Book - Quain's Embryology 6

From Embryology

Development of the Nervous System

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 Cerebro-spinal Centre

From what has been previously stated it will have been seen that the rudiment of the cerebro-spinal the medullary canal, not yet closed, shows at the cephalic extremity a partial division into the three jirimary cerebral vesicles ; and at the posterior extremity a rhoniboidal enlargement. Six proto-vertebral divisions are visible ; so, the upper division of the blastoderm, sp, the lower division.



Fig. 544. — Embryo of the Dog seen from above, with a portion of the Blastodekm attached (from Bischoff).


Fig-. 545. — Embryo of the Dog more advanced, seen from above (after Bischoff).

The medullaiy canal is now closed in ; c, the anterior encephalic vesicle ; o, the primitive optic vesicle in communication with the anterior encephalic ; au, the primitive auditory vesicle opposite the third encephalic vesicle ; am, the cephalic fold of the amnion enclosing the anterior third of the embryo ; ov, the omphalo-mesenteric vein entering the heart posteriorly ; in-, the proto- vertebral divisions, now become numerous.

nervous centre is formed more immediately from the thickened medullary plates of the involuted epiblast, the ridges of which, rising from the surface of the blastoderm, become united dorsally along the middle line into a hollow medullary tube of a cylindrical form. This tube becomes dilated at its anterior or cephalic extremity, and this dilated portion becomes divided by two partial constrictions into the three primary cerebral or encephalic vesicles, which, as representing fundamentaf portions of the brain, have been termed the fore-brain, mid-brain and hind-brain. The spinal portion retains its more uniform cylindrical shape, escepting towards the caudal extremity, where it is longer of


Fi<r. ZiPj.


Fig. 546. — Transverse Section THROUGH THE EjlBRYO OF THE ChICK, AND

A, Blastoderm at the end of the First Day. Magnified FROM 90 TO 100 TIMES (from Kolliker).

d'/> tUuh d d i,a-p c/i /i, epiblast ; (?c?, hypoblast ;

sp, mesoblast ; Pv, medullary

groove ; m, medullary plates ;

c7i, chorda dorsalis ; utrp, proto-vertebral plate ; uw7i, commencement of division of mesoblast into its upper and lower laminte ; between i?/ and h the dorsal laminte or ridges which by their approximation close in the medullary canal closing, and forms for a time a flat open rhomboidal dilatation. The continuous cavity enclosed within the primitive medullary tube is the same with that which, variously modified, afterwards constitutes the central ventricles of the brain and canal of the spinal cord.

The formative cells composing the medullary substance are at first spherical, but they afterwards become elongated and spindle-shaped, and increase rapidly by multiplication. They represent at first the grey substance, or the nerve-cells and non-medullated fibres. The cylindrical cells which, from the first, line the whole canal, remain permanently in the part of it which forms the central canal of the spinal marrow, and frequently present the ciliated structure.



Fig. 547. — Transverse Section of the Embryo Chick on the Second Day. Magnified FROM 90 to 100 times (from Kolliker).

The explanation of the letters is in part the same as in the foregoing figure, mr, the medullary tube now closed along the dorsal line and covered in by continuous epibkist ; nwh, hollow of the proto-vertebral mass ; mp, mesoblast external to the protovertebroe dividing into hpl, somatopleure, and df, splanchnopleure ; ao, one of the primitive aortas ; iinff, intermediate mass connected with the origin of the Wolffian body.


The internal grey substance of the spinal marrow is first formed; the white substance is produced later on the exterior. The sides acquire considerable increased thickness, while the dorsal and ventral parts remain comparatively thin, so that the cavity assumes the appearance in section of a slit, which becomes gradually narrower as the lateral thickening increases ; and at last the opposite surfaces uniting in the middle divide the primary central canal into an anterior or lower and posterior or upper part (see figs. 547 and 548).

The lower of these divisions becomes the permanent central canal, the upper or dorsal is afterwards so far obliterated that it is filled with a septum of connective tissue belonging to the pia mater, and becomes the posterior fissure of the cord (in human anatomy). (Lockliart Clarke, Phil. Trans. 1862.)

In birds and mammals there is no distinction to be seen at first between the outer or corneous layer of the involuted epiblast and the cells which by their increase more immediately constitute the medullary plates. In batrachia, however, the dark colour of the corneous layer shows it to be distinct from the more strictly nervous layers. In osseous fishes there is no medullary groove or canal at first, but an involution of a solid column of cells, which is subsequently hollowed out for the formation of a ventricular cavity.

The masses of grey matter first formed in the spinal marrow correspond chiefly to the anterior columns ; these are succeeded by lateral masses or columns, and somewhat later by small posterior columns. There are at first no commissures except by the passage of the deepest layer of cells across the middle line, but the fibres from the roots of the nerves when formed are traceable into the grey substance of their rcs])cctive anterior and posterior columns.


Fig. 548. — Transverse Section of the CervicaIi Part of the Spinal Cord of a Human Ejibryoop Six Weeks (from Kolliker). 'f

Tins and the following figure are only sketched, tJie white matter and a part of the gre.y not being shaded in. r, central canal ; e, its epithelial lining, at e (inferiorly), the i)art which becomes the anterior comniissure ; at c (superiorly), the original place of closure of the canal ; a, the white substance of the anterior columns, beginning to be separated from the grey matter of the interior, and extending round into the lateral column, where it is crossed by the line from [/, which points to the grey substance ; p, posterior column ; ar, anterior roots ; j5r, posterior roots.


The white substance is formed external to or on the surface of the deeper grey substance j but it is not yet determined whether it is developed out of the cells composing the grey matter or from separate blastema to which the mesoblast may in part contribute. It is combined with connective tissue elements, and its structure is <lifferent from that of the grey substance, which is undoubtedly produced by multiplication and differentiation of the involuted epiblastic cells.


Fig. 549.— Transverse Section of Half THE Cartilaginous Vertebral Column AND THE Spinal Cord in the Cervical Part of a Human Embryo op from NINE to ten weeks (from Kolliker) '~

c, central canal lined with epithelium ; a, anterior column ; ^j, posterior column ; p', band of Goll ; [/, ganglion of the posterior root ; p r, posterior root ; a r, anterior root passing over the ganglion ; d m, dura-matral sheath, omitted near p r, to show the posterior roots ; b, body of the vertebra ; cl), chorda dorsalis ; n a, neural arch of the vertebra.


On the fifth and sixth days in the chick, according to Foster and Balfour, the white columns increase rapidly in size, and the anterior median fissure begins to be formed between the anterior columns by their swelling outwards and leaving its interval between them. It is at first wide and shallow and soon receives a lining of vascular connective tissue or pia mater. The commissures are now also formed ; the anterior grey commissure first, then the posterior grey, and somewhat later the anterior white commissure.

In the further increase of the anterior and lateral white columns as they thicken, they become more united together on each side, so that they can only be arbitrarily distinguished ; the fibres of the roots of the nerves are traced through them into the grey matter ; the cornua of grey matter become more and more developed, and the fissures between the white columns deepen, while the connective tissue or pia-matral septa run more completely inwards through the white substance.

Angular cells with radiating processes make their appearance in the grey matter, and the nerve-fibres both of the grey and white matter become more distinct.

The cylindrical cells lining the central canal retain their distinctness, and they are more completely separated from the grey matter by the delicate tissue of the ependyma. Throughout the greater part of the spinal marrow the dorsal part of the primary medullary hollow is obliterated to form the fissure, but in the sacral region of birds it opens out in the rhomboidal sinus, and in the filum tcrminale of the human spinal marrow the whole primary medullary cavity remains.


The spinal cord has been found by Kolliker already in the form of a cylinder in the cervical region of an embiyo four weeks old. Un-united borders have been seen by Tiedemann in the ninth week towards the lower end of the cord, the perfect closing of the furrow being delayed in that part, which is slightly enlarged, and presents a longitudinal median slit, analogous to the rliomboidal sinus in bii-ds.

The anterior fismirc of the cord is developed very early, and contains even at fii-st a process of the pia mater.


Fig. 550. — Transverse Section OF Half of the Spinal Cord of the Chick OF Seven Days (from Foster and Balfour). Magnified.

pciv, posterior, lew, lateral, and acw, anterior •white columns ; pc, jjosterior cornu of grey matter with small cells ; ac, anterior grey cornu with large cells ; ep, epithelium of the canal ; c, the upper part now open and filled with tissue in the posterior fissure ; S2')c, the lower division of the primitive medullary cavity, which remains as the permanent canal ; «/, anterior fissure left between the ]3rojecting anterior columns ; ajc, anterior grey commissure.


aye

The cervical and lumhar cnlarricmcnts, opposite the attachments of the brachial and crural nei-ves, appear at the end of the third month : in these situations the central canal, at that time not filled up, is some'ndiat larger than elsewhere (see figs. 556 and 5.58).

