Book - Quain's Embryology 5
The Skeleton and Organs of Voluntary Motion
- Development of Particular Organs and Systems
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Sharpey W. Thomson A. and Schafer E.A. Quain's Elements of Anatomy. (1878) William Wood and Co., New York.
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- 1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital
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The morphological development of the skeleton and organs of voluntary motion is closely in accordance with the general plan of development which belongs to the whole vertebrate body. The first steps are connected with the formation of the strictly axial part, consisting of the enclosing walls of the cranio-vertebral cavity for containing the rudiments of the brain and spinal marrow, and for the issue of the successive pairs of nerves arising from them. These are succeeded by the formation of the walls of the great visceral cavities of the head and trunk, in which the facial and costal arches are to be distinguished ; and lastly, the appendicular parts, or the limbs and limb-arches, are developed. The permanent forms of these parts are only produced in the process of ossification ; but the rudiments of most of them are already to be distinguished in the masses of cartilage or formative tissue which precede the ossifying change.
As the mode of ossification of the several bones has been described in the osteological part of the work, and the histological view of the process of formation of bone has been given in the part on General Anatomy, the morphological view of the development will alone be referred to in this place, in which will be included the more important phenomena of the preparation of the matrix or formative material for the various parts of the skeleton.
Fig. 525. — Embryo of the Dog seen from above wIth a Portion of the Blastoderm attached.
The medullary canal is not yet closed, but shows the dilatation at the cephalic extremity with a partial division into the three primary cerebral vesicles ; the posterior extremity shows a rhomboidal enlargement. The cephalic fold crosses below the middle cerebral vesicle. Six primordial vertebral divisions are visible ; so, the upper division of the blastoderm ; sp, the lower division.
Vertebral Column and Trunk
Relation of Vertebral Rudiments to the Notochord
It has already been shown (General Phenomena of Development, p. 692), that all the parts of the skeleton ow^e their primitive formative material to mesoblastic elements, and that the bodies and arches of the vertebras, and the adjacent part of the cranial walls are formed from continuous blastodermic substance lying below and around the primitive medullary canal. A part also of the basis of the cranium has this in common witli the vertebral axis, that its formative substance surrounds the notochord, extending forward from the column of the vertebral bodies into the occipito-sphenoid part of the cranial basis, which is there composed of the formative substance termed the invesiing mass of Ratlike.
It is to be remembered, however, that closely as the formative tissue of the bone elements appear to surround the notochord, that structure does not itself, nor by its sheath, contribute to the formation of the vertebral or basi-cranial bones, but merely lies within them ; and the formative material, out of which the bones are produced, is derived from mesoblastic substance which passes inwards from the primordial vertebral plates, and envelopes the chorda external to its sheath. The formation of the notochord, therefore, precedes that of the formative bone-elements which afterwards envelope it, and the remains of the notochord, unaffected directly by any ossifying change, are found in the interior of the commencing bones, and may be traced even for a long time throughout the whole length of the column of the bodies of the vertebrre.
This important fact was first demonstrated by H. Miiller, of Wm-tzbrn-g-, who showed fui-ther that the notochord did not pass through the anterior arch of the atlas, but was traceable directly from the body of the axis vertebra through its odontoid process, and thence into the basi- occipital and basi-sphenoid bones, reaching as far as the pituitary fossa. (Heinrich Miiller, " Ub. d. Vorkommen von Eesten des Chorda Dorsalis b. Menschen nach der Geburt," in " Zeitsch. fiir Rat. Med.," von Henle u. Pfeifer, 1858, b. ii. See also Gegenbaur, " Untersuch. ub. Vergleich. Anat. Das Kopfskelet der Selachier," Leipzig, 1872. W. Miiller, " Bau der Chorda dorsalis," in '■ Jenasch. Zeitsch." b. vi. E. Dursy, " Zur Entwick. des Kopfes," 1869, and Mihalkovics, On the Chorda and Pituitary Body," in "Archiv filr Miki-oscop. Anat.," b. xi., 1875.)
It may be mentioned further, as the result of H. Miiller's observations, that though in general the chorda passes through the middle of the vertebral bodies, the position was found subject to variation in the caudal portion of the column, where it sometimes passed above, and at other times below, the vertebral bodies.
The notochord itself has been generally held to be produced from an intruded central column of mesoblastic cells, and this seems to be the mode of origin in birds ; but it may be doubtful whether it is the same in all animals. In sharks Balfour finds that there is no median column derived from the mesoblast, and attributes the origin of the notochord to the hypoblast (Quart. Joum. of IMicroscop. Sc, Oct. 1874). The same origin is ascribed to it in mammals by Hensen, who finds that the notochord is late of being formed in the rabbit, — an observation confii-med by Kolliker : Mihalkovics, on the other hand, is inclined to refer it in all animals to the epiblast. However this may be, the tendency of recent research appears to be to show that the notochord may be more nearly alUed to epithelial structiu'es than to cartilage with which it has generally been previously associated. It is at all events important to note that it is in many respects different from the parts ascertained to proceed from the mesoblast, that it never combines with their elements, and that there is no penetration of its substance by connective tissue or blood-vessels, as happens in all other parts derived from the mesoblast.
The interesting observations of Kowalevsky on the existence of a chorda dorsalis in Ascidia (Mem. de TAcad. de St. Petersboiu'g, torn. x. and xi., 1867 and 1868), would appear to show that this structure, and the type of development which accompanies it, are not confined to vertebrate animals, and that in them the notochord may present more of a merely vestigial character than constitute an important element in the formation of the skeleton. The constancy of its position and relations, however, is an important fact regai-ding its history.
The notochord does not undergo transverse segmentation in the same manner as the protovertebral plate does. It remains undivided to the last, but in the course of vertebral ossification it shows alternate diminutions and enlargements of its diameter, corresponding in number and position with the vertebral divisions. One of these enlargements is found between the odontoid process and the basi-occipital bone, and another has been observed by Mihalkovies in the interval betwecn the basi-occipital and the basi-sphenoid bones or their cartilagenous matrices.
Fig. 526. — View from above of the Embryo-Chick in the Fig. 525. first half of the second day.
1, 2, the three primary enceplialic vesicles ; in front and to the sides the cephalic fold ; crossing at 2 the fovea cardiaca ; 3, the caudal extremity of the medullary canal dilated into a rhomboid form ; 4, 4, six primordial vertebral divisions.
