CHAPTER XV. THE DEVELOPMENT OF THE NERVOUS SYSTEM

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

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The nervous system of the adult, including the cerebrospinal axis and nerves, and the sympathetic system of ganglia and nerves, is made up of the essential neural elements, the neurons, together with the supporting framework or stroma.

The neurons and a part of the stroma result from the specialization of the ectodermic layer of the embryo. The ecto<lermic origin of the nervous system acquires certain interest in view of the conditions that obtain in some of the lowest and simplest organisms. For example, in the ameba, the single protoplasmic cell which constitutes the entire individual possesses the several fundamental vital properties of protoplasm, such as resi)iration, metabolism, contractility, motility, etc, in ecpial degree, no single property being more highly developed than the others, and no particular part of the cell exliibiting greater s])ecialization than the other parts. In other words, the j)n)t()plasmic substance of the animal is at once a respiratory mechanism, a nervous apparatus, and an organ for the execution of the various other vital functions.

In somewhat more highly developed creatures, as the infusoria, although there is no differentiation into separate tissues and probably not even into separate cells, there is seen some progress toward the sj)ecialization of certain parts of the orgimism for the performance respectively (»f the different functions of life. For example, the central part of the animal has digestive functions, while it is by the superficial portion alone that the creature is brought into relation with the outside world, the sensitiveness or irritability of the surface, by which the animal is made responsive to external impressions, being the nearest approach to the function of a nervous system that it possesses.

' The neuronti are the units of which tlic nervous Bvstem is made up. Each neuron consists of a nerve-cell with everything belonging to it -that is, with its various processes, including the nxis-rylinder process or iieuritf which becomes the axis-cvlindcr of a nerve-ri))er.

This primitive function of the surface of the organism is suggestive as to the origin of the nervous system of higher type creatures. It will be seen, indeed, that not only is the nervous system proper derived from the ectoderm ic cells of the embryo but that the peripheral parts of the organs of special sense, as the olfactory epithelium, the organ of Oorti, and the retina, have the same origin.

The alteration of those cells of the ectodermic stratum that are to specialize into nervous elements begins prior to the fourteenth day in the human embryo, in the stage of the blastodermic vesicle. The change consists in a gradual modification of the form of the cells, the cells common to the general surface of the germ assuming the columnar type. The process affects the cells of the median line of the embryonic area in advance of the primitive streak, resulting in the production of a thickened longitudinal median zone. This thickened area is the medullary plate (Fig. 41, p. 70). On each side of the plate — which is apparent at the fourteenth day — the adjoining ectodermic cells become heaped up to form the medullary folds, which latter therefore bound the medullary plate laterally. The medullary plate becomes concave on the surface, forming the medullary groove (Fig. 137). By the deepening of the groove, the lateral edges of the plate approach each other (Fig. 138), and finally they meet and unite, thus producing a tube, the neural tube or canal.

Since the medullary folds similarly meet and unite with each other — their union slightly preceding that of the edges of the plate — the neural tube comes to lie entirely beneath the surface-ectoderm and soon loses all connection with it. The closing of the tube and the union of the medullary folds occur first near the anterior end of the embryonic area, in a position that corresponds with the region of the future neck,

flnil fnim this jwint it proceeds Iwlh cephalad and caudad. Sinw the nie»!ullar>' fnUis at their caiidal extremity embrace

Fig. is;.— TrBDirene lecllon of > ■Ixteen-Rnd'a-l

the primitive streak {Y\g. -11, p. 70), the latter structure i» inchidefl within the cjuidiil end of the neural tube by the \

isHTie BBcUiiii of II fitl<;i;n-mnl a-hslf-dnj sheap embryo Kvca lomltci (Bonnet).

coming together of the folds, and thus the blastopore, which was previously the external aperture of the archonteron.

comes to constitute the neurenteric canal, or an avenue of communication between the neural canal and the primitive intestine.

The neural canal then is a tube composed of columnar cells, which is formed by the folding in of the ectoderm and which occupies the median longitudinal axis of the embryonic area and consequently of the future embryonic body. From this simple epithelial canal the entire adult nervous system is evolved.

The evolution of the highly complex cerebrospinal axis from such a simple structure as the neural canal is referable both to the principle of unequal growth — the walls of the tube becoming thickened by the multiplication of the cells — and to the formation of folds.