At first the cord occupies the whole length of the vertebral canal, so that there is no Cauda equina. In the fourth month the veitebraB begin to grow more rapidly than the cord, so that the latter seems as it were to have been retracted within the canal, and the elongation of the roots of the nerves which gives rise to the Cauda equina is commenced. At the ninth month, the lower end of the cord is opposite the third lumbar vertebra. (Kolliker, Ent'wickelungsgeschichte ; Lockhart Clark in the Phil. Trans. 1862; Bidder und Kupfer, Untersuch. iib d. Eiickenmark, Leipz., 1857. Foster and Balfom-, Elements of Embryology.)

Till lately it was believed that the roots and ganglia of the spinal nerves are at first distinct from the medullary substance of the cord, ;and that they originate by differentiation of cells in the mesoblastic f5ubstance of the protovertebral plate. But recent observations, to bo more particularly referred to hereafter, have shown that they arise in 2)art at least in close connection with the spinal cord itself.


The Brain 0r Encephalo

1. — General phenomena of development as ascertained in birds and mammals. — A reference has previously been made to the simple form in which the brain at first presents itself in the anterior dilated portion of the primitive medullary tube, and its partial division into the three primary cerebral vesicles. This is placed within simple cranial walls formed by the cephalic inflection of the blastoderm, without face or any other parts ; so that the head of the embryo consists at first of no more than the wider part of the medullary tube and the simple enclosing wall.


Fig. 551. — Four Views of the Brain of an Embrto-kitten in the Stage of First Division into the Five Cerebral Eudiments, magnified Three Diameters (from Reichert).

A, from above ; B, from the side ; C, vertical section showing the interior ; D, from below.

1, Cerebral hemisphere, prosencephalon ; 2, thalamencephalon ; 3, mesencephalon, still single ; 4, cerebellum, epencephalon ; 5, mj-elencephalon, medulla oblongata ; o, optic nerves ; V, fifth ^pair ; "VIII, eighth pair or glossopharyngeal and pneumogastric ; i, infundibulum ; v, v', general ventricular cavity, opening at v, into the lateral ventricle by the foramen of Monro.

In the base of this wall, it will be remembered that the notochord extends forward beneath the posterior and middle of the vesicles, and occupies, therefore, the part of the cranium corresponding to the occipito-sphenoidal basis, while the trabecute cranii, developed forwards



Fig. 552. — Vertical Sections of Embryonic Brains in two Stages of Transition from the Rudimentary Condition, Magnified THREE Diameters (from Eeichert).

A, Brain of the embryo pig in commencing state of transition. 1, Right cerebral hemisphere ; 2, thalamencephalon and position of the pineal gland ; 3, midbrain, with a large cavity ; /, foramen of Monro ; i, infundibulum ; 4, cereliellum ; 5, medulla oblongata.

B, Brain of the embryo of the cat more advanced, c, Cerebral hemisphere passing backwards so as to cover the other parts in succession ; I, olfactory bulb ; II, optic ner^'e ; th, thalamus opticus ; /, foramen of Monro ; cc, cor25US callosum ; ^3, pineal gland ; i, infundibulum ; cq, corpora quadrigemina, not yet divided ; 3, third ventricle ; cr, crura cerebri, the aqueduct of Sylvius, now reduced in width ; c', cerebellum ; 4, fourth ventricle ; jpv, Pons Varolii ; mi, medulla oblongata.

from below the anterior vesicle, are prolonged in the anterior or sphenoethmoidal part. The latter cerebral rudiment, therefore, which corresponds to the thalami optici and third ventricle, and which may with Huxley be conveniently called thaJamcncepiialon, is at first the foremost part of the brain, and the region of the ijituitary fossa lying below it is the foremost part of the cranial basis. The manner in which the development of the trabecule and other elements of the face modifies the form of this re,e,'ion of the head has already been adverted to, aud need not be repeated here.

As regards the earliest phenomena of development in the brain itself, there are three changes which mainly tend to modify its form in the most marked degree, viz., 1st, the development from the anterior vesicle on each side of the primitive ocnlar vesicle ; 2nd, the expansion from another part, somewliat later, of the vesicles of the cerebral hemispheres; and 3rd, the formation in the forepart of the posterior encephalic vesicle of a new cerebral rudiment corresponding to the cerebellun).


Fig. 553. — Sketches op the Primitive Parts OF THE Human Brain (from Kolliker).

1, 2, and 3 are from the human embryo of about seven weeks. 1, view of the wliole embryo from behind, the brain and spinal cord exposed ; 2, the posterior, and 3, the lateral view of the brain removed from the body ; h, the cerebral hemisphere (prosencephalon) ; i, the thalamenceiihalou ; i', the infundibulum at the lower part of the same ; m, the middle ^jrimary vesicle (mesencephalon) ; c, the cerebellum (epencephalou) ; m 0, the medulla oblongata. Figure 3 shows also the several curves which take place in the development of the parts from the primitive medullary tube. In 4, a lateral view is given of the brain of a human embrjo of three months : the enlargement of the cerebral hemisphere has covered in the optic thalami, leaving the tubercula quadrigemina apparent.

The formation of the primitive ocnlar vesicles, by an evolution of the lateral wall of the primitive medullary tube, gives to the first vesicle and the adjacent part of the head a much greater lateral width ; but the cranial wall, though pushed out by the enlarging oculai: vesicles, does not follow closely the inflection of their surfaces. As the subsequent contraction of the stalk of the ocular vesicles progresses, these vesicles are thrown more backwards and downwards by the change next to be described.

Tills is the evolution or expansion of the wall of the anterior encephalic vesicle into the two cerebral hemispheres, which takes place in front and at each side, so that the vesicles of the right and left hemispheres are from the first separate and distinct. As these vesicles become dilated, the cranial wall undergoes a corresponding expansion in the forepart of the head, and the vesicle of the thalamencephalon, which was at first the foremost part of the embryo-head, is thrown downwards and backwards into a deeper position.

The middle encephalic vesicle, increasing greatly in size, takes the most prominent part of the head superiorly, both from its own greatei relative magnitude, and from the sudden bend which the head now takes below this vesicle in the great cranial curvature.

The formation of the cerebellum begins by a thickening in the upper and lateral walls of the part of the posterior primitive vesicle which is next to the midbrain, and is accompanied by a deep inflection of the medullary tube between it and the remaining part of the vesicle which forms the medulla oblongata.


There are thus distinguished the rudiments of five fundamental constituents of the brain, under which it will be found convenient to bring the notice of the development of the several parts forming the full grown organ, and which m^y in this association be shortly enumerated as follows, viz. : —

1. The cerebral hemispheres, with their ventricular hollows or lateral ventricles, the corpora striata, and the olfactory lobes, — a set of parts to which, as a whole, the name of proceredrum or j)rosejice2}haIo?i may be Q-iveu.



Fig. 554. — Sketches of the early form OF THE PARTS OF THE CeREBRO-SPINAl Axis in the Human Embryo (after Tiedemann).

A, at the seventh week, lateral view ;

1, spinal cord ; 2, medulla oblongata ; 3, cerebellum ; 4, mesencej^halon ; 5, 6, 7, cerebrum. B, at the ninth week, posterior view; 1, medulla oblongata; 2, cerebellum ; 3, mesencephalon ; 4, 5, tlialami optici and cerebral hemispheres. C and D, lateral and posterior views of the brain of the human embryo at twelve weeks, a, cerebrum ; b, corpora quadrigemina ; c, cerebellum ; d, medulla oblongata ; the thalami are now covered by the enlarged hemispheres. E, posterior view of the same brain dissected to show the deeper parts. 1, medulla oblongata ; 2, cereliellum ; 3, corjDora quadrigemina ; 4, thalami optici ; 5, the hemisphere tuined aside ; 6, the corpus striatum embedded in the hemisphere ; 7, the commencement of the aorpus callosum. F, the inner side of the right half of the same brain separated by a vertical median section, showing the central or ventricular cavity. 1, 2, the spinal cord and medulla oblongata, still hollow ; 3, bend at which the pons Varolii is formed ; 4, cerebellum ; 5, lamina (supierior cerebellar peduncles) passing up to the corpora quadrigemina ; 6, crura cerebri ; 7, corpora quadrigemina, still hollow ; 8, third ventricle ; 9, infundibu lum ; 10, thalamus, now solid ; 11, optic nerve ; 12, aperture leading into the 1 ventricle ; 13, commencing corpus callosum.