In front of this last swelling, the chorda is bent down below the base of the skull, and tapering to a fine filament, ends or is lost in the floor of the pituitary fossa. The enlargements now mentioned have some interest in connection with the question of the vertebral constitution of the skull in number and position to the vertebral segments of the body (somatomes of Goodsir) ; each one comprising superficially a segment of the muscular plate, and more deeply a pair of intervertebral ganglia and nerves, as well as the parts of the skeleton which lie before and behind them.
Segmentation of the Protovertebrae
The transverse vertebrate segmentation which occurs in the primary vertebral plates affects only that part of these plates which is formed of mesoblast. It begins at a very early period, as already stated, even before the close of the primary medullary canal, in the form of one or two, or it may be three short transverse transparent lines which separate a corresponding number of dark or condensed quadrilateral masses of the primitive vertebral plates. These quadrilateral masses, the so-called primordial vertehrcc (Urwirbel of the Germans) (fig. 526, -4, 4), do not, however, correspond merely to the vertebrae of the skeleton, nor are they directly converted into theii- rudiments, but they are rather divisions equivalent
Fig. 527. — Cervical part of the Primitive Vertebral column and adjacent parts of an embryo of the Sixth Day, showing the division of the Primitive Vertebral Segments (from Kolliker after Remak).
1, 1, chorda dorsal is in its sheath, pointed at its upper end ; 2, points by three lines to the original intervals of the primitive vertebrae ; 3, in a similar manner indicates the places of new division into permanent bodies of vertebrte ; c indicates the body of the first cervical vertebra ; in this and the next the primitive division has disappeared, as also in the two lowest represented, viz. , d and the one above ; in those intermediate the line of division is shown : 4, points in three places to the vertebral arches ; and 5, similarly to three commencing ganglia of the spinal nerves : the dotted segments outside these i^arts are the muscular plates.
The more obvious protovertebral segmentation docs not extend into the mesoblastic tissue beyond the commencement of the basis of the cranium, the mass of blastema which there surrounds the prolongation of the notochord (the investing mass of Rathke) remaining one and undivided, or being devoid at least of the marked cleavage which occurs in the strictly vertebral part.
Fig. 528. — Sections op the Vertebral Column of a Human Fcetus of eight weeks (from Kolliker).
A, transverse longitudinal section of several vertebrre. 1, 1, cliortla dorsalis, its remains thicker opposite the intervertebral discs ; 2, is placed on one of the bodies of the permanent vertebrce ; 3, on one of the intervertebral discs.
B, transverse horizontal section through a part of one dorsal vertebra. 1, remains of the chorda dorsalis in the middle of the body ; 2, arch of the vertebra ; 3, head of a rib.
It is from this protovertebral plate on each side, whether in its entire primitive condition, or in its later and divided state, that the material is derived for the formation of the bodies and laminae of the vertebra? and the muscles which cover them. This is effected by the rapid increase of the mesoblast, and by the extension of that structure beyond the immediate confines of the vertebral laminae in an inward and downward direction, so as to throw a quantity of new mesoblastic material round the notochord, and inwards and upwards, so as to pass in between the primary medullary canal and the enveloping layer of epiblast.
The muscular plate
Shortly after this extension of the mesoblast in the two directions before mentioned, another separation, or rather differentiation, is observed to take place in the direction of its length, in the formation along the dorsal surface, and below the epiblast, of a series of circumscribed plates which form the foundation of the erector muscles of the spine, and the great dorsal muscles of the trunk. These constitute together the muscular, or rather the musculo-cutaneous plate, for it appears also to include the formative rudiment of the true skin.
There is thus deposited the formative material for the vertebral bodies, the vertebral arches, and the muscles which immediately surround them, together with the general integument.
Meanwhile the vertebral segmentation goes on progressing from before backwards, extending through the dorsal, lumbar, sacral and coccygeal vertebrfB, till the process is complete ; but this is accompanied by other changes having reference to the separation of the nerveroots and ganglia from their formative tissue, and the development of the elements of the permanent vertebra?.
In the outer portion of each protovertebral mass a transverse partition arises which separates the anterior part, as ganglion and nerve root, from the posterior, as matrix or forerunner of the bone and other structures which belong to the vertebral column. Each nerve then comes thus to be placed in front of the future permanent vertebra with the protovertebral division of which it was originally connected. In the inner or central part of the primordial vertebrae a different kind of change occurs, first, by the amalgamation or fusion of the protovertebral masses, and subsequently by their subdivision in such a manner that the intervertebral disc arises on a level with or opposite the centre of each protovertebral mass, and the blastema, out of which a permanent vertebral body is formed, is made up by the union of two parts, an anterior and a posterior, the first of these being derived from the hinder part of the preceding protovertebral mass of the same number, the other part being supplied by the anterior section of the protovertebral mass immediately following.
Fig. 529. — Transverse section through the Dorsal region of an embryo-chick, end of third day (from Foster and Balfour).
Am, amnion ; mp, muscle plate ; c v, cardinal vein ; Ao, dorsal aorta at the point where its two roots begin to join ; Ch, notochord ; Wd, Wolffian duct ; Wh, commencement of formation of Wolffian body ; tip, epiblast ; so, somatopleure ; liy, hypoblast. The section passes through the place where the alimentary canal Qiy) communicates with the yolk-sac.
This is a somewhat complicated change ; and the more difficult to be followed that it would appear that the original division between the protovertebral masses disappears previous to a new segmentation taking place. Thus it results that, as respecta the centrum or body part, the posterior half of one protovertebra is thrown into connection w4th the anterior half of the one next following, and thus each permanent body is formed from parts of two protovertebral masses ; while in respect to the arches, each one proceeds from the hinder segment of the anterior of the two protovertebra3 concerned, the spinal gangiion and root being thrown into connection with the hinder part of the permanent vertebra immediately in front of the protovertebra of which they originally formed a part.
Formation of Vertebral Matrices
While the material for the vertebral bodies is laid dowu round, the notochord, a further extension of mesoblastic substance from the primordial vertebral plates takes place at the sides and round the medullary cavity for the matrix of the vertebral arches, and in due course, by differentiation of the formative cells, chondrification of the substance occurs in the form of the strips which constitute the first rudiments of the vertebral arches, and the accompanying transverse and other processes. The first ossification of these bones is from cartilage, but doubtless in them, as in other bones, much of the subsequent growth and extension of the bone substance proceeds from sub-periosteal deposit. It is also to be remarked that in some bones originating in membrane, cartilage may subsequently contribute to the growth and extension of the bone, as appears to occur in the lower jaw and clavicle.