The portion of the neural canal — approximately one-half — that is devoted to the formation of the brain is delimited from the part that produces the spinal cord by the dilatation of the anterior or head-end of the tube, and the subsequent division of this dilated sac-like portion into three communicating sacs called respectively the fore-brain, mid-brain, and hind-brain vesicles (Fig. 142). These three vesicles give rise to the brain, while the remaining part of the neural canal forms the spinal cord.

The Development of the Spinal Cord

In the growth of the spinal cord from the spinal portion of the neural canal we have to consider the evolution of a cylindrical mass of nerve-cells and nerve-fibers with the supporting stroma from a simple epithelial tube.

The wall of the neural tube, although consisting at first of a single layer of epithelial cells, is not of uniform thickness throughout its circumference. While the external outline is oval, the lumen of the tube is a narrow dorsoventral fissure (Fig. 45, p. 73). The cavity is therefore bounded on the sides by thickened lateral columns, while the dorsal and ventral walls, which connect the lateral columns with each other, are thinner and are called respectively the roof-plate and the floor-plate.

After a short time, the walls of the tube having thickened by the multiplication of the cells, the shape of the lumen alters, two laterally projecting angles being addetl (Fig. 139). The effect of tiiis change is to partiallydivide each lateral half into a dorsal and a ventral region. The neural canal at tlii,-i stage may be said to consist of six columnri of cells, the two dorsal zones connected with each other liy the roof-plate, and the two ventral zones united by the floor-plate. These regions are also distinguishable, with . certain characteristic modifi<a- I tions, in the head-region of tlw I tnl>e. They are important in their bearing upon the further development of the atrnctnre, since the dorsal and ventral zones are related respectively to the dorsal or sensory and the ventral or motor roots of the spinal nerves.

The differentiation of the cells of the neural tube into two ^ kinds of elements, one of which gives rise to susteutative tissue or neuroglia wliile the other produces the nerve-cells, is ( observed at about the end of the third week. The single , layer of columnar cells which at first comjMises the wall of the tube, the long axes of the cells being radially arranged, soon exhibits near the lumen a row of roond cells, jirobably the first offspring of the columnar cells. The round cella are the genn-cells or Kerminating cells, from which are develoiied ihe neuroblasts or young nerve-colls as well as the neuroglia cells. All the other cells, known as the spongioblfista or ependymal cells, are concerned in producing susten

36<rWim Kaillkcr): c.ccDtraJ t, its eplthdUI nntng: Me' ortyl, Iho original placu of ot (he cuul ; a, the wblte lU of the anterior coluouu : a. g atance of BnlerolUiiral born terlar column ; or. miterlui pr, iMatarior roola.

Tiie stroma of the central nervous system includes two constituents — a connective-tiBsue element, and a part, the neuroglia, which is of epithelial origin, and which is not to be regarded, therefore, as connective tissue. The connectivetissue portion of the stroma is produced by the ingrowth of the pial processes from the pia mater, and is hence of mesodermic origin.

The neuroglia is derived from the spongiobhtsts, which result from the specialization of the large colnmnar cells of which the wall of the neural canal is composed. These cells, whose length comprises the entire thickness of the wall of the tube in the earliest stages, undergo partial absorption and disintegration, each cell being transformed into an elongated system of slender processes or trabecule, and each such system

no. IW.-CrOBB-Bectlon through the ipinal cord of ■ Tertebnite embryo {after His}: a. outer nmltlng membrane; b, outer DeurogUa layer. re«lon of future white matter; e, germ^cells ; d, central canal: e. Inner UmitlnE membrane or ependymal t>rer 1 /, spongioblaiU : a, neuroblaata (mantle tajer) : A, anterior root-flben.

being a completed spongieblaet or ependymal cell (Fig. 140). Thp inner ends of the spongioblasts coalesce with each other, forming thus the Inteinal limiting membrane, while the peri])heral extremities interlace with each other to form a close network, the marginal velum. As the walls of the neural tube increase in thickness, the spongioblasts become mors and more broken up to form the delicate neurogliar network with interspersed nucleated glia cells, which latter are derived from some of the round cells noted above as lying near the hmien of the neural tube and which have taken a position in the marginal velum. Such of the spongioblasts as border the cavity of the neuml tube become the cells of the later ependyma of the central canal of the spinal cord and of the ventricles of the brain. The cells of the ejwndyma become ciliated in the human fetus in the fifth week.