2. The thalamencephalon with its cavity or third ventricle, the primary ocular pedicles, and the infundibulum.

3. The mcsenceplialon, which is the same with the original middle vesicle, and comprises the corpora quadrigemina and crura cerebri with its contracted internal hollow, the iter a tertio ad quartuni ventriculum of human anatomy.

4. The next part in succession is the cerehellimi, along with which is included the pons Varolii and the fourth ventricle.

5. The hinder part, which passes into the spinal marrow, is the medulla oblongata, with the continuation of the medullary cavity in the fourth ventricle and into the central spinal canal.

In these five fundamental parts or rudiments of the brain, arising out of very simple modifications of the primitive medullary tube, it is mainly by an increased thickening of the medullary wall in some of the parts, and the relative thinning, or even the removal of the substance in others, that the changes accompanying the formation of the cerebral masses are effected, while as a consequence of these and other modifications of form, the several parts of the internal cavity or ventricles of the brain acquire the different degrees of expansion and contraction, or the comparatively closed or open condition which they exhibit in after life. Thus the cerebral hemispheres, and corpora striata are the main masses formed by the lateral thickening and expansion of the medullary walls of the procerebrum, while the corpus callosum and fornix are formed later by a deeper median development in connection with these parts : the thalami optici are the most solid parts of the lower and lateral region of the second rudiment, while in the upper wall the pineal gland, and in the lower the infundibulum with the hypophysis cerebri, are added : the corpora quadrigemina are thickenings of the upper wall of the third rudiment, while the crura cerebri arise by increased deposit in its lower part ; the cerebellum is a large deposit in the upper wall of the fourth radiment, while the pons Varolii is a thickening of its lower wall ; and the parts composing the medulla oblongata are principally formed by increased deposit in the lower and lateral wall of the fifth rudiment.

Thus, also, the lateral ventricles are two lateral expansions of the forepart of the original ventricular cavity which follow the dilatation of the vesicles of the right and left cerebral hemispheres, and communicate with the central or third ventricle by the common foramen of Monro. The central or third ventricle, originally the foremost part of the medullary hollow, is narrowed on the sides by the increased development of the thalami optici, while inferiorly it is prolonged and projects downwards as infundibulum into the pituitary fossa ; and above the wall of this ventricle comes to be opened up by the thinning away of its medullary substance, excepting at the place where it is crossed by the pineal gland. The continuation backwards of the original ventricular hollow, greatly narrowed by the ultimate thickening of the substance of the corpora quadrigemina and crura cerebri, forms the aqueduct of Sylvius, or the iter a tertio ad quartura ventriculmn, and is succeeded by the more expanded cavity of the fourth ventricle, lying between the cerebellum and the lower wall. The upper wall of the latter cavity undergoes great thinning, like that of the third ventricle, so as to be reduced m the part before the cerebellum to the valve of Vieussens, and in the part behind it to be covered only by membrane, and to present an opening from the cavity into the posterior sub-arachnoid space.



Fig. 555. — Vertical Section of thr Brain of a Human Embryo of fourteen weeks, Magnified THREE Diameters (from Reicliert).

c, cerebral bemispliere ; cc, carpus callosum beginning to pass back , /, foramen of Monro ; p, membrane over tbe third ventricle and the pineal gland ; tk, thalamus opticus ; 3, third ventricle ; I, olfactory bulb ; cq, corpora quadrigemina, mesencephalon : cr, crura cerebri, and above them the aqueduct of Sylvius still wide ; c', cerebellum, and below it the fourth ventricle ; _/)c, Pons Varolii ; m, medulla oblongata.


From what has before been said of the relation of the fundamental parts of the brain to the basis of the skull, it will be seen that the cerebral development is intimately connected also with the great cranial flexure which occurs at the pituitary fossa ; for while the infundibular prolongation of the thalaraencephalon projects down into this fossa, and the lamina terminalis rises in front in the position of the original foremost part of the encephalon, certain parts of the brain may be considered as situated posterior to this point, viz., the mesencephalon with crura cerebri, cerebellum with pons Varolii, and medulla oblongata, while the cerebral hemispheres, with the corpora striata, corpus callosum, and fornix, notwithstanding their enormous proportional development, may be considered as formed by forward expansion, and as situated in front of this turning point. But though the connections of the cerebral hemispheres with the rest of the brain may thus be considered as anterior to the cranial centre, and while in their earlier stages, and still of small size, they are actually placed as in the lowest Vertebrates, entirely in front of it, yet by the later great proportional development in the higher animals, and especially in man, the cerebral hemispheres come to progress backwards, and successively to cover superiorly the thalami, corpora quadrigemina, the cerebellum, and the medulla oblongata.

The connection of the several parts of the brain, with the five fundamental parts respectively, may be stated in the following tabular form :

I" Cerebral Hemispheres, Corpora Striata,

{1. Prosencephalon.* s Corpus Callosum, Fornix, Lateral Ven(. tricles, Olfactory bulb (Rhiuencephalou). 2. Thalamencephalon. f Thalami Optici,Pinealglaud,Pituitai-y body, (Diencephalon.) ( Third Ventricle, Optic nerve (iirimarily).

ir Middle urimarv Vesicle •? Mesenceiilnlon J Corpora Quadrigemina, Crura Cerebri, Aque11. -MKime primal y vesicle, o. Jiebencepiiaion. j duct of Sylvius, Optic nerve (secondarily).


III. Posterior primary Vesicle,

Cerebellum, Pons Varolii, anterior part of f- 4. Eiicncephalon. \ the Fom-th Ventricle.

"j ( Medulla Oblongata, Fourth Ventricle, Au 1.5. Meteneephalon. ( ditory nerve.

The changes which take place in the growth of the brain were first elaborately described by Tiedemann ; they have been investigated by Von Baer, Bischoff, Remak, Reichert, Kolliker, and others. (Tiedemann. " Anatomic und Bildiingsgeschichte des Gehu-ns," Nilremberg, 1816 ; Reichert, Bau des Menschlichen Gehirns," Leipzig, 1859, 1861 ; F. Schmidt, '• Beitragez. Entwick. des Gehims," in " Zeitschr. f. Wissen. Zool.," 1862 ; Kolliker, " Entwicklmig,sgeschichte," 1861,)

FARTHER DEVELOPMENT OF THE BRAIN IN MAN AND MAMMALS.

The medulla oblongata is not completely closed in above bj^ nervous matter. The open part of the medullary tube constitutes the floor of the fourth rent ride, which communicate.^ below with the canal of the spinal cord at the place where the calamus scriptorius is eventually formed, and opens posteriorly into the subarachnoid space.

  • TliLs and the four following terms are adopted as applicable to the principal secondary divisions of the primordial medullary tube, and a.s corresponding to the commonly received names of the German embryologists, viz., Vorderhirn, Zwischenliirn, Mittelhirn, Hinterhirn, and Nachhirn ; or their less used English translations, viz., forebrain, interbraiu ('tweenbrain), midbrain, hiudbrain, and afterbrain.


The three constituent parts of the medulla oblongata beg-in to ]je distrnguisheJ about the thii-d month ; first the rr.stiform bodies, which are connected with the commencing cerebellum, and afterwards the anterior pjTamids and olives. The anterior puminidx become prominent on the surface and distinctly defined in the fifth month ; and by this time also their decussation is evident. The oUranj fasciculi are early distinguishable, but the proper olirary hochj. or tubercle, doe's not appear till about the sixth month. The fam'wlai c'merea; of the fourth ventricle can be seen at the fomth or fifth month, bub the nvhltc strue not until after birth. •

Cerebellum

In the human embryo the cerebellum exists at the end of the second month, as a delicate medullary lamina, forming an arch behind the corpora quadrigemina across the widely open primitive medullary tube.