The chondrification of the formative matrix of the bones in the human embryo takes place chiefly during the fifth and sixth weeks of foetal life, and in the seventh and eighth, ossification has begun in several of the long bones. But even before this time an ossific deposit shows itself in the fibrous matrix of the clavicle and lower jaw. By the ninth week the greater number of the bones have begun to ossify.
The formation of cartilages for the arches of the vertebrae begins first in those of the dorsal region, and spreads from these forwards into the cervical vertebrae and basis of the skull, and backwards into the lumbar and sacral vertebrae : but the extension of the matrix upwards ceases in the lower sacral and coccygeal region where the arches are deficient.
A small cartilaginous band forms the matrix of the subcentral portion or anterior arch of the atlas vertebra, quite distinct from that of the body of the axis, and out of the line of prolongation of the notochord.
In the lateral plates the cartilaginous matrices of the ribs are formed in connection with those of the transverse processes, and in the vertebral part of the ribs themselves ossification is comparatively early ; but a considerable part remains unossified in the sternal portion, or costal cartilages, in connection with their special use in the mechanism of the respiratory movements.
Certain portions of the transverse parts of the cervical and lumbar vertebrae are undoubtedly homologous with ribs ; but we give the name only to those costal bars which are separately articulated to the vertebra, and the first of the vertebras with which a rib reaching the sternum is articulated is reckoned as dorsal. Among the thoracic ribs a certain number, as elsewhere stated, of the cartilaginous matrices behind the first, are in the commencement united together at their ventral extremities into a strip of cartilage on each side, and thus the matrix of the sternum is at first cleft in two behind the pre-sternal portion. Subsequent fusion of these two lateral strips unites them into one ; and the transverse division of the bone only appears from the result of ossification in successive distinct centres. This fact possesses an interest in connection with the tendency of the meso-sternuui and xiphi-sternum to divide and to produce various degrees of the malformation termed fissura sterni.
Here aud elsewhere, unless otherwise explained, the terms used to indicate position apply to the primitive proue position of the embryo as it lies in the blastoderm, the dorsal aspect upwards and the ventral downwards.
In the lumbar region there is reason also to look upon part of the transverse processes as representing costal elements, but it is only in cases of abnormal formation that they are found distinct from the rest of the vertebra. (See the Descriptive Anatomy, Vol. I., p. 22.)
The sacrum is peculiar in presenting, thrust in and compressed between its strictly vertebral elements and the iliac bone with which it is united, several bony pieces which may be regarded as interposed ribs. The ossification of two of these occurs as early as the fifth or sixth month of foetal life.
The Head
The head of the embryo consists at first, as already stated, of the cranial part alone, the face, nose, and mouth being absent. Below the cranium, and extending as far forward as the point of junction of the anterior with the middle encephalic vesicle, is situated the pharyngeal portion of the primitive alimentary canal, closed in anteriorly by the inflection of the blastodermic layers. It is at this place that subsequently the opening of the alimentary canal to the exterior takes place in what constitutes ultimately the isthmus of the fauces ; and in front of this the buccal cavity, not yet existing, is afterwards formed.
In the progressive development of the head the principal changes by which its fundamental parts come into shape may be enumerated shortly as follows, viz., First, increase of deposit and textural differentiation of the mesoblastic substance for the formation of the cranial walls in their basilar, lateral, and upper portions ; second, the interpolation of the sense-capsules as connected with the formation of the rudiments of the nose, eye, and ear ; third, the development of the cerebral hemispheres and other parts of the brain from the three primary encephalic vesicles; fovrtli, the occurrence of the several cranial inflections ; and fifth, the new formation of outgrowths for the development of the parts of the face.
The Cranium
The basal portion of the cranium consists primarily of two fundamental parts. Of these the posterior is distinguished by the presence of the prolongation of the notochord within it as far forward as the part of the skull which afterwards becomes the pituitary fossa. This portion, which may be named occipito-sphenoid, is originally formed by the undivided investing mass of Ratlike, which surrounds the anterior extremity of the notochord, and contains the matrix of the future basi-occipital and basi-sphenoid cartilages. By its later extension to the sides, it forms the matrix of the exoccipitals and the periotic mass of cartilage which surrounds the primary auditory vesicles. The main part extends forward below the posterior and middle primary encephalic vesicles, ending at the pituitary fossa.
The anterior portion of the basis cranii may be called sphcno-ethmoid, as containing the matrix of the pre-sphenoid, and the ethmoid cartilages. It is mainly produced in connection with the trabeculae cranii, which contain between their separated limbs the pituitary fossa. This part of the cranial basis contains no prolongation of the notochord ; it lies below the anterior encephalic vesicle (thalamencephalon), and becomes greatly modified in connection with the expansion of the cerebral hemispheres and primary ocular vesicles, and the development of the nasal fossae and mouth, together with the other parts of the face.
Fig. 529.
Fig. 530. — The Lower or Cartilaginous part op THE Cranium op a Chick op thk Sixth Dat (from Huxley).
1, 1, chorda dorsalis ; 2, the shaded portion here and forwards is the cartilage of the base of the skull ; at 2 the occipital part ; at 3 the prolongations of cartilage into the anterior part of the skull called traheculce cranii ; 4, the pituitary space ; 5, parts of the labyrinth.
The primary parts of the three principal sense organs, it may here be stated, the nose, eye, and ear, formed in connection respectively with the cerebral hemispheres, the thalamencephalon, and the third primary vesicle, are interpolated between the rudimentary parts of the head as follows, viz., 1. The nose between the frontal, intermaxillary and ethmoid ; the eye between the frontal, sphenoid, ethmoid and maxillary ; and the ear between the basi-occipital, exoccipital and alisphenoid. "While the base of the cranium, to the extent already mentioned, is
Fig. 531. — View from below op the Cartilaginous Base of the Cranium with its Ossific Centres in a Human Foetus op about Five Months (from Huxley, slightly altered).
The bone is dotted to distinguish it from the cartilage, which is shaded with lines. 1, the basilar part, 2, the condyloid or lateral pvarts, and 3, 4, the tabular or superior part of the occipital surrounding the foramen magnum ; 5, centres of the pre-sphenoid on the inside of the optic foramen ; 6, centres of the post-sphenoid ; 7, centres of the lesser wings or orbito-sphenoid ; 8, septal cartilage of the nose ; 9 & 10, parts of the labyrinth.
cartilaginous in its origin, the lateral and upper walls are chiefly of membranous formation, as in the squama occipitis, the squamo-zygomatic of the temporal, the parietal and the frontal bones.