The nerve-cells of the spinal cord — as also of the brain — are the specialized descendants of the germ-cells referred to above. The proliferation of the germ-cells produces the neuroblasts, or young nerve-cells (Fig. 140). The latter elements move away from the primitive position of the germcells near the lumen of the tube and, taking up a position between the bodies of the ei)endymal cells and the periphery of the neural tube, develop into the nerve-cells. The transition is effected by the accumulation of the cell's protoplasm on the distal side of the nucleus and its elongation into a process. This ])rocess is a neurit or axon or axis-cylinder process and is the beginning of a nerve-fiber. The dendrites or protoplasmic processes apj>ear considerably later. Some of the fibers thus prcHluced grow out from the neural tube to constitute the eiferent filxTs of the jxTiphcral nerves, that is, the ventral roots of the spinal nerves, while others contribute to the formation of the fiber-tracts of the cord.

After the appearance of the neuroblasts and developing nerve-cells, the wall (►f the neural tube is divisible into three layers (Fig. 140): an inner or ependsrmal layer, next the lumen of the tube; adjoining this, the mantle layer, made up of neuroblasts ; and a peripherally situated neuroglia layer or marginal velum, which occupies the position of the future tracts of white fibers (►f the cord.

The alterations in the form and size of the sj)inal cord go hand in hand with the histological changes noted above. While th(»se areas that have been mentioned a> the dorsal and ventral zones in{.Teas<» greatly in thi<'kness, the floor-plate and the roof-plate — the ventral and dorsal walls of the neural tube — remain thin i Fig. 141). They are n<'ver invaded by the nerve-cells, but consist of thin layers of neuroglia which later become penetrated by nerve-fibt-ra thatpro"' fnim oiif side to the other. They thus represent the anterior and posterior white commiBsmes of the cord. Thetie plates remain relatively fixed in position because of their failure to expand, while the liitcral walls iif the tulie undergo great expansion, in both the ventral and dorsal directions, as well as laterally. In this way a median longitudinal cleft is produced on the ventral wall of the spinal c<>r<l and a similar one on the dorsal wall. These are the anterior and posterior median fissures. Since the so-called jKisterlor median tissiiit is not a true fissure but merely a eeptiim, it differs from the anterior fissure, and It is held by some authorities that this septum is formed by the gmwing together of the walla of the dorsal part of the central canal.

The flber-tracts or white matter of the spinal cord develop in the outer or neuroglia layer, each filjer being the elongated neurit of a nerve-cell. Some of the fibers originate from the nerve-cells of the cord while others grow into the c«rd from other sources. As examples of the former method may be cited the direct cerebellar tract, composed of the axons of the cells of the vesicular column of Clark, and the tract of ,*

GrowtT, made up of the axons oi' cells t>f the dorsal gray horn ; while the direct und crosat-d |iyniniiilul tracts ai-e the axons of cells in the cortex of the ct-rebrum. and the tracta of Goll and of Biinlach are composed largely of the axons >] of the cells of the Hpinal ganglia (see p. 318), The devcl- 1 opmont of these filwr-tracts is not complete until the tilwrs ap(|iiirc their niyeltn-sheaths (see p. 414), The myelination of the tracts of Biirdach and of Goll occurs In the latter part of the fourth month and in the tit\h month ; of the : direct cerehellar tract, in the seventh month ; of ihe jiyramidal tracts, at or soon after birth.

As the walls of the neural canal thicken through the mul-^ tipliciition of the cells, the cavity of the tube is gradually] encroached upon almost to obliteration. Whea development f is complete, all that remains of the cavity is the sm&U ceatral f canal of the spinal cord.

The lengtb of the spinal cord in the fourth fetal month \ corresponds with that of the spinal column, Its lower termi- J nation being opposite the last coccygeal vertebra. From this 1 time forward, however, the cord grows less rapidly than does J the spinal column, so that at birth, the cord terminates at the4 last lumbar vertebra, and in adult life at the second lumbar 1 vertebra. This gradually acquired disproportion in thai length of the two structures explains the more oblique 1 direction of the lower spinal nerves as comi«ired with those g

E higher up. In the early condition of the cord, each pair of ' Uerves passes almost horizontally outward to the cprrespoading intervertebral foramina, but us the spinal column gradn- ] ally outstrips the cord in growth, the lower nerves necessarily ^ pursue a successively more oblique course to reach their j foramina, the lower nerves being almost vertical in direction and constituting, collectively, the canda equina. -J (lev dih ves nei

THE DEVELOPMENT OF THE BRAIN.