According to Bischoff, the cerebellum does not commence, as was previously supposed, by two lateral plates which grow up and meet each other in the middle line ; but a continuous deposit of nervous substance takes place across this part of the medullary tube, and closes it in at once. This layer of nervous matter, which is soon connected with the coi-pora restiformia, or inferior peduncles, increases gradually up to the fourth month, at which time there may be seen on its under sui'face the commencing corpus th-ntatvm. In the fifth month a division into five lobes has taken place ; at the sixth, these lobes send out folia, which are at first simple, but aftei-wards become subdivided. Moreover, the hem i. '<])?! eres of the cerebellum are now relatively larger than its median portion, or worm. In the seventh month the organ is more complete, and thejfocoihis and 2Mt<terior rein in, with the other parts of the mferior vennifoiin process, are now distinguishable, except the amyrjdalo', which are later in theii' appearance. Of th.ej}eil-incles of the cerebellum, the inferior pair (corpora restiformia) are the first seen — viz., about the thii-d month ; the middle peduncles are perceptible in the foui-th month ; and at the fifth, the superior pedmicles and the Vieussenian valve. The pons VfiroHi is formed, as it were, by the fibres from the hemispheres of the cerebellvun embracing the pyramidal and olivary fasciculi of the meduUa oblongata. According to Baer, the bend which takes place at this part of the encephalon thrusts down a mass of nervous substance before any fibres can be seen ; and m this substance transverse fibres, continuous with those of the cerebellum, are afterwards developed. From its relation to the cerebellar hemispheres the pons keeps pace with them in its gi'owth ; and. in conformity with this relation, its transverse fibres are few, or entirely wanting, in those animals in which there is a con-esponding deficiency or absence of the lateral parts of the cerebellum.


Fig. 556. — Brain and Spinal Cord exposed feom behind A FcETus OF THREE MONTHS (from Kolliker).

h, the hemispbei-es ; in, the mesencephalic vesicle or corpora qnadrigemina, c, the cerebellum ; below this are the medulhi oblongata, mo, and fourth ventricle, with remains of the menibrana obturatoria. The spinal cord, s, extends to the lower end of the sacral canal and presents the brachial and crural enlargements.


Parts connected with, the Mesencephalon. — The eorjioni qvndricfemina are formed in the upper part of the middle cephalic vesicle ; the hollow in the interior of A^diich communicates with those of the first and thhd vesicles. the coi-pora qnadrigemina, in the early condition of the human embryo, are of great proportionate volume, in harmony with what is seen in the lower vertebrata ; but subsequently they do not grow so fast as the anterior parts of the encephalon, and are therefore soon overlaid by the cerebral hemispheres, which at the sixth month cover them in completely. Moreover, they become gradually solid by the deposition of matter within them : and as, in the meantime, the ccrchval jwdunclcs are increasing rapidly in size in the floor of this

Fig. 557. — Brain of the Human EmBRYO OF THREE MONTHS. NATURAL SIZE (from Kolliker).

In 1 the view is from above, the Upper fiart of the cerebral hemispheres and mesencephalon having been removed. /, fore-part of the divided wall of the hemisphere ; /', hind part of the same turned in which becomes the hippocampus ; est, corpus striatum ; tha, thalamus opticus.

In 2 the lower surface is represented \ to, ti'actus opticus ; and in front of this the olfactory bulbs and tracts ; c m, single mass of the corpora mammillaria not yet divided ; p, pons Varolii. The cerebellum and medulla oblongata are seen behind and to the sides in both figures.

middle cephalic vesicle, the cavity in its interior is quickly filled up, with the exception of the narrow passage named the SijJrian aqueduct. The fillet is distinguishalble in the fourth month. The corpora quadrigemma of the two sides are not marked off from each other by a vertical median groove until about the sixth month ; and the transverse depression separating the anterior and posterior pau-s is first seen about the seventh month of intra-uterine life.


Fig.. 558. — Brain and Spinal Cord op a Foetus of four MONTHS, seen from BEHIND (from Kolliker).

h, hemispheres of the cerebrum ; m, corpora quadrigemina or mesencephalon ; f, cerebellum ; m o, medulla oblongata, the fourth ventricle being overlapped by the cerebellum ; s s, the spinal cord with its brachial and crural enlargements.


Thalamencephalon

From the sides of this vesicle, as has already been described, the optic vesicles are fonned. and from its forepart on the two sides the vesicles of the cerebral hemispheres are developed. Reichert first pointed out that there is left between the hemu2)?ier(Tcslclcs of opposite sides a wedge-shaped interval, which forms the thu-d ventricle. He points out that the tenninal extremity (lamina terminalis) of the cerebrospinal tube is at the tip of this wedge, and is placed immediately in front of the optic commissure, at the lamina cinerea ; and that therefore the infundibulum is not that extremity, as had been previously supposed by Baer, but is an expansion of the vesicle downwards. The formation of the pituitary body has akeady been described. The infundibulum of the thalamencephalon

becomes connected with it superiorly, and seems for a time even to form a part of it.

The pineal gland, according to Baer, is developed from the back part of the thalami, where those bodies continue joined together ; but it is suggested by Bischoff that its development may be rather connected with the pia mater. It was not seen by Tiedemann until the fourth month in the human foetus ; subsequently its growth is very slow : and it at first contains no gritty deposit : this, however, was found by Sommeiiing at birth.


The two optic thalamic fonned from the posterior and outer part of the anterior vesicle, consist at first of a single hollow sac of nervous matter, the cavity of which communicates on each side in front with the anterior of the commencing their interior behind, below, and at the sides, the thalami become solid, and at the same time a cleft or fis&ure appears between them above, and jjenetrates down to the internal cavity, which continues open at the back part opposite the entrance of the Sylvian aqueduct. This cleft or fissure is the tJiird rentriclc. Behind, the two thalami continue united by the jyoxfrrinr C(i/ii»iix.\ure, -which, is distinguishable about the end of the third month, and also by the ped7i?icle.i of the 2>i»ii glnnrl. The Koft commissure probably exists from an early period, although it could not be detected by Tiedemann until the ninth month.


Fig. 559. — Remimagrammatic Views of the Inner Surface op the Right Cerebraii Hemisphere of the Fcetal Brain at various stages of development (from Schmidt).

1, 2, and -3, are from foetuses of the respective ages of eight, ten, and sixteen M'eeks ; 4, from a foetus of six months, a, lamina terminalis or imrt of the first primary vesicle which adheres to the sella turcica ; b, section of the cerebral i)eduncle as it passes into the thalamus and corpus striatum ; the arched line which surrounds this bounds the gi-eat cerebral fissure ; c, anterior part of the fornix and the sej)tum lucidum ; d, inner part of the arch of the cerebrum, afterwards the hippocampus major and posterior part of the fornix ; (, corpus callosum very short in 3, elongated backwards in 4 ; in 4, /, the superior marginal convolution ; /', fronto-parietal fissure ; g, gyrus fornicatus ; p', the internal vertical fissure descending to meet the fissure of the hippocampus ; I, olfactory bulb ; F, P, 0, T, frontal, parietal, occipital and temporal lobes.

cerebral hemispheres, and behind with that of the middle cephalic vesicle (corpora quadrigemina). Soon, however, by increased deposit taking jilace in

Fig. 560. B



560. — The Surface of the Fcetal Brain at Six Months (from R. Wagner).

This figure is intended to show the commencement of the formation of the principal fissures and convolutions. A, from above ; B, from the left side. F, frontal lobe ; P, parietal ; O, occipital ; T, temporal ; a, q, a, slight appearance of the several frontal convolutions ; s, the Sylvian fissure ; s', its anterior division ; within it, C, the central lobe or convolutions of the island ; r, fissure of Rolando ; p, the vertical fissure (external part).



At an early period the ojriie tracts may be recognised as hollow i^rolongations from the outer part of the wall of the thalami while they are still vesicular. At the fourth month these tracts are distinctly fonned. They subsequently are prolonged backwards into connection with the coi-pora quadiigemina.

Prosencephalon

Each hemispherevesicle becomes divisible into two parts : one of these is the part which from the interior appears as the corpus striatum, and from the exterior as the island of Eeil, or central lobe ; the other fonns the expanded or covering portion of the hemisphere, and is designated by Reichert the mantle. The aperture existing at the constricted neck of the hemispherevesicle, Schmidt and Reichert have recognised as the foramen of Monro.


The corpora striata, it will be observed, have a different origin from the optic thalami ; for, while the latter are formed by thickening of the cu-cumferential wall of a part of the first cerebral vesicle, and thus correspond in their origin with all the parts of the encephalon behind them, which are likewise derived from portions of the cerebro-spinal tube, the corpora striata appear as thickenings of the floor of the hemisphere-vesicles, which are lateral off-shoots from the original cerebro-spinal tube. On this account, Reichert considers the brain primarily divisible into the stem, which comprises the whole encephalon forwards to the taenia semicircularis, and the hemisphere-vesicles, which include the corpora striata and hemispheres.


Fig. 561. — View of the Inner Surface of the Right Half of the Foetal Brain of about Six Months. (from Reichert).