The trabeculai stretch forward to the anterior extremity of the head, and maintain the foremost place as the seat of the nasal cartilages and external apertures of the nose. Behind these the coalesced trabeculae form a narrow cthmo-vomerine cartilage, the nasal septum, round the back of which the vomer is formed as a bony splent covering ; while in the hinder lyre-shaped interval of the separated trabeculge is placed the infundibulum in connection with the pituitary body.
Fig. 532. — Basilar part op the primordial cranium of a human foetus of three months seen from above (from Kolliker).
a, iipi)cr half of the squama occipitis ; h, lower half of the same ; c, cai-tilaginous plate extending into it ; d, (in the foramen magnum) the exoccipital ; c, basi-occipital ; /, peti'ous, with the meatus anditorius interuus ; g, dorsum selliB, with two nuclei belonging to the basi-sphenoid bone ; /;, nuclei in the anterior clinoid processes ; i, great wing nearly entirely ossified ; I; small wings ; I, crista galli ; m, cribrethmoid ; n, cartilaginous nose ; o, strip of cartilage between the sphenoid and the parietal ; p, osseous plate between the lesser wings and the cribriform plate.
From the side of the presphenoid cartilage the matrix of the orbitosphenoids or lesser wings, containing the optic foramina, is developed ; and from the sides of the basi-sphenoid proceeds the matrix of the greater wings, which are also cartilaginous in their origin.
In the periotic or cartilaginous rudiment of the temporal bone three centres of formation are distinguished by Huxley, viz., 1. OpisthoUc, or that surrounding the fenestra rotunda and cochlea ; 2, prootic, or that which encloses the superior semicircular canal ; and 3, epiotic, or that which surrounds the posterior semicircular canal and extends into the mastoid portion. They soon unite into one so as to form the petromastoid bone.
Fig. 533. — Longitudinal Section through the Head of an Embryo op Four Weeks (from Kolliker). V, anterior encephalic vesicle, cerebral portion ; z, interbrain ; m, midbrain ; h, cerebellum ; n, medulla oblongata ; no and a, optic vesicle ; o, auditory depression ; t, centre of basi-cranial flexure ; t', lateral and hinder parts of tentorium ; p, the fold of epiblast which forms the hypophysis cerebri.
The styloid process and the auditory ossicles are of cartilaginous origin.
The squamo-zygomatic and tympanic are produced from membrane.
The Cranial Flexures
The earliest and the most important of the cranial flexures is that which takes place at the anterior extremity of the notochord and in the region of the mid-brain or middle encephalic vesicle. Here, as previously stated, the notochord extends into the substance of the basis of the cranium as it is prolonged forwards in the line of the vertebral bodies. At this place the medullary tube, and the substance forming the wall of the cranium especially, undergoes a sudden bending downwards and forwards, so as to cause the projection of the thickened cranial base in a marked manner upwards. This coincides with the place where the investing mass and the trabeculae meet, and where inferiorly the pituitary body, and superiorly the infundibulum are afterwards formed. The investing mass of blastema, in which the anterior extremity of the notochord is enclosed, and the notochord itself, terminate here behind the pituitary fossa, or what afterwards becomes that part, in a place corresponding to the dorsum selliB of human anatomy. Above and behind this, the middle cerebral vesicle forms the most prominent part of the cranium, which remains a characteristic feature of this part of the embryo head for a considerable time. Another early flexure of the cranium accompanies the development of the cerebellum from the third primary vesicle, a cleft now appearing behind and below the rudimentary cerebellum, in the region of the fourth ventricle, and above the medulla oblongata, and this flexure is necessarily attended with a convexity forwards, or another flexure in the place of the pons Varolii.
Fig. 534. Longitudinal Section of the Human Embryo at the sixth or seventh week
1, cerebral hemispheres
2, vesicle of the third venti'icle ; 3, mid-hrain ; 4, cerebellum ; 5, medulla oblongata ; c7i, notochord passing up through the bodies of the vertebrae into the basis cranii and terminating in the head between the infundibulum and the sac o£ the hypophysis cerebri ; s, the vertebral spines ; n, the spinal cord ; p, the phannix ; 7i, the heart ; I, the liver ; ?', the stomach and intestine; cl, the cloaca ; r, the urinary bladder and pedicle of the allantois ; it, u', the umbilicus containing the vitellointestinal duct, urachus and vessels ; between i, and /, superiorly, the Wolffian body is shown.
The great cranial flexure thus marks the division between the strictly basi-cranial, or occipito-sphenoidal, and the basi-facial, or sphenoethmoidal part, the chorda terminating between those two portions of the cranial base, with a conical and sharp point. Here the chorda is itself lient downwards and forwards, and terminates in a spot which corresponds to the post-sphenoid body, or dorsum sellas. According to Mihalkovics, who has recently investigated the subject with care (see Archiv filr j\Iikroskop. Anat., vol. xi., 1875,) in connection with the formation of the pituitary gland in mammals and birds, the chorda tapers off to a fine point in front of this spot, but presents a slight swelling just at the place of the future occipitosphenoidal suture.
The formation of the mouth, and its opening by the fauces into the pharyngeal or first part of the primitive alimentary canal, are phenomena of development intimately connected with the formation of the central part of the cranium and sella turcica, but they are also associated with the development of the face, which is next to be considered.
Formation of the Mouth and Hypophysis Cerebri
Along with the changes which accompany the formation of the principal cranial flexure, is associated in a remarkable manner the origin of a body (the pituitary gland or hypophysis cerebri) the nature and uses of which in the adult are entirely imkno-mi, but the constancy of whose presence, and the imiformity of its connections in the whole series of vertebrate animals, points to some important morphological relation.
Fig. 535.— Vertical Section of the Head in Early Embryos of the Rabbit, magnified {from Mihalkovics).
A. From an embryo of five millimetres long.
B. From an embryo of six millimetres long,
C. Vertical section of the anterior end of the notochord and i^ituitary body, &c., from an embryo sixteen millimetres long.