The encephalic portion of the neuml iuIk, — that part devoted to the production of the bniin — after undci^ing dilatation, becomes marked "ff into ihe thnc ])riman' brainvesicle«, the fore-brain or prosencephalon, the mid-brain or mesencephalon, and llie hind-brain or rhombencephalon, by constrictions in the lateral walls of the tube (Fig. 142). the constricted part of the hind-brain that adjoins the midbrain is the isthmns. This division occurs at an early stage, before the closure of the tube is everywhere complete. The vesicles communicate with each other by rather wide openings. As in the spinal part of the neural canal, the walls of the primary brain- vesicles consist of epithelial cells, and it is by the. muliiplicalion of these cells in unequal degree in different regions^ and by the fo)*r)iation of folds in certain localities, that the various parts of the adult brain are developed from these simple epithelial sacs.

The stage of three vesicles is soon succeeded by the fivevesicle stage, the primary fore-brain vesicle undergoing division into two, the secondary fore-brain (telencephalon) and the inter-brain (thalamencepalon) or diencephalon, and the primary hind-brain vesicle likewise dividing, a little later, into the secondary hind-brain (meteneephalon) and the after-brain (myelencephalon).

The division of the primary fore-brain is preceded by the appearance upon each of its lateral walls of a small bulgedout area which soon assumes the form of a distinct diverticulum. This is the optic vesicle, the earliest indication of the development of the eye (Fig. 142). In the further process of growth the base of attachment of the optic vesicle becomes lengthened out into a relatively slender pedicle, which remains in connection with the lower }>art of the hiteral wall of the brain- vesicle. Following the appearance of the optic vesicle, the anterior wall of the primary fore-brain vesicle projects as a small ovagination, which latter is then distinctly marked off from

Fig. 142.~Diagrram8 illustrating the primary and secondary segmentation of the brain-tube (Bonnet).

the parent vesicle by a groove on either side. This anterior divertieuhim is the secondary fore-brain vesicle or the vesicle of the cerebrum, and the original or ]>riniary fore-brain vesicle is now the vesicle of the inter-brain.

The division of the primary hind-brain is eflTected by the development of a constriction of its lateral wall, this resulting in the production of the secondary hind-brain or the vesicle of the cerebellum, and the after-brain or the vesicle of the medulla oblongata.

While the three primary vesicles at first lie in the same straight line, they begin to alter their relative positions shortly before division. The change of position is coincident with the flexures of the body of the embryo that occur at this time. Three well-marked flexures appear, the result being

InUr-brain.

Fore-brain.

Cerebral portion of Pontine pituitary body. fltxure.

Fig. 143.— Diagram shuwin^ relations t)f l)raiii-vesiclvs ami flexures (Bonnet\

that the fore-brain is bent over ventrad to a marked degree. The most anterior of these flexures, and the first t4) develop, is the so-called cephalic flexure (Fig. 143), the primary forebrain, in the advanced stiite of the curvature, being bent around the termination of the chorda dorsiUis so as to form a right angle, and later, after its division, an acute angle with the floor of the mid-brain. This curvature makes th(» mid-brain very prominent as regards the surface of the embryonic body, producing the parietal elevation or the prominence of mid-brain. In the region of the future pons Varolii, on the floor or ventral wall of the secondary liind-brain, is a second wullmarked an^^iilarity. This is the pontal flexure. Its convexity projects forward.

A third bond, the nuchal flexure, is a less pronounced cnrvature at the jniicture of the after-brain with the spinal part (if the neural tube.