F, frontal lohe ; P, parietal ; 0, occipital ; T, temporal ; I, olfactory bulb ; II, right optic nerve ; f p, fronto-parietal fissure ; p, vertical fissure ; ;/, internal vertical fissui'e ; /(, hippocampal fissure ; ff, gyrus formicatus ; c, c, corpxis callosum ; s, septum lucidum ; /, placed between the middle commissure and the foramen of Monro ; V, in the upper part of the third ventricle immediately below the velum interpositum ami fornix ; v', in the back part of the third ventricle below the pineal gland, and pointing l)y a line to the aqueduct of Sylvius ; v" , in the lower part of the third ventricle above the infundibuhim ; )•, recessus pinealis passing backwards from the tela choroidea ; p r,pons Varolii ; C c, cerebellum.


The cerebral hemispheres enlarging, and having their walls increased in thickness, fonn, dming the fourth month (Tiedemann), two smooth shell-like lamellae. which include the cavities afterwards named the lateral ventricles, and the parts contained within them. Following out the subsequent changes affecting the exterior of the cerebral hemispheres, it is found that about the foiuth. month the first traces of some of the conrolvtions appear, the intermediate sulci commencing- only as very slight depressions on the hitherto smooth sm-face. Though the hemispheres continue to grow quickly upwards and backwards, the convolutions at first become distinct by comparatively slow degrees ; but towards the seventh and eighth months they are developed with gi-eat rapidity, and at the beginning of the last month of intra-uterine life, all the principal ones are marked out.


The Sf/!r!fi)i _p\\:finv, which afterwards separates the anterior from the middle lobe of each hemisphere, begins as a depression or cleft between them about the fourth month, and, after the great longitudmal, is the first of the fissures to make its appearance. It is followed by the fissure of Rolando, and the vertical or parieto-occipital fissure, and somewhat later by the internal fronto-parietal fissui-e. After this, the various subordinate fissures dividing the convolutions gradually make their appearance. By the end of the third month the hemispheres have extended so far backwards as to cover the thalami ; at the fourth they reach the corpora quadrigemina ; at the sixth they cover those bodies and great part of the cerebellum, beyond which they project still fiuther backwards by the end of the seventh month.


During the growth of the hemisphere the aperture of the foramen of Monro is extended backwards ; the arched margin of this opening is curved downwards at its extremities, and forms anteriorly the fornix, and posteriorly the corpus fimbriatum and hippocampus major ; above the margin a part of the wall of each hemisphere comes into contact with its fellow, and in the lower part forais the septum lucidum, while above this the hemispheres are united by the development of the great commissure, the coipus callosum.


The corpus callosum is described by Tiedemann as being first seen about the end of the third month, as a narrow vertical band, extending across between the forepart of the two hemispheres, and subsequently growing backwards. With this view the observations of Schmidt coincide. Reichert, however, maintains that the commissural structure seen at the forepart of the hemispheres is the anterior white commissure, and that the coiiaus callosum appears in its whole extent at once.


The corpora alhtcantui at fixst form a single mass : so also do the anterior pillars of the fornix, which are distinguished before the posterior pillars. The posterior pillars are not seen until the fourth or fifth month. At that period the hippocampus minor is also discernible.

Nerves

Spinal Nerves

Very little is yet known as to the mode of origin of the nerves. In their peripheral extension the great majority of them seem to arise more immediately from mesoblastic formative elements, and the manner in which this takes place has been adverted to in the General Anatomy at p. 161. The ganglia and roots of the spinal nerves are first seen to make their appearance in some very close association with the protovertebral segments. In this the ganglion comes to be distinguishable as a mass by itself, and the anterior and posterior roots follow, with their jimctiou in the part forming the nerve-trunk beyond the ganglion. But according to recent observations by Balfour in ElasmoBranch fishes (Scyllium and Torpedo), it would appear that both the anterior and posterior roots may arise iu these animals in a closer connection with the nervous centre than was previously believed, and as independent outgrowths from the involuted epiblast of the neural canal. The posterior roots are the first to appear, and commence by an outgrowth at the summit (dorsal median groove) of the neural canal, and gradually pass outwards from thence to reach their permanent place of origin in a posterior lateral furrow. A subsequent division of the nerve rudiment takes place into root, ganglion, and a part of the nerve beyond.

The anterior roots spring by an outgrowth from the antero-lateral angles of the cord, one for each muscular plate nearly in the place which they permanently occupy, and after they have attained some size they iniite with the posterior roots beyond the ganglion.

But although the roots of the nerves may thus be traced in their commencement to elements of epiblastic nature, it is probable that their sheaths and blood-vessels arise from mesoblastic tissue introduced later into them. The ganglia are at first of proportionally very large size, causing even a considerable part of the projections on the surface of the body, as in the human embryo of from four to seven weeks, which are usually attributed to the vertebrate segmentation (Kolliker). The union of the roots with the grey matter soon becomes apparent, being most obvious in the anterior roots.

The nerves, like the other elementary parts, are at first composed entirely of cells, but fibres are soon formed by transformation of the cells. On the sixth day in the chick, Foster and Balfour found the fibres developed, but were unable to trace them into connection with the ganglionic cells, but at a later period the connection was observed by Lockhart Clarke to be formed.

The very early development of the trunks of the nerves of the limbs, and their progress outwards into the first part of the commencing limb, were observed and figured by Remak.

Cranial Nerves

The optic nerve and the retina, arise from epiblast by an extension of the primary medullary wall of the brain, as already described, and may therefore, in some sort, be regarded as an extension of the brain itself. The olfactory tract and bulb are still more to be looked upon as constituent parts of the cerebrum ; but the manner in which the peripheral olfactory nerves which pass through the cribriform plate into the nose are formed has not been yet observed.

Among the other cranial nerves there are four important ones of which the rudiments are seen to be formed at an early period ; taking their origin as has been supposed in the mesoblastic wall of the cranial cavity, and extending thence into the facial or visceral plates. These are the fifth pair or trifacial, the facial or portio dura of the seventh pair, the glossopharyngeal and the pneumogastric nerves. The two first of these are situated in front, and the two last behind the otic vesicle and tympanoeustachian passage ; and according to Parker each of these nerves forks or divides into two above one of the visceral clefts. Thus the fifth pair gives its naso-maxillary division in fi-ont, and its infero-maxillary division behind the oral cleft ; the facial gives its vidian or superior petrosal before and its descending part behind the tympano-eustachian passage ; the glosso-pharyngeal has its lingual and pharyngeal branches divided by the first branchial cleft, and in animals the pneumo-gastric is similarly divided at the second and succeeding clefts.

The third, fourth and sixth pairs of nerves are of subordinate importance, and may be considered as related, the two first to the fifth pair, and the last to the facial nerve. Their peripheral parts are developed in connection with the muscles of the eyeball, but the mode of the formation of their roots in connection with the nervous centres has not been ascertained.

The hypoglossal nerve, although it passes through the exoccipital bone in man, may be compared to a spinal nerve, and probably takes its origin much in the same manner.

The Sympathetic Nerves. — Remak observed the development of the great sympathetic nerves from the lateral plates in loops or arches connecting them with the spinal nerres ; wlnle the great sympathetic itself consisted at first of a chain of rounded masses representing; the ganglia, connected together, but so closely set that scarcely any intervening nervous cord was at first perceptible. He also observed the separate formation in the mesentery of birds of the large visceral nerve which he discovered in these animals. The gangliated cord of the sympathetic has been described and figured by Kiilliker in the human foetus of eight or ten lines long. The peripheral sympathetic nerves are also formed at a very early period, and are perceptible in a foetus of three months. In the hinder part of the abdomen their origin appears to be intimately connected in some way with the formation of the suprarenal bodies.

Development of the Eye

The embryonic structures forming the eyeball and its contents may be considered as proceeding from three sources, viz., 1st, by evolution or expansion from the medullary wall of the first encephalic vesicle (thalamencephalon), giving rise to the retina, in its nervous and pigmental structure and optic nerve ; 2nd, by involution or depression and development of a part of the cuticular epiblast, forming the foundation of the lens and the epithelium of the conjunctiva ; and 3rd, by the intrusion of mesoblastic elements between and around the other parts, so as to furnish the materials out of which are formed the general coverings of the eyeball, cornea and sclerotic, the fibrous and vascular choroid, the ciliary apparatus and iris, the capsule of the lens and the capsulo-pupillary membrane, the vitreous humour, and all the fibrous and vascular parts of the organ.