In A, the faucial opening is still closed ; in R, it is formed ; c. anterior cerebral vesicle ; mc, meso-cerebrum ; mo, medulla oblongata ; co, corneous layer ; to, medullary layer ; if, infundibuhim ; avi, amnion ; spe, spheno-ethmoidal, be, central (dorsum sellte), and spo, spheno-occipital parts of the basis cranii ; /;, heart ; /, anterior extremity of primitive alimentary canal and opening (later) of the fauces ; i, cephalic jrortion of primitive intestine ; tha, thalamus ; p', closed opening of the involuted part of the pituitary body (py) ; ch, notochord ; pk, pharynx.
The general nature of this body, in its joint connection with the infundibulum of the brain on the one hand, and a diverticulum of the alimentary canal on the other, was first pointed out by Eatlike (Miiller's Archiv, 1838. p. 482), although he afterwards abandoned the view there set forth. It was, however, fully confirmed by others ; and, among recent observers, we owe more especially to William Miiller an elaborate investigation of the whole subject (Jenaische Zeitschr., vol. vi., 1871), who traced most carefully the nervous and diverticular elements in their development, and their union with mesoblastic elements in the formation of the gland, Goette next ascertained that the diverticulum from below is connected with the buccal cavity and epiblast, and not with the pharynx and hypoblast, as was previously supposed (Archiv fur Miki-oscop. Anat., vol. ix., p. 397). The observations of Mihalkovics on Mammals complete the history of this point in development, and will be mahily employed in the following description.
The formation of this body may be shortly described as consisting in the meeting and combination of two outgrowths from very different fundamental parts ; one cerebral or medullary from above, and the other corneous or epiblastic (glandular), from below, in a recess of the cranial basis which afterwards becomes the pituitary fossa (fig. 53.5, B, <f,'py). The cerebral outgrowth, the posteiior of the two parts, takes place by the fonnation of a pointed projection downwards of a portion of the lower medullaiy waU of the vesicle of the third ventricle, and its firm adhesion to the base of the cranium. This is the commencement of the infundibulum. Meanwhile, a little in front of the same place, there is projected upwards from below a part of the basilar surface of the cranium, so as to form a deep recess lined by the corneous layer from the back and upper part of the future mouth. This recess is the commencement of the hypophysis or pituitary- body in its glandular portion, which is not, as has been supposed, a recess from the pharynx, seeing that it is in front of the opening which is afterwards formed for the fauces. The depressed and sharpened out anterior part of the notochord is directed downwards and forwards, while the sac of the hypophysis is can-ied upwards and backwards, and, according to Mihalkovics, the attenuated end of the chorda soon disappears from between the infundibulum and the hypophysis, previous to the occurrence of the intimate union which follows between these two bodies. The anterior extremity of the chorda, therefore, is lost in the floor of the pituitary fossa, and the swollen or dilated portion of the chorda which succeeds, and which comes then to form the apparent termination, occupies the interval between the basi-occipital and the basi-sphenoidal cartilages. The chorda traced back from this point, presents another swelling at the junction of the basi-occipital cartilage with that of the odontoid process, into which last it passes. The third swelling of the chorda lies between the odontoid cartilage, and that of the body of the axis vertebra.
Fig. 536. — Cranium and Human Embryo seen (from Ecker).
Face of the from before
A, from an embryo of about three weeks : 1, anterior cerebral vesicles and cerebral hemispheres ; 2, interbrain ; 3, middle or fronto-nasal process ; 4, superior maxillary plate ; 5, the eye ; 6, inferior maxillary or mandibular plate (first postoral) ; 7, second plate ; 8, third ; 9, fourth, and behind each of these four plates their respective pharyngeal clefts. B, from an embryo of five weeks : 1, 2, 3, and 5, the same as in A ; 4, the external nasal or lateral frontal process ; 6, the superior maxillary plate ; 7, the mandibular ; x , the tongue ; 8, the first phaiyngeal cleft, which becomes the meatus auditorius extenius.
The base of the skull, therefore, consists of two parts, one the posterior, in which the chorda is imbedded, and corresponding to the futui-e basi-occipital and basi-sphenoidal parts, the other in front of this, into which the chorda does not penetrate, the sphcno-ethmoidal, and which, according to the researchee of Parker and Gegenbaur, is of a later fonnation, and is more immediately related to the development of the face.
The flask-like outgrowth of the buccal epiblast which gives rise to the hypophysis cerebri, is now gradually shut off from the corneous layer and cavity of the mouth, iirst by the constriction, and subsequently by the closure of its place of communication. There remains however, for a considerable time, a longish thread of union between the two (fig. 535, C, 7/). The epithelium of the enclosed portion subsequently undergoes development into glandular coeca and cell-cords, and its internal cavity becomes gradually obliterated. This fonns the anterior part or lobe of the pituitary body. The posterior part owes its origin to the combination with mesoblastic tissue of a. widened extension of the infundibular process of the brain, which is thrust in between the sac of the pituitary body and the dorsum sella;. The nervous structure of this posterior lobe afterwards disappears in the higher animals, but in the lower it retains its place as a part of the brain.
Fig. 537. — Outline Plan view of the Upper Part of the Body of an Embryo Pig, two-thirds of an inch in length. Magnified seven diameters (from Parker).
Fig. 538. — Plan op the Skull, &c., of the same Ejibryo seen from below. Magnified ten diameters (from Parker).
In this and the preceding figure the letters, where present, indicate the following parts : —
c' to 0^ , the five primary divisions of the brain ; a, the eye ; n, the nose ; m, the mouth ; , cartilage of the trabeculae ; ctr, cornua trabecularum ; pn, prenasal cartilage ; 2 pterygo-palatine cartilage ; mn, the mandibular arch with Meckel's cartilage ; te, first visceral cleft which becomes the tympanoeustachian passage ; au, the auditory vesicle ; hi/, the cerato-hyoid arch ; Ir, the branchial bars and clefts, 1 to 4; tItJi, the thyro-hyoid ; py, the pituitary fossa ; ch, the notochord in the cranial basis, .surrounded by the investing mass (ii') ; vii, facial nerve ; ix, glosso-pharyngeal ; X, pneumog;istric ; xii, hypoglossal nerve.