The Metamorphosis of the Fifth Brain-vesicle.— The fifth brain-vesicle, the caudal division of the primary hind-brain, (lifferentiates into the strnctnres whioh surnmnd the lower half of the fourth ventricle, these stnictures con

Fig. 144.— Diagram of > sinrlttal section or the brain of ■ mammal, sbowlng tbe trpc of stroctiirc and tlic pailB that develnji from the Beveral bialn-TeBlclei (modified rrnm Edltim^r).

stituting the mTelenceplialoii (Pig. 144). The Ustological clianges eorresjjond essentially with those that occur in the spinal segment of the neural tube, the aerve-cells and fibers and the neuroglia resulting from the diilbreiitiation of the original ectoderniic epithelium of which the wall of the tube is composed, and the coimective-tiflaue stroma growing into these from the surrounding mesoderm.

There is a marked disproportion between the rate of growth of the tube in different parts of its circumference. The great thickening of the ventral and lateral walls produces the several parts uf the mednlla oblongata. In the dorsal wall growth occurs to such slight extent that the wall in this region remains a thin layer of epithelium. As a consequence, the cavity of the neural tube is not encroached upon on its dorsal side and the central canal of the spinal cord therefore expands in the myelencephalon into a much larger space, the lower half of the future fourth ventricle. This relative expansion of the central canal begins to be apparent in the third week in the human embryo, from which period it continues to increase. A cross-section through the lower part of the developing medulla shows a cavity which is narrow laterally but which has a considerable anteroposterior extent. A section at a higher level disclosc^s a triangular space, the base of the triangle being the dorsal wall of the cavity.

At the time when the cavity of the after-brain acquires a distinctly triangular shape — about the third week — each thickened lateral half of the tube is divisible into a ventral and & dorsal segment, these being known respectively as the basal lamina and the alar lamina (Fig. 145).

The first indication of the longitudinal fiber-tracts of the medulla is presented by two bands of fibers which appear upon

the surface of the alar lamina and which constitute the ascending root of the fifth nerve and the ascending root (funiculus solitarius) of the vagus and glossopharyngeal nerves. These are later covorcil in by the folding over of the dorsal part of the alar lamina (Fiff. 146) and thnaighupiHT part core- tlius coiiic to ()crn)>y tlicir permanent bviiar rtri<m. of tho po^jticm ill tlic interior of the medulla.

fourlh venlrielo <.f an * /» i i i •

omhryo (His): r, roof of 1 hc parts of the alar lamina^ that are

mv.rai ;a""i : "/. "i"r f^^i^j^.^j ^,^,^.,. j^^ ^\^^ manner referred

Iniiiina ; W, basal luiiiina ;

r. ventral ixjrdiT. to diiliTeiitiatc for the most part

into the restiform bodies or inferior peduncles. These are distinguishable in the third month. The anterior pyramidal tracts develop from the ventral parts of the basal lamiiue and are recognizjible in the fifth month. CoiiK*i<leiitally with the f\)rmation of the fibers, the gray matter of th(* medulla assumes its prrmancnt form and arrangement. This gray matter, although in part ])eculiar to the

nie<1iilla, is in great measure hut the continuation of the gray matter of the spinal cor*l rearranged and differently related because of the motor and sensory decussations and of the dorsal expansion of the central canal. A notable feature of this

{Ills) : V, ventml border : (, tenls : ot, otic vesicle ;

rearrangement is the presence of masses of gray matter immediately beneath the floor or ventral wall of the now expanded cavity or fourth ventricle.

As stated above, the dorsal \vall of the aftcr-brain vesicle remains an extremely thin epithelial lamina, and the cavity in consequence expands toward the dorsal surface. Owing to the excessive delicacy of this dorsal wall of the cavity, it ia easily destroyed in dissection, with the effect of disclosing a triangular fossa (Fig. 151) on the dorsal surface of the medulla, which in connection with a similar depression on the dorsal surface of the pons, constitutes the rbomboidal fossa, or the foortli Tentride of the brain.

It is often stated in descriptions of the medulla and fourth ventricle that the latter is produced by the opening out of the central canal of the cord to the dorsal surface. It should be borne in mind, however, that the central canal does not, in reality, open out to the surface, although it may appear to do so l)ecause of the attenuated condition of its dorsal boundary. The thin epithelial roof or dorsal wall of the aflerbrain l>ecomes adherent to the investing layer of pia mater, thus forming the tela choroidsa inferior, which roofs over the lower half of the fourth ventricle (Fig. 144). The piamater invnj^inUcs ilu» epithelial layer to form the choroid plexuses of the fourth ventricle. Although apparently within the cavity of the ventricle, the choroid plexuses are excluded from it hy the layer of epithelium, the morphological roof of the after-hrain, which they have pushed before them.

