification.
Fig. 111. - Model of Right Half of Mandible of a Fetus 95 mm. in Length, seen from the mesial surface. C 1 and C 2 , Accessory cartilages; Ch. T., chorda tympanijO., cartilage for coronoid process; Cy., cartilage for condyloid process; Mai., malleus; M.C., Meckel's cartilage; N. Al., inferior alveolar nerve; N. Aur., auriculo-temporal nerve; N.L., lingual nerve; N.Mh., mylo-hyoid nerve; N.T., trigeminal nerve; Sy., symphysis. - (Low.)
Fig. 112. - Diagram showing the Categories to which the Bones of the Skull . . Belong. The unshaded bones are membrane bones, the heavily shaded represent the chondrocranium, while the black represents the branchial arch elements. AS, Alisphenoid; ExO, exoccipital; F, frontal; Hy, hyoid; IP, interparietal; Z, zygomatic; Mn, mandible; Mx, maxilla; NA, nasal; P, parietal; Pe, periotic; SO, supraoccipital; Sg, squamosal; St, styloid process; Th, thyreoid cartilage; Ty, tympanic.
Each half of the mandible thus ossifies from a single center, and is essentially a membrane bone replacing a cartilaginous precursor. At birth the two halves are united at the symphysis by fibrous tissue, into which ossification extends later, union occurring in the first or second year.
The upper part of the cartilage of the second branchial arch also comes into relation with the tympanic cavity and ossifies to form the styloid process of the temporal bone. The succeeding moiety of the cartilage undergoes degeneration to form the stylo-hyoid ligament, while its most ventral portion ossifies as the lesser comu of trie hyoid bone. The great variability which may be observed in the length of the styloid processes and of the lesser cornua of the hyoid depends upon the extent to which the ossification of the original cartilage proceeds, the length of the stylo-hyoid ligaments being in inverse ratio to the length of the processes or cornua. The greater cornua of the hyoid are formed by the ossification of the cartilages of the third arch, and the body of the bone is formed from a cartilaginous plate, the copula, which unites the ventral ends of the two arches concerned.
Finally, the cartilages of the fourth and fifth branchial arches early fuse together to form a plate of cartilage, and the two plates of opposite sides unite by their ventral edges to form the thyreoid cartilage of the larynx.
The accompanying diagram (Fig. 112) shows the various structures derived from the branchial arch skeleton, as well as some of the other elements of the skull, and a re'sume' of the fate of the branchial arches may be stated in tabular form as follows, the parts represented by cartilage which becomes replaced by membrane bone being printed in italics, while membrane bones which have no cartilaginous representatives are enclosed in brackets:
(Styloid process of the temporal. Stylo-hyoid ligament. Lesser cornu of hyoid .
3d arch Greater cornu of hyoid.
4th and 5th arches Thyreoid cartilage of larynx.
The Development of the Appendicular Skeleton
While the greater portion of the axial skeleton is formed from the sclerotomes of the mesodermic somites, the appendicular skeleton is derived from the somatic mesenchyme, which is not divided into metameres. This mesenchyme forms the core of the limb bud and becomes converted into cartilage, by the ossification of which all the bones of the limbs, with the possible exception of the clavicle, are formed.
Of the bones of the pectoral girdle the clavicle requires further study before it can be certain whether it is to be regarded as a purely cartilage bone or as a combination of cartilage and membrane ossification (Gegenbaur). It is the first bone of the skeleton to ossify, two centers appearing for each bone at about the sixth week of development. The tissue in which the ossifications form has certain peculiar characters, and it is difficult to say whether it is to be regarded as cartilage which, on account of the early differentiation of the center, has not yet become thoroughly differentiated histologically, or as some other form of connective tissue. However that may be, true cartilage develops on either side of the ossifying region, and into this the ossification gradually extends, so that at least a portion of the bone is preformed in cartilage.
The scapula is at first a single plate of cartilage in which two centers of ossification appear. One of these gives rise to the body and the spine, while the other produces the coracoid process (Fig. 113, co), the rudimentary representative of the coracoid bone which extends between the scapula and sternum in the lower vertebrates. The coracoid does not unite with the body until about the fifteenth year, and secondary centers which give rise to the vertebral edge (b) and inferior angle of the bone (a) and to the acromion process (c) unite with the rest of the bone at about the twentieth year.
The humerus and the bones of the forearm are typical long bones, each of which develops from a primary center, which gives rise to the shaft, and has, in addition, two or more epiphysial centers. In the humerus an epiphysial center appears for the head, another for the greater tuberosity, and usually a third for the lesser tuberosity, while at the distal end there is a center for each condyle, one for the trochlea and one for the capitulum, the fusion of these various epiphyses with the shaft taking place between the seventeenth and twentieth years. The radius and ulna each possesses a single epiphysial center for each extremity in addition to the primary center for the shaft, the proximal epiphysial center for the ulna giving rise to the tip of the olecranon process.