The very early formation of the primary optic vesicles by the expansion of the lower and anterior parts of the wall of the anterior primary encephalic vesicle has already been described, and the manner in which each of these vesicles forms a hollow pediculated chamber communicating by its stalk with the general ventricular cavity of the primitive brain. The first important change which the primary optic vesicles undergo is connected with the depression of the rudimentary lens from without, and consists in a doubling back or inwards of the medullary wall of each vesicle, so as to form a depression or cup at its lower and anterior part, into which the commencing lens in part sinks. This depression has been called the secondary optic vesicle, or the opfic cup (Foster and Balfour). From a very early period the outer fold of this cup undergoes a much greater thickening by the rapid development of its cellular constituents than the inner or that towards the brain, and from this, as well as the increase of the inward folding, the original cavity of the primary optic vesicle becomes in a great measure obliterated or narrowed, and the outer and inner folds are closely approximated, while the stalk or pedicle becomes proportionally much diminished. The continued increase of cellular development in the outer fold of this cup leads to the formation of the various elements composing the retina ; while in the thinner inner fold only pigment cells are formed. The transition at the bend from the thick nervous part tc


Fig. 562. — Section of the Head through the Primitive Optic Capsule uf one side in an Embryo-Calf of 9 mm. long, MAGNIFIED (from Julius Arnold).

To the right is seen the optic capsule with its contracted pedicle and its outer wall depressed by the thickening of the corneous layer which forms the commencement of the formation of the lens. The optic stalk is in communication with the thalamencephalon. Mesoblast is seen between the optic capsule and the lens rudiment.



Fig. 563. — Section through the Eye op an Embryo-Calf OF TWELVE 3IM., OR HALF AN INCH, LONG, MAGNIFIED (froui Julius Amold).

The lens follicle Ls now closed in and detached fi'ora the corneous laj'er, and its cavity contains loose cells which are the remains of the superficial corneous cells. The oj^tic vesicle or cajisule is now completely doubled back, so as to present towards the lens side the secondary ocular capsule or cup ; its outer wall now much thickened by the commencement of the development of retina. Mesoblastic tissue is seen to have passed in from the periphery between the optic capsule and the lens, as well as in front of the lens. The commencement of a vascular circle is shown round the exterior of the ocular capsule, and extending also between the lens follicle and the cuticle.

the thin pigmental part is quite sudden, and as soon as pigment cells begin to be developed, a very marked distinction is perceptible between it and the nervous structure of the retina. These cells were formerly regarded as a part of the choroid membrane, but they are now looked upon as belonging rather to the retina, — a view which is supported by the mode of development now described.

Fig. 564. — Diagrammatic Sketch op a Vertical Longitudinal Section THROUGH THE EyEBALL OP A Human Foetus OF FOUR WEEKS (after Kolliker).

The section is a little to the side so as to avoid passing through the ocular cleft, c, the cuticle, where it becomes later the cornea ; I, the lens ; o ij, optic nerve formed by the pedicle of the primary oj)tic vesicle ; v 2'>, primary medullary cavity of the optic vesicle ; p, the pigment-layer of the outer wall ; r, the inner wall form- . ing the retina ; v s, secondary optic vesicle containing the rudiment of the vitreous humour.



The fold which produces the optic cup proceeds from above downwards, and when the lens is formed it seems as if it enclosed the lens but left for a time an aperture or depression below. This is the choroidal fold or fissure, which may easily be distinguished in the embryo-head after pigment has been deposited, from the circumstance that the pigment is absent from the cleft, which thus appears for a time as a broad white line, particularly obvious in the embryo bird, running from the circumference in upon the lens.

The lens is developed in the part of the cuticle opposite to the -most projecting part of the primary optic vesicle, or at the place where this vesicle comes in contact with the surftice of the head. In this situation there is seen from a very early period a thickening of the epiblast, which seems to reside chiefly in its deeper layer of cells, and in birds and mammals it would appear that an actual involution of the cuticle takes place, so that first an open follicle and next an enclosed ball of cuticle is formed. Although, however, both the corneous and the deeper layer (sensory of Strieker) of the cuticle are enclosed, it is only the cells of the deeper layer which undergo development into the fibres of the lens. The ball of the lens separating from the external cuticle, which passes freely over the surface, a cavity filled with loose cells, the remains of cells of the corneous layer, exists for a time within the lens ball. Then the cells of the hinder or inner wall are seen to rise from the bottom by their elongation, and thus a rapid growth of fibres from that side of the ball takes place, while the anterior or outer wall undergoes no similar change, but retains its simply cellular structure. Figures 565, and 566, show sufficiently clearly the manner in which the fibres thus developed from cells rise fi'om the bottom of the lens ball and come to constitute the solid part of the lens.

The optic cup receives the enlarging lens in its anterior and lower opening, and the reflected margins of the cup closely embrace the margin of the lens ; but there is a considerable space intervening between the lens and the hollow of the optic cup (or secondary ocular vesicle), which comes to be occupied by the vitreous humour. Into this space connective tissue and blood-vessels developed from mesoblastic



Fig. 565. — Sectiojt of the Eye in an Embryo-C^lf OF 18 MM. LONG, MAGNIFIED (from JuUus Amold).

The posterior or inner v.-all of the lens follicle is now much thickened by the elongation of its fibres, each of which presents a nucleus, and the whole causing a bulging of the posterior wall. The outer la^'er of the lens capsule consists of columnar cells. The cavity of the lens follicle is still visiljle, but is now widened and flattened. Two layers of mesoblastic tissue are now visible between the lens and the cuticle, viz., a deeper vascular layer and a superficial non-vascular one containing nuclei. The secondary optic capsule is now occupied bj' connective tissue nuclei and numerous blood-vessels. The retinal section of the primitive ocular vesicle is now thicker. Pigment begins to ajapear in the choroidal section, and numerous bloodvessels surrovmd the whole exterior.


elements are projected from below, so as ..'^,|y| to furnish the materials for the formation

^ '"^r of the vitreous humour and the blood-vessels

-W which pass through it to the lens, and also to surround the lens with vascular and fibrous elements, out of which are produced the capsulo-pupillary membrane, and probably also the capsule of the lens. It results from the observations of Lieberkiihn that in mammals the fold which produces the ocular cup or secondary vesicle runs back into the stalk so as to fold in the optic nerve for a considerable space, and by the simultaneous intrusion of mesoblastic tissue, thus to lead to the introduction of the central blood-vessels of the retina within the nerve. But in birds,, according to the same observer, no such infolding of the stalk occurs, so that in them the vessels are excluded from thenerve. The malformation termed coloboma iridis is to be attributed to a persistence of the choroidal cleft or fold, and the pecten of birds, close to the optic nerve, with the vascular fold farther forwards, and the falciform fold of the eyes of fishes are to be regarded as fibro-vascular structures formed by original projection through the same fold.

The further development of the parts of the eye may be briefly stated as follows : —

The expansion of the ocular cup continuing to proceed, the chamber for the vitreous humour enlarges, and that structure gradually comes to occupy its space between the retina and the lens.

The marked distinction between the nervous and the pigmental portions of the primitive ocular vesicle goes on increasing by the continued deposit of pigment in the latter, and its proportional thinning, and by the great addition to the thickness and the textural differentiation of the substance of the former. Thus the cells in the retinal or nervous portion, by their rapid multiplication, soon l^ecome several layers thick ; certain parts of these cells assume the spindle shape, and exhibit elongation into fibres, while others retain the nuclear form, and thus there is foreshadowed the division into the fibrous, ganglionic, and nuclear layers of the retina. On the exterior a limiting membrane


Fig. 566. — Section of the Eye of AN EjIERYO-CaLP of 30 MM. LONG, MAGNIFIED (froiu Julius Arnold).

the cavity of the lens is much reduced in size from the increased development of fibres from behind. The intersection space begins to be formed posteriorly, and the zone of nuclei is thrown forward. The cornea is now formed, covered externally by the cuticular epithelium, and with the separation of the aqueous chamber internally. Close to the lens is the caiDsulo-pupillary membrane, which is continuous with the vascular structure occupying the secondary ocular capsule or cavity for the vitreous humour, and with the choroid membrane round the margin of the ocular capsule, where

iris and ciliary processes will afterwards be formed. The layer of pigment is now more developed, and the tissue of the sclerotic is begun to be formed. The eyelids are beginning to project as folds of the skin.

makes its appearance, and in connexion with it the rudiments of the cells composing the layer of rods and cones. The space between the retinal and pigmental layers rapidly contracts, and finally the rods and cones are closely united with the layer of pigment cells.