Subcranial, Facial, or Pharyngeal Plates or Arches
In man, and all vertebrates, there are developed below and on the sides of the cranial part of the head, a series of processes or bars in pairs, which contribute to the formation of the subcranial structures constituting the face and jaws, and the hyoid and other parts intervening between the head and trunk. These bars first received attention from their discovery by Eatlike in 182G, published in the Isis of that year, and were named by him the branchial arches, from the relation ■which some of them bear to the gill bars of branchiate vertebrates. Their nature and transformations were fully investigated by Eeichert in 1837 (Miiller's Archiv, 1837). From later researches it appears that other processes, with somewhat similar relations to the cranium, occurring further forward, may be associated with those described as branchial by Eatlike, and it will be expedient therefore to describe the whole of the subcranial outgrowths together at this place. In this the views of Huxley and Parker will be chiefly followed. (See "On the Stracture and Development of the Skull in the Pig," by W. K, Parker, in Trans. Eoy, Soc. 1873, p. 289 ; Huxley, in Elements of Compai\ Anat., 1864, and Manual of Compar. Anat., 1871 ; also Gegenbaur, Das Kopfskelet, &c., 1872). According to these views the parts of the head situated in fi*ont of and above the future mouth, are formed from two pairs of plates, w^hich may thence be called preoral, in one pair of which the bars are the same "with the trahecukc cranii of Eatlike surrounding the pituitary gland, and are the basis of formation of the pre-sphenoid, ethmoid, nasal, and pre-maxillary portions of the skull, while in the other pair, consisting in each of a deeper and a superficial part, the bars form the foundation of the pterygo-palatine wall of the nose and mouth, and the superior maxillary bone. The nasal pits or primary nasal depressions, which extend themselves afterwards into the nasal fossae, and remain permanently open to the exterior, are formed by a depression of the surface of the epiblast in the anterior prolongation of the head as constituted by the ends of the trabeculas, and the harder structures of the septum and walls of the nasal cavities, as well as the turbinated structures on which the olfactory nerves are distributed, are all derived from the anterior parts of these trabecule ; — a mesial union giving rise to the nasal septum, while lateral parts circling round the nasal pits, form the alar enclosures of these depressions (see figs. 537 and 538).
Fig. 539. — Outlines showing the early changes in the form of the Head of the Human Embryo.
A, profile view of the head and fore i)art of the body of an embryo of about four weeks (from nature, '^^) : the five primary divisions of the brain are shown, together with the primai-y olfactory and optic depressions, and a, the auditory vesicle ; 1, marks the mandibular plate, and behind this are seen the three following plates with the corresponding pharyngeal clefts. B, from an embryo of about six weeks (from Ecker, f ) : the cerebral hemispheres have become enlarged and begin to spread laterally ; 1, the lower jaw ; 1', the first pharyngeal cleft, now widening at the dorsal end, where it forms the meatus externus ; the second cleft is still visible, but the third and fourth clefts are closed and *he corresponding plates have nearly disappeared. C, from a human fojtus of nine weeks (from nature, \) ; the features of the face are now roughly formed ; the first pharyngeal ■cleft is now undergoing conversion into the meatus, and the auricle is beginning to rise at its outer border.
The second preoral subcranial plates have received the name pterygopalatine from the nature of their deeper connection with the bar in which the pterygoid and palate-bones are afterwards formed. These enclose the posterior nasal apertures, and advancing from the two sides, at last meet each other in the palate, and in front meet the pre-maxillary process to complete the palate and upper jaw.
But these are only the deeper parts of the structure out of which the upper jaw is formed, there being on the surface of the head, and behind the depression of the eye on each side, a bulging process known as the superior maxillary process, in which the upper jaw and malar bone are formed, and which has externally the appearance of bending round the angle of the mouth in continuity with the mandibular or inferior maxillary.
The formation of the superficial parts of the face, as seen from before, may be described as follows, viz. : — In the middle, there descends in what now forms the region of the forehead, a mesial portion, the fronto-nasal plate, which forms the integument of the nose, as far as the inside of the nostrils, and the columella of the nose, together Avith the mesial part or lunula of the upper lip, that is, all the part lying inside the depression of the nostrils. On the outside of these depressions, a short lappet surrounds the orifice of the nostrils, as wings forming the external nasal processes. It is towards the outside of these last plates that the ocular depression is situated, that depression being thus interposed between the lateral or external nasal plate,- and the maxillary plate, and forming the fissure which has been tailed the ocular fissure, but which afterwards becomes more fetrictly the lachrymal in its anterior part (see figs. 536 and 539).
The great buccal aperture now passes across the face, having the middle and external nasal with the superior maxillary plates above and in front, and the inferior maxillary plates below and behind. It will be remembered, however, that the cavity of the mouth is thrown forward by the outward development of the subcranial plates, which deepen more and more the buccal cavity as they grow outwards from the primitive cranium.
The postokal pairs of pharyngeal visceral plates are four in number ; the first being that already mentioned as following immediately behind the mouth, and forming the mandibular or inferior maxillary.
At an early period of foetal life in the human foetus, and in that of all vertebrates, plates of this description are found, but in the lower vertebrates a greater number exists than in the higher. The number of four pairs belongs to man in common with all the non-branchiate vertebrates. Behind each of these plates there are formed from within, in the course of development, clefts which penetrate the wall of the pharynx ; these clefts, or so-called branchial apertures, running completely through the wall of the pliarynx and the external wall of the body of the embryo (see figs. 537 and 538).
The auditory pit, or primary depression from the epiblastic surface which forms the rudiment of the labyrinth of the ear, is situated immediately above the upper or proximal end of the two first postoral plates, and consequently on a level with the first postoral cleft. And this proximity of position is connected with the intimate relation in which two sets of parts stand to each other : for the part called the first branchial cleft is afterwards converted into the external and middle passages of the ear, (meatus, tympanum, and Eustachian canal), the membrana tympani being at a later period thrown across the passage. It forms thus the tympauo-eustachian cleft or canal. The tympanic bono is of membranous origin and is formed round the first cleft. The external auricle is of integumental origin, and is formed in the second postoral bar posteriorly and externally to the aperture of the first cleft.
The second postoral cleft is the first true water passage, or the first of those which serves as a gill aperture in branchiate vertebrates, and which may in the lower classes be increased to a greater number.
Although the description of some of the changes which the several pharyngeal plates or branchial arches undergo in the further process of development, belongs to a different part of the subject, yet it may be useful to describe sliortly the more important of them in this place.
In the first or mandibular arch a strong cartilaginous bar is formed known as the cartilage of Mcclcel, on the exterior of which, but not in its own substance, throughout a considerable part of its distal length, the lower jaw-bone is afterwards developed. The proximal part next the cranium, which comes later to be connected with the auditory capsule, becomes in mannnalia the malleus, in birds and reptiles chiefiy the OS quadratum. (Seefigures in connection with development of the ear.)