Fig. 113. - The Ossification Centers of the Scapula. a, b, and c, Secondary centers for the angle, vertebral border, and acromion; co, center for the coracoid process. - (Testut.)
Fig. 114. - Reconstruction of an Embryonic Carpus.
c, Centrale; cu, triquetral; lu, lunate; m, capitate; p, pisiform; sc, navicular; t, greater multangular; tr, lesser multangular; u, hamate.
The embryological development of the carpus is somewhat complicated. A cartilage is found representing each of the bones normally occurring in the adult (Fig. 114), and these are arranged in two distinct rows: a proximal one consisting of three elements, named from their relation to the bones of the forearm, radiate, intermedium, and ulnar e; and a distal on^composed of four elements, termed carpalia. In addition, a cartilage, termed the pisiform, is found on the ulnar side of the proximal row ^nd is generally j^g&rded as a sesamoid cartilage developed in the /tendon of the flei ulnaris, and furthermore a number of inconstant carti been observed whose significance in the majority of cast less obscure. These accessory cartilage^either disappc stages of development or fuse with neighboring cartilages^ cases, ossify and form distinct elements of the carpus, however, occurs so frequently as almdK to deserve^ classification as a constant element; it \p known asvthje ceniraie (Fig. 114, c) and occupies a position between the/car\ua!Â§;es of the proximal and distal rows and apparently correspond ~r&. a cartilage typVally present in lower forms and o^fying*to~fÂ»rai a distinct bone. Iri tha human carpus its fate varies, wfe it may\eitnfer disappear or unitp with other cartilages, that with wpich it most usually fuses b'eing probably the radiale. There is evraence also to sfrftw that another ofJthe accessory cartilages unites/with the ulnar element of the distatsAw, representing the carpale v typically present in lower forms.
Each of the eleinents corresponding to an adult bone ossifies from a single centerwith the exception of carpale iv-Xwhich has two centers, a furtherindication of its composite character. The relation of the cartrteg&s to the adult bones may be seen from the table given on page ??
With regard toYhe metacarpals and phalanges; it need merely be stated that each develops from a single primary center for the shaft and one secondary epiphysial center. The" primary center appears at about the middle of the shaft excepJ in the terminal phalanges, in which it appears at the distal enfr of the cartilage. The epiphyses for the metacarpals are at the distends of the bones, except in the case of the metacarpal of the ihumb, which resembles the phalanges in having its epiphysis at the proximal end.
Each innominate bone appears as a somewhat oval plate of cartilage whose long axis is directed almost at right angles to the vertebral column and which is in close relation with the fourth and fifth sacral vertebrae. As development proceeds a rotation of the cartilage, accompanied by a slight shifting of position, occurs, so that eventually the plate has its long axis almost parallel with the vertebral column and is in relation with the first three sacrals. Ossification appears at three points in each cartilage, one in the upper part to form the ilium (Fig. 115, il) and two in the lower part, the anterior of these giving rise to the pubis (p), while the posterior produces the ischium (is). At birth these three bones are still separated from one another by a Y-shaped piece of cartilage whose three limbs meet at the bottom of the acetabulum, but later a secondary center appears in this cartilage and unites the three bones together. The central part of the lower half of each original cartilage plate does not undergo complete chondrification, but remains membranous, constituting the obturator membrane which closes the obturator foramen. In addition to the Y-shaped secondary center, other epiphysial centers appear in the prominent portions of the cartilage, such as the pubic crest (Fig. 115, c), the ischial tuberosity (d), the anterior inferior spine (b) and the crest of the ilium (a), and unite with the rest of the bone at about the twentieth year.
The femur, tibia, and fibula each develop from a single primary center for the shaft and an upper and a lower epiphysial center, the femur possessing, in addition, epiphysial centers for the greater and lesser trochanters (Fig. 94). The patella does not belong to the same category as the other bones, but resembles the pisiform bone of the carpus in being a sesamoid bone, developed in the tendon of the quadriceps extensor cruris. Its cartilage does not appear until the fourth month of intrauterine life, when most of the primary centers for other bones have already appeared, and its ossification does not begin until the third year after birth.
Fig. 115. - The Ossification Centers of the os innominatum. a, b, c, and d, Secondary centers for the crest, anterior inferior spine, symphysis, and ischial tuberosity; il, ilium; is, ischium; p, pubis. - (Testut.)