The optic nerve, as already described, is at first connected by its origin with the vesicle of the third ventricle or thalamencephalon, and for a time it retains its original hollow form. But as the cerebral hemispheres are developed forwards, the eye and the optic nerve are thrown backwards and downwards, and a new connection is established between the optic nerve (or tract) and the vesicle of the midbrain (mesencephalon) : the rudiment of the optic commissure is at the same time formed by the median approximation of the stalks and the growth of one over the other. Each stalk then becomes more and more solid by the development of nerve fibres along with the intruded connective tissue which forms the sheath substance of the nerve.

Lens

The development of fibres from the hinder wall of the primitive lens-follicle continuing to take place, the cavity of the follicle is first greatly narrowed and then completely filled up by the lengthening fibres, and the lens takes more and more of its fall spherical shape. The new fibres continue to be formed towards the margin of the lens ; each fibre retaining its nucleus, so as to produce the "nuclear zone which runs through the whole lens. This zone is at first situated far back in the lens while the fibres are still short, but as they elongate, its place is advanced, so that it comes to be situated considerably in front of the equatorial plane of the lens. It is most distinct towards the margin where the fibres are newly formed. The anterior wall of the lens-follicle remains as a simple cellular layer. The greater number of the fibres now follow the general curve of the surface of the lens, presenting therefore their concavity towards its centre, but the curvature gradually diminishing in those nearest the middle, where they are straight or nearly so. Only the external short and recently formed fibres present a concavity towards the exterior. The intersecting stars of the anterior and posterior poles of the lens now make their appearance by the collection of cells in the peculiarly shaped triradiate space in these two situations, and the ends of the fibres are now traceable to the edges of these spaces, so that the fibres gradually take the arrangement round the poles of the lens which belongs to the adult.


Fig. 567.— Transverse Vertical Section of the Eyeball op a Human Embryo of four weeks (from Kolliker). '""

The anterior half of the section is represented. pr, the remains of the cavity of the primary optic vesicle ; ii, the inflected part of the outer layer, forming the retinal pigment ; r, the thickened inner part giving rise to the columnar and other structures of the retina ; r, the commencing vitreous humour within the secondary optic vesicle ; v', the ocular cleft through which the loop of the central blood-vessel, a, projects from below ; 1, the lens with a central cavity.

The capsule of the lens, according to Lieberkiihn's and Julius Arnold's most recent observations, owes its origin to the thin innermost pellicle of mesoblast which is introduced atan early period of development between the lens and the secondary ocular vesicle.


Cornea

There is at first no aqueous chamber in the eye, and even after the solution of continuity which gives rise to this space lias occurred, the cavity is not dilated with fluid, till near the time of ])irth. Even then it is very shallow and the lens is placed very near to the cornea. The formation of the cornea is due to a differentiation of the tissue in the layer of mesoblast whichis introduced from the neighbouring wall of the head, between the primitive lens-follicle and the corneous epiblast, the cavity of the aqueous humour arising by the separation of the corneous part from a layer of the mesoblastic tissue lying within it. The latter gives rise to the anterior part of the vascular capsulo-pupillary membrane, while a still deeper lamina closely embracing the lens, remaining non-vascular, is converted into the lens capsule. Along with the latter is also formed the suspensory ligament of the lens.

Vitreous humour

The enlargement of the space for the vitreous humour progressing, the cells of the mesoblast which form its foundation become stellated and very sparse from the effusion of a large quantity of fluid, and the hyaloid membrane surrounding this structure takes its origin from the same mesoblastic elements.

Choroid and other membranes

The mesoblastic substance which surrounds the ocular vesicle externally is the source of a number of important parts. Among these may be mentioned first the choroid membrane, the cellular (membrana fusca), fibrous, and vascular layers of which are developed out of the deeper division of the mesoblastic substance, and to the same source may be traced in a later stage of development the ciliary processes, ciliary muscle and iris ; while the zonula ciliaris may be regarded as a part of the deeper mesoblastic tissue connected with the formation of the hyaloid membrane and membrana capsulo-pupillaris. The folds of the ciliary processes gradually increasing, encroach upon the space outside the margin of the lens and indent the zonula ciliaris and canal of Petit.


Fig. 568. — Blood-vessels of the CapSULO-PUPILLARY MEMBRANE OF A New-born Kitten, magnified (from Kolliker).

The drawing is taken from a preparation injected by Tiersch, and shows in the central part the convergence of the net-work of vessels in the pupillary membrane.



The sclerotic coat is due to a process of differentiation occurring in the outer layer of the enveloping mesoblastic tissue, which occurs considerably later than those which bring the choroid and the cornea into existence, but there is from the first continuity between the corneal tissue and that of the sclerotic coat.


The capsulo-pupillary membrane, already referred to, may be looked upon as at first a complete fibro-vascular investment of the lens, which owes its origin to the deepest pcart of the intruded mesoblast. The vessels of this membrane are svipplied by a branch of the central artery of the retina, which passes foi"n-ards in the axis of the globe, and breaks up at the back of the lens into a brush of rapidly subdividing twigs. The forepart of this tunic, adherent to the pupillary margin of the iris, forms the jnipUhir// mcmhrane by which the aperture of the pupil is closed in the middle periods of foetal life. In the human subject, the whole tunic, to<^ether with the artery which supplies it, becomes atrophied, and is lost sight of before birth, but in some animals it remains apparent for a few days after birth. According to Kolliker, the anterior chamber expands only a short time before birth by the intervention of the aqueous humour between the iris and cornea.


The eyelids make their appearance as folds of integument, subsequently to the foi-mation of the globe. "When they have met together in front of the eye, their edges become closely glued together ; and they again open before birth.

The lachrymal canal may be regarded as a persistently open part of the fissure between the lateral frontal process and maxillary lobe of the embryo.


The first discovery of the mode of development of the eye as it is now generally understood was made by Husclike in 1832, and was published in Meckel's Archiv for that year. In addition to the vaiious systematic works on Development previously quoted, the reader is refen-ed to the following, viz., Lieberkiihn, Uber das Auge des Wirbelthier-enibryo, 1872 ; and Julius Arnold, Beitrage zur Entwick. de^ Auges, Heidelberg, 1874.


Development of the Ear

The first origin of the organ of hearing as an involuted follicle from the superficial epiblast of the head, constituting the primary auditory



Fig. 569. — Outlines showing the Formation of the External Ear in the Fietus.

A, head and upper part of the body of a human foetus of about four weeks (from nature). \' Four branchial plates (the first, forming the lower jaw, is marked 1), and four clefts are shown ; the auditory vesicle (a), though closed, is visible from the transparency of the parts, and is placed beliind the second branchial plate.

B, the same parts in a human fcetus of about six weeks (from Ecker). \ The third r.nd fourth plates have nearly disappeared, and the third and fourth clefts are closed ; the second is nearly closed ; but the first (1') is somewhat widened posteriorly in connection with the formation of the meatus externus.

C, human fcetus of about nine weeks (from nature). \ The first branchial cleft is more dilated, and has altered its form along with the integument behind it in connection witli the formation of the meatus externus and the auricle.

or otic vesicles, has already been shortly described. From numerous cbservations there is now no doubt that both in birds and mammals


Fig. 570. — Transverse and slightly Oblique Section of the Head op a Fietal iSlikep, in the Eegion op the Hind Erain (from Foster and Balfour after Boettclier).

HD, inner surface of the thickened walls of the hind brain ; RL, recessus vestilnili ; VB, commencing vertical semicircular canal ; CC, canalis cochlew, Avith the cavity of the primitive otic vesicle. On the left side parts only of these structures are seen ; GC, cochlear ganglion of the right side ; on the left side, Gr', the ganglion, and N, the auJitoiy nerve connected with the hind brain.

the otic vesicle forms at first for a time a follicle open to the surface, ami that it has therefore no original connection with the nervous centre. Its position is at the side of the medulla oblongata, and in a l)lace opposite to the interval between the first and second postoral visceral arches. The outer opening of the follicle very soon contracts and becomes entirely closed. The follicle sinks down towards the basis of the cranium, and becomes imbedded in the formative mesoblastic tissue lying between the basioccipital and alisphenoid matrices, undergoing chondrification and ossification at a very early period, as has been already described under the development of the head. There are therefore to be distinguished from an early period a part corresponding to the internal membranous labyrinth proceeding from the epiblast, and an outer cartilaginous or bony and fibrous wall, together with other adventitious structures arisingin the mesoblast. Labyrinth. — In the development of the primary otic vesicle after it becomes completely closed, a series of very remarkable changes by extension of its cavity in different directions gives rise to the formation of the different parts of the labyrinth. The first complication which the vesicle exhibits is by the extension of a process upwards and backwards, which remains permanent in the lower vertebrata, but in mammals is obliterated, its vestiges remaining in the aqueduct of the vestibule. The semicircular canals next appear as elongated elevations of the surface of the primary vesicle : the middle portion of each elevation becomes separated from the rest of the vesicle by bending in of its walls under it, and thus the elevation is converted into a tube open at each end, vyhich subsequently becomes elongated and acquires an ampullar dilatation. The cartilage which forms the osseous labyrinth is continuous with that of the rest of the primordial cranium. The cartilaginous walls of the cavity are united by connective tissue to the vesicle ; this connective tissue, according to Kolliker, becomes divided into three layers, of which the outer forms the lining periosteum, the inner forms the external walls of the membranous labyrinth, while the intervening; layer swells up into gelatinous tissue, the meshes of which become wider and wider, till at last the space is left which ultimately contains the perilymph.