AVhile, for the most part, the roof of the after-brain consists of the thin epithelial layer referred to above, there are slight linear thickenings, the ligulsB, along its latei'al margins, and at its lower angle, the obex. At the up|)er margin of the roof, at the place of junction with the hind-brain, there is also a thicken<>d area, the inferior medullary velum. These regions of thick<T tissue serve to eife<'t the transition from the thin epithelial layer that helps to form the inferior choroidal tela to the more massive boundaries of the rhomboidal fossa.

The Hind-brain Vesicle or Metencephalon

The metencephalon consists of the pons, the cerebellum with its superior and middle peduncles, and the valve (valve of A"ieuss(Mis). the>e structures are j)r()duced by the thickening of the walls of the fourth or hind-brain vesicle.

Th(» pons is forni<Ml by the thickening of the ventnd wall of the vesi(rlc. Its tnmsverse libers become recognizable durintr the fourth mouth.

The cerebellum grows iVom tin? posterior part of the nK)f or dorsid wall of the vesicle (Fig. \A\). The lirst indication of its development is seen as a thick transverse ridge or fold on th(; po>terior extremity of the <lorsid wall (Fig^*. 147, 1 4S). In tli<* tliinl mouth the <M'ntral j)art of this ridge, now grown larger, )>r('S('uts iour deep transverse grooves with the n'sult oi* dividing the originid eminence into five transverse ridges. The grooved ))art of the ridge is the ]>ortion that subse<|ueutly becomes the vermiform process or median lobe of the cerebelluui, while the smooth lateral ])ortious becom<' the lateral hemispheres. As the vermiform process increases in bulk, two of the ridges come to lie ujM)n its upper surface and three? on the inferior aspi»ct. These ridg(»s and furrows j)ersist throughout lifi^ as the principal convolutions and fissures of the vermiform process (Figs. 14J», 1 :»<)).

The lateral parts of the primary ridge inercaso in size and eventually, in the hiiin&n bmin, outstrip the niwlian lobe in pritwlh. They acquire their chief transverse fifisures in the fourth or fifth iiKinth, and the smaller sulci later.

The thickened cerebellar ridge nn the roof of the hindbrain vesicle being continuous with the lateml walls, the continuity of the cerebellar hemispheres with the jHins through the middle and superior cereliellnr |>eiliincles and with the medulla by means of the inferior pcdnnrles. is easily thickens and dcvelops into the cerebellum, all the remaining part of this roof remains relatively thin and becomes the anterior medullary velum or the valve of Vieussens (Fig. 144). The relations of this structure in the mature brain, stretching across, as it does, from one sujHjrior cerebellar })eduncle to the other and l)eing continuous posteriorly with .the white matter of the cerebellum, ixw. easily explained in the light of the fact that all these parts are but the specialized dorsal and lateral walls of the hind-brain vesicle. Since the roof of the hind-brain vesicle is continuous with that of the after-brain or fifth vesicle, it will be seen that the cerebellum must be in continuity with the roof of the medullary part of the fourth ventricle. The transition from the cerebellum to the epithelium of the tela choroidea inferior is eifected by a pair of thin crescent-shaped bands of white nerve-matter which i>ass downward from the central white-matter of the cerebellum, and which are collectively known as the inferior or posterior medullary velum. Thus, as the result of unecpial growth, there are ])ro<luced from the continuous dorsid walls of the fourth and fifth vesicles the thin laminar medullary velum or valve, the massive cerebellar lob(»s, the thin bands known as the infi^rior niedullarv velum, and the single layer of epithelimn which, with a layer of pia mater, constitutes the inferior choroidal tela.

Although the fourth and fifth brain-vesicles are at first delimited from each other by a constriction, this constriction, as development goes on, <Iisappears, the cavity of the fourth vesicle and that of the iifth together constituting the fourth ventricle of the brain.

The walls of th<» fourth or hind-brain vesicle then give rise vent rally to the j)ons, latendly to the superior and middle cerebellar peduncles, and dorsally to the valve* and the cerebellum, while its cavitv beciunes the anterior half of the fourth ventricle.