The tarsus, like the carpus, consists of a proximal row of three cartilages, termed the tibiale, the intermedium, and the fibulare, and of a distal row of four tarsalia. Between these two rows a single cartilage, the centrale, is interposed. Each of these cartilages ossifies from a single center, that of the intermedium early fusing with the tibiale, though it occasionally remains distinct as the os trigonum, and from a comparison with lower forms it seems probable that the fibular cartilage of the distal row really represents two separate elements, there being, properly speaking, five tarsalia instead ot four. The fibulare, in addition to its primary center, possesses also an epiphysial center, which develops at the point of insertion of the tendo Achillis.
A comparison of the carpal and tarsal cartilages and their relations to the adult bones may be seen from the following table:
f Tibiale \ Intermedium
Fuses with navicular
1 st Cuneiform
Carpale IV 1 Carpale V J
( Tarsale TV I Tarsale V
The development of the metatarsals and phalanges is exactly similar to that of the corresponding bones of the hand (see p. 185).
The Development of the Joints
The mesenchyme which primarily represents each, vertebra, or the skull, or the skeleton of a limb, is at first a continuous mass, and when it becomes converted into cartilage this also may be continuous, as in the skull, or may appear as a number of distinct parts united by unmodified portions of the mesenchyme. In the former case the various ossifications as they extend will come into contact with their neighbors and will either fuse with them or will articulate with them directly, forming a suture.
When, however, a portion of unmodified mesenchyme intervenes between two cartilages, the mode of articulation of the bones formed from these cartilages will vary. The intermediate mesenchyme may in time undergo chondrification and unite the bones in an almost immovable articulation known as a synchondrosis (e. g., the articulation of the first rib with the sternum) ; or a cavity may appear in the center of the intervening cartilage so that a slight amount of movement of the two bones is possible, forming an amphiar thro sis (e. g., the symphysis pubis); or, finally, the intermediate mesenchyme may not chondrify, but its peripheral portions may become converted into a dense sheath of connective tissue (Fig. 116, c) which surrounds the adjacent ends of the two bones like a sleeve, forming the articular capsule, while the central portions degenerate to form a cavity. The bones which enter into such an articulation are more or less freely movable upon one another and the joint is termed a diarthrosis (e. g., the knee- or shoulder-joint).
In a diarthrosis the connective-tissue cells near the inner surface of the capsule arrange themselves in a layer to form a synovial membrane for the joint, and portions of the capsule may thicken to form special bands, the reinforcing ligaments, while other strong fibrous bands, which may pass from one bone to the other, forming accessory ligaments, are shown by comparative studies to be in many cases degenerated portions of what were originally muscles.
In certain diarthroses, such as the temporo-mandibular and sternoclavicular, the whole of the central portions of the intermediate mesenchyme does not degenerate, but it is converted into a fibro-cartilage, between each surface of which and the adjacent bone there is a cavity. These interarticular cartilages seem, in the sterno-clavicular joints, to represent the sternal ends of a bone existing in lower vertebrates and known as the precoracoid, but it seems doubtful if those of the temporo-mandibular and knee joints have a similar significance, the most recent observations on their development tending to derive them from the intermediate mesenchyme.
Fig. 116. - Longitudinal Section through the Joint oe the Great Toe in an Embryo of 4.5 cm. c, Articular capsule; i, intermediate mesenchyme which has almost disappeared in the center; p 1 and p 2 , cartilages of the first and second phalanges. - (Nicholas.)
From their mode of development it is evident that the cavities of diarthrodial joints are completely closed and their walls, except where they are formed by cartilage, are lined by a continuous layer of synovial cells. Ligaments or tendons, which, at first sight, appear to traverse the cavities of certain joints, are in reality excluded from them, being lined by a sheath of synovial cells continuous with the layer fining the general cavity. Thus, the tendon of the long head of the biceps, which seems to traverse the shoulder-joint is, in the fetus, entirely outside the articular capsule, upon which it rests. Later it sinks in toward the joint cavity, pushing the articular capsule before it, so that it lies at first in a groove in the capsule, which later on becomes converted into a canal and, finally, separates from the rest of the capsule except at its two extremities, forming a cylindrical canal, open at either end, traversing the joint cavity and containing the tendon of the biceps.
The ligamentum teres of the hip-joint is similarly excluded from the joint cavity by a sheath of synovium, which extends outward around it from the bottom of the acetabular fossa to the depression in the head of the femur, and in the knee-joint the crucial ligaments are also excluded from the cavity by a reflection of the synovium. This joint, indeed, is in the fetus incompletely divided into two parts, one corresponding to each femoral condyle, by a partition which extends backward from the patellar ligament to the crucial ligaments, remains of this partition persisting in the adult as the so-called ligamentum mucosum.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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