Fig. 571. — Labyrinth of tee Human Fcetus of FOUR aveeks, magnified (from Kolliker).

A, from behind ; B, from before ; v, the vestibule ; rv, recessus vestibuli, giving rise later to the aqueduct ; r.s', commencement of the semicii'cular canals ; a, upper dilatation, belonging perhaps to another semicircular canal : c, cochlea.


Fig. 572. — Transverse Section of the Head op a Fcetal Sheep of four-fifths of AN INCH IN LENGTH (from Foster and Balfour after Boettclier).

RV, recessus vestibuli ; VB, vertical semicircular canal ; CC, cochlear canal ; G, cochlear ganglion ; HB, horizontal canal.


Fig. 573. — Tr.ANsvERSE Section of THE Cochlea in a Fcetal Calf, magnified (from Kolliker).

C, the wall of the cochlea, still cartilaginous ; c c, canalis cochlear ; I s, placed in the tissue occupying the place of the scala vestibuli, indicates the lamina spiralis ; n, the central cochlear nerve ; (/, the jjlace of the spiral ganglion ; S, the body of the sphenoid • ch, remains of chorda dorsalis.


Fig. 574. — Views of the Cartilage of Meckel and parts connected WITH THE First and Second Branchial Plates.


A (after Kolliker), bead of a fcetus of about eighteen weeks, showing the cartilage of Meckel in connection with the malleus and the surrounding parts.

M, placed upon the lower jaw indicates the cartilage of IMeckel of the right side.

B (from nature). An enlarged sketch explanatory of the above view ; z, the zygomatic arch ; ma, the mastoid process ; ml, jDortions of the lower jaw of which the parts near the angle and the symphysis have been removed ; M, the cartilage of Meckel of the right side ; M', a .sratdl part of that of the left side, joining the left cartilage at s, the symphysis ; T, the tympanic ring ; m, the malleus ; i, the incus ; s, the stapes ; sta, the stapedius muscle ; st, the styloid process ;p,A,<7, the stylo- j)haryngeus, stjdohyoid, and stylo-glossus muscles ; stl, stylo-hyoid ligament attached to the lesser cornu of the hyoid bone ; hij, the hyoid bone ; th, thyroid cartilage.



The cochlea appears at first as a prolongation downwards from the auditory vesicle, but afterwards becomes tilted forwards. This prolongation of the auditory vesicle is the rudimentary canalis membranacea. Close to it is placed the cochlear nerve, with a gangliform extremity. The canal becomes elongated in a spiral direction, and the ganglion, which is elongated with it, becomes the ganglion spirale. Between the canal and the cartilaginous wall which afterwards surrounds it a large amount of connective tissue intervenes, and in this tissue the cavities of the scala vestibuli and scala tympani gradually appear at a later period, precisely as does the space for the perilympli, in the vestibule. The modiolus and spiral lamina, according to Kulliker, are ossified without intervention of cartilage. Within the canalis membranacea Kulliker finds in the embryo a continuous epithelial lining, thin on the membrane of Reissner and on the outer wall, but forming a thick elevation in the position of the rods of Corti, and a larger elevation more internally, filling up the sulcus spiralis. On the surface of this latter elevation he has observed a transparent body, the membrane of Corti.


The auditory nerve is said to be developed, separately from both the brain and the labyrinth, in the intermediate mesoblastic wall of the head ; the canal termed meatus auditorius internus being left in the bones round it and the facial nerve. The auditory nerve becomes secondarily connected with the medulla obloug-ata in a manner not yet ascertained, and its fibres are extended into relation with the delicate terminal structiu-es formed in the membranous labyrinth.


Middle and External Cavities of the Ear

It has been already explained that the external meatus, the tympanic cavity, and the Eustachian tube, are formed in the posterior or upper part of the first postoral visceral cleft, which remains permanently open as the tympano-eustachian passage, except at the place where it is interrupted by the formation of the membrana tjmipani ; and also that the malleus is formed in the first visceral plate from the proximal part of Meckel's cartilage, and the incus, stapes, and stapedius muscle and the styloid process probably in the second plate. It is pointed out by Kolliker that during the whole period of foetal life the tympanic cavity is occupied by connective tissue, in which the ossicles are imbedded ; and that only after respiration has been established this tissue recedes before an expansion of the mucous membrane.


The pinna is gradually developed on the posterior margin of the first visceral cleft. It is deserving of notice that congenital malformation of the external ear, with occlusion of the meatus and greater or less imperfection of the tympanic apparatus, are observed in connection with abnormal development of the deeper parts of the first and second visceral plates and the intermediate cleft ; while cases have been observed of the persistence in the neck of the adult of one or more of the branchial clefts situated behind the first. (Allen Thomson, Proceed. Eoy. Soc. of Edin. 1844, and Edin. Journ. of Med. Sc. 1847.)


Development of the Nose

The organ of smelling, as was first pointed out by V. Baer, owes its origin, like the primary auditory vesicle and the crystalline lens of the eye, to a depression of the integument, or what may be more precisely designated as epiblast. This depression, the primary olfactory groove, is at first encircled by a uniform wall, and is unconnected with the mouth. This staple has been observed by Kolliker in the human embryo of four weeks. The olfactory groove is enclosed in the anterior extremity of the nasal cartilages prolonged forward from the trabeculas cranii (Parker). Soon, however, by the unequal growth of the surrounding parts, the groove so formed, descending from above, passes into the mouth. Thus the middle frontal process is isolated between the grooves of opposite sides, while the lateral frontal process separates the nostril from the eye. The maxillary lobes, growing forwards from behind the eyes, complete the boundaries of the nostrils, which then open into the fore part of the mouth. Kolliker observed this stage in the latter half of the second month. The palate subsequently grows inwards to the middle line, as has been elsewhere stated, and separates the nasal from the buccal cavity ; leaving only the small communication of the incisor foramen. Meanwhile, with the growth of the face, the nasal fossae deepen, and the turbinated bones make their appearance as processes from their walls. The ethmo-turbinal cartilages are at first simple, but rapidly extend themselves to take the more or less complex shape which they present in different animals or in man.



Fig. 575. - Views op the Head op Human Embryoes, illustrating the Development of the Nose.


A, Head of an embryo of three weeks (from Ecker). 1, anterior cerebral vesicle ; 2, middle vesicle ; 3, nasal or middle frontal process ; 4, superior maxillary process ; 5, eye ; 6, inferior maxillary process or first visceral plate, and below it the first cleft ; 7, 8, and 9, second, third, and fourth plates and clefts.

B, Head of an embryo of about five weeks (from Ecker). 1, 2, 3, and 5, the same parts as in A ; 4, the external nasal or lateral frontal process, inside which is the nasal groove ; 6, the superior maxillary process ; 7, the inferior maxilla ; x , the tongue seen within the mouth ; 8, the first visceral cleft which becomes the outer part of the meatus auditorius externus and tynipano-eustachian passage.

C, View of the head of an embryo of eight weeks seen from below, the lower jaw having been removed (from Kolliker).

n, the external nasal apertures ; , premaxillary or incisor process, and to the outer side of this the internal nasal aperture ; m, one of the palatal processes of the upper jaw, which, advancing inwards from the sides, form the partition between the mouth and nose ; p, common cavity of the nose, mouth, and pharynx.


Observations are still wanting to determine whether the olfactory nerves are developed from the bulbs, and have thus a cerebral origin, or are separately formed from peripheral blastema like all other nerves, with the exception of the optic.



1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital



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