The Mid-brain Vesicle

The third brain-vesicle or the vesicle of the midbrrain or mesencephalon gives rise to the structures surroun<ling the acpieduct of Sylvius, the ])ersistent part of the cavity constituting the aqueduct itself.

The thickeninir of the ventral wall of the vesicle results in the formation of the crura cerebri and the poaterior peribrated lamina nr space included between them. The crura first become apparent in the third month as a j»air of rounded longitudinal ridges on the ventral siirface of the vesicle. These remain relatively small until the fifth month, when the longitudinal fibers of the pons begin to grow into them. After this occurrence their increase in size is comparatively rapid, their ventral parts or cmsta becoming separated from each other ami iiidndiiig between tiieni the posterior perforated lamina.

The roof or dorsal wall of the mid-brain vesicle undergoes considerable thickening (Fig, 147), especially in the Sauropsida (birds, reptiles, fishes). In the fifth week a longitudinal ridge appears upon the dorsal wall, which in the third month is replaced by a furrow. The expansion of the wall on each side of the furrow produces a pair of rounded eminences (Figs. 148-151), which, in birds, attain to a much

Fig. .— Brain aire; f 6, foro-brafu ; lb, brain: P, ruliln uf pLa

greater development than in mammals and constitute the corpora bigemina or optic lobes. In the human emhr}'o, each of these elevations is divided into two by an oblique groove, and thus arc formed the coipora qtiadiigemina, which are peculiar to man and other mammals.

The part of the dorsal wall of the vesicle that underlies the corpora quadrigemina is the lamina qnadrigemina.

The thickening which the walls of the vesicle undergo to produce the several parts of the micl-bmin encroaches so miicli iiiK)n its cavity thatan I'.xceedinj^'ly small cjiual, the aqueduct of Sylvius, remains. It is scarcely necessary to piHiit out llijtt llii>; canal is a part of the ventricular system of the iiniii), ost:ihli'-hiu<;a ctminiunicatiou l>etweeu the fourth ventricle ami the thini ventricle or tyivity of the intcr-braiu. The Metamorphosis of the Inter -brain "Vesicle. — The inter-limiii vesicle results fn.m the division of the primary fore-brain vehicle, comprising what in lell of the latter after the outgrowth from it uf the diverticulum that l>ecome8 the secondary fore-brain. The thickening of the walls of the inter-brain vesicle produces the sirueture-s which surround the third ventricle in the mature eoudition, and which constitute collectively the thalomencephalon or inter-brain, the cavity of tJie vesicle persisting as the adult third ventricle. These Btructures are the optic thalajoi, which iirc iorincd from the lateral walls; the velum interpoBitum and the pineal bod7> which develop from the roof; and the lamina cinerea, the

Fig. ]4S,~A. mualsl icrtiDii IhroURh bniiii <i[ a huiunii rclUB of two-Bud-a-bfttf months (Hla): cA. cerebral lii'mlnphi-'ru ; o. ituWc UinliiiDue:/Hi. ri>niin«n of Monro; o{f, olOwtory tobc: p, pllultar; body ; no, minlulU (iblongaU: eq, corpora quadrlpmlDai tb, eerebetlum, B, brain of human Celui of (hrve montbi (HI*): olf, olftntory Inlie; rM, rnrpUB sltUlum; eq, corpora quadriffemlna ; eft, eerebBllnrnj inn, mcdiinn obloiiKOU.

tuber cinereum, the infiindibuluin, the posterior lobe of the pituitary body and the corpora albicantia, which are differentiatoil fiiuii the floor of the vesicle.

The lateral walls of the vesicle undergo the most marked thickening. The cell-multiplication here is so r.ipid that each lateral wall is converted into a large ovoiil inaris of cells with iiiterminglctl bands of fibers, the optic thalamus.

The roof of the inter-brain vesicle, in nutahle coiitra.st with the lateral walls, remains extremely thin throiighnnt the greater part of its extent (Fig. 144). Fmm ihe Ijack part of the roof, at a point immediately in front of the lamina iiiiadrigeiuina of the mid-brain, a diverticulum grows otit and becomes metamorphosed into the pineal tody. With this e.\ception, the roof of the vesicle reinains a single layer of epithelium, just aa in the ease of the roof of the afterbrain. This epithelial layer adheres closely to the pia mater, which covers it in common with the other parts of the