In the second or hyoid arch are developed the styloid process, the stylo-hyoid ligament, the lesser or upper coruua of the hyoid bone, the series of parts which connect them with the basis of the skull, being united like the first to the auditory capsule : but the proximal part of this arch would appear also to have the incus formed in it, and to be connected with the stapes and stapedius muscle.
The third arch is the thyro-hyoid, and is related to the formation of the lower or great cornua, and the body of the hyoid bone. It corresponds with the first true branchial arch of amphibia and fishes, in which animals the clefts and bars behind this arch become more numerous than in the higher vertebrata.
The fourth arch, which has no special name, but might be called suhhyoid or cervical, does not seem to form the basis of any particular organ, but is situated exactly at that part of the body which becomes elongated as the neck, — a part which may be considered as absent in the foetus, and the formation of which by a simple process of elongation gives rise to some peculiar features in the anatomy of the parts composing it.
Relations of Cranial Nerves
The rudiments of four cranial nerves, besides the optic and auditory afterwards to be referred to, are found at a very early period in connection with the plates now under consideration, and the following is the relation in which, according to Parker, they stand to these plates in all vertebrate animals. These nerves are the fifth pair or trip-eminus, the facial, the glosso-pharyngeal, and the pneumo-gastric. The two first are situated iu front, and the two latter behind the auditory sac. These nerves all divide or fork above a visceral cleft, one division going to the posterior face of the arch in front of the cleft, the other to the anterior face of the arch behind it.
Fig. 540. — Embryo of the Ciiioii at tub end of the Fourth Day (from Foster and Bcalfour).
The amnion has been removed ; Al, allantois ; Clf, cerebral hemispheres ; FS, thalamencephalon, with Pn, the pineal gland projecting from its summit; MB, midbrain ; Cb, cerebellum ; IV, V, fourth ventricle ; Z, lens ; chs, choroid slit ; C'en V, iiuditory vesicle ; sm, superior maxillary process ; IF, 2F, &c., first, second, third and. fourth visceral folds ; V, fifth nerve in two divisions, one to the eye, and the other to the first visceral arch ; VII, seventh nerve passing to the second visceral arch ; Gph, glosso-pharjmgeal nerve passing to the third visceral arch ; Pg, pneumo -gastric nerve passing to the fourth arch ; / V, investing mass ; ch, notochord ; Mt, the heart ; HIP, muscle plates ; W, wing ; HL, hind limb.
The orbito-nasal and the palatine divisions of the trigeminus belong to the trabecular arch, the former above, the latter below the optic nerve. Of the other division, one part (the superior maxillary nerve) follows the palato-pterygoid arch, the other (inferior maxillary nerve) accompanies the mandibular arch.
The facial nerve (portio dura of seventh pair) divides above the tympano-eustachian passage, its anterior part (chorda tympani) going to the posterior side of the mandibular arch, and its posterior part (descending branch of facial) to the outer or anterior side of the hyoid arch.
The glosso-pharyngeal nerve, by a similar division, goes by its inner or anterior branch (lingual) to the inner or posterior side of the hyoid arch, and by its other division (pharyngeal) to the front of the first branchial or thyro-hyoid arch.
In the higher animals the pneumo-gastric nerve shows no close relation to the clefts, but in branchiate vertebrates it is continued past the gills, and sends forked branches to the gill arches in front and behind each of the clefts.
Origin and Formation of the Limbs
The close connection of the limb-arches with certain vertebral segments of the trunk has been previously referred to in the morpholoo-ical remarks, given under the description of the bones and muscles m the first volume ; and although the vertebral homology of the parts of the limb proper is not so apparent, at least in the proximal segments, yet in the quinquifid division of the more remote parts, in the preaxial and postaxial arrangement of these divisions, and in their relation to the nerves and some other circumstances, we can scarcely fail to perceive some very near relationship between the structure of the limb as a whole, and a certain number of the vertebral segments of the trunk.
Fig. 541. — Human Embryo of about four weeks (froiu Kijllikei', after A. Thomson). | , the anterior limb rising as a semicircular plate from the lateral ridge. (The figure is elsewhere described. )
The limbs do not exist from the earliest time of the formation of the cranio-vertebral part of the trunk, but only begin to be formed when the development of the axial part of the body has made some advance, as in the first half of the fourth day of incubation in the chick, and at the commencement of the fourth week m the human embryo.
They first make their appearance as two pan-s of buds from the side of the vertebral part of the trunk, in the form of flattish lateral elevations with curved free margins projecting from the exterior of the body, outside the thickened ridge (sometimes called the Wolffian ridge) where the division of the mesoblast into somatopleure and splanchnopleure take place, and near the outer margins of the muscular plates. The anterior pair of limbs appears earlier than the posterior, and for a long time is always more advanced in the development of its parts.
The place of formation of the anterior and posterior limbs does not vary to any great extent throughout the vertebrate animals,— and this fact may be looked upon as one of the most marked features ot vertebrate organisation. .
The thickened plate which forms the commencing limb, by its increased growth, projects still more from the side, so as to take the form of a flattened lappet with a semicircular free margin ; presenting then two surfaces which may be named dorsal and ventral with reterence to their correspondence to the like surfaces of the trunk, constituting respectively the primitive extensor and flexor surfaces of the limb ; while the anterior margin of the semicircular lappet corresponds to the preaxial and the posterior margin to the postaxial borders of the future limbs.
The whole thickness of the somatopleure division of the mesoblast is involved in this primary limb-bud, and it is of course also covered with the epiblast or cuticular layer, in the substance of which there is considerable increase of thickness at the most prominent part of the margin.
As the limb-buds increase in size, the lateral limb-plate, or Wolffian ridge, which is at first very prominent in its whole length, becomes less, and gradually flattens down into the more even surface of the wall of the trunk.
The part of the limb which appears first, corresponds more immediately with the hand or foot than with the other divisions of the limb. Along with this, however, at a very early period there is an indication of the formation of the limb-girdles as folds passing off from the side
Fig. 542. — Diagrammatic Outline of the Profile VIEW OF the Human Embryo OF ABOUT seven weeks, TO SHOW THE PRIMItive relations of the Limbs to the Trunk. (Allen Thomson.) f
r, the radial (preaxial), and u, the iilnar (postaxial) border of the hand and forearm ; t, the tiljial (preaxial), and /, the fibular (postaxial) border of the foot and lower leg. (The foot is represented at a somewhat more advanced stage than the rest of the embryo).
of the trunk. As the projection of the limb increases from the side of the body, the distal or terminal segment becomes slightly notched off from the part next the trunk. This terminal part, forming nearly three-fourths of a somewhat circular flattened plate, contains the rudiments of the hand or foot. The next change which takes place is in the division of the proximal part, or rather the preaxial border and ventral surface, by a notch which separates the fore-arm and lower leg from the upper arm and thigh at the elbow and knee joint respectively. In the third stage the notched division of the free lateral curved margin, with intermediate slightly tubercular projections of the substance, shows the commencement of digital development, in which it soon becomes apparent that the pollex and hallux occupy the preaxial position in their respective limbs, and are followed by the series of other fiugers to the fifth, which is placed on the postaxial border. From these it is easy to trace, by reference to the simple original position of the limbs, the preaxial position afterwards held by the radius in the one and the tibia in the other, and the postaxial position of the ulna or fibula. In the meantime the internal differentiation of texture takes place, by which is brought out the more complete distinction of the segments of the limbs, and the various component parts of each, which gradually appear in the cartilages for the bones, muscular plates extended from the general muscular sheath of the trunk, prolongation of the cutaneous layer of the integument, the formation of nerves, blood-vessels, &c., the consideration of all which belongs to the history of more advanced development.
In order also to complete the history of the formation of the limbs, it is necessary to take account of the changes of attitude the anterior and posterior respectively undergo, as compared with the primary embryonic position. In this the elbow comes first to be turned outwards and then directed backwards, bringing the flexor surface of the upper arm forwards, while the position of the flexor surface of the fore-arm and hand, though generally and naturally inwards, may, by supination, be brought forwards, and by pronation backwards, the "latter being the permanent position given to the manus in most animals. In the hinder limb, again, the thigh is turned inwards, so that in the higher animals the flexor surface looks backwards, and in all animals the lower division of the limb is turned inwards and the sole of the foot downwards, so that the extensor surface and dorsum look forwards. (See vol. i., p. 122.)
Development of the Muscles
The muscles of the trunk derive their origin from the muscular plates previously referred to as being separated by differentiation of the formative cells in the outer or superficial part of the protovertel)raI masses. Some difference of o]iinion exists, however, among embryologists, as to how far the hypaxial (hyposkeletal of Huxlc}') as well as the epaxial muscles, proceed from this source alone, or whether only the latter are traceable to the muscular plate formed by the protovertebral differentiation, and the hyj^axial may be supposed to proceed from a deeper source.
Fig. 543. — Section through the Lumbar IIkgion of an EiiBRYO-CnicK of Four Days (from Foster and Balfour).
nc, neural canal ; jir, posterior root and ganglion of a spinal nerve ; ar, anterior root ; ■»«/>, muscle-plate; cfi, notochord ; WB, Wolffian ridge; AO, aorta; Vca, cardinal vein ; Wd, Wolffian duct ; Wb, Wolffian body with glomeruli ; ge, germinal epithelium ; Md, depression forming the commencement of the Miillcrian duct ; d, alimentary canal ; M, mesentery ; >S'0, somatopleure ; SP, splanchnopleure ; 1', blood-vessels ; P2h pleuro-peritoneal space.
Recent observations seem to show that a downward extension of the mesoblast from the protovertebrge may also give rise to the hypaxial muscles.
Being developed from the segmented protovertebral elements, the muscular plate shows at first the same division into segments, which ai-e separated for a time by intermuscular septa (myotomes) as occurs during life in a considerable number of them in fishes and amphibia.
The formation of the longer muscles of the trunk proceeds from the disappearance of the septa, and the longitudinal union of the fasciculi of successive myotomes. In the trunk the direction of these remains for the most part chiefly longitudinal, but those connected with the limb-girdles change their direction with the development of the limb.
The formation of the muscles of the limbs themselves has not been traced in detail. The greater number of these muscles appear rather to arise independently in the blastodermic tissue of the limb-bud, than to be prolonged from the sheets of trunk-muscles (Kiilliker).
The facial muscles and the platysma, belong to the subcutaneous system, and are developed along with the skin.
The diaphragm is at first wanting. It arises soon after the formation of the lungs, from two parts which spring from above and the sides, and which divide the pleural and peritoneal cavities, which were jDreviously in one, from each other.
The muscles begin to be formed in the human embryo in the sixth and seventh week.
Formation of the Joints
With regard to the formation of the joints, very little is known. It would appear that the cavities of the synovial joints are not yet formed at the time when chondrification has taken place in the matrix of the bones. It is therefore by a secondary process of solution of continuity that these cavities are produced. The articular cartilages remain as the coverings of the opposed surfaces of the bones, and the various ligamentous and other parts belonging to the joints arise by processes of textural differentiation which it is unnecessary to particularise here.
Distinction of Bones according to their Cartilaginous or Membranous Origin
There is here appended for the assistance of the reader a note of the distinction as regards their origin from cartilage or fibrous membrane of the .several permanent bones of the skeleton.
Bones arising from Cartilage
fl, In the Head.
Basi-occipital. ex-occipital, and part of the supra-occipital or squama occipitis.
The whole sphenoid except the cornua sphenoidalia.
The p.etro-mastoid or periotic portion of the temporal bone.
The mes-ethmoid and ethmo-turbinal.
The pterygo-palatine.
The malleus (quadi-ate of animals) with Meckel's cartilage.
The incus and stapes, with the stylo-hyoid.
The thyro-hyoid.
b. In the Trunk.
The bodies, arches, and processes of the vertebraa. The ribs and sternum.
c. In the Limbs.
The scapula and coracoid. The clavicle in part, and all the other bones of the upper limbs (excepting sesamoid).
The ilium, ischium, and pubis, and all the other bones of the lower limbs, including the patella, but excepting sesamoid of toe.
Bones arising from Fibrous Membrane
a. In the Head.
Part of the squama occipitis.
The frontal.
The parietal.
The squamo-zygomatic and tympanic of the temporal.
The nasal and lachrjTual.
The maxillaries and i3re-maxillaries.
The vomer and comua sphenoidalia.
The inferior or maxillo-turbinal.
The malar or jugal.
The inferior maxillary or mandibular.
b. In the Trunk. None.
c. In the Limbs.
The clavicle in part.
(The marsupial bone of animals.)
The smaller sesamoid bones of tendons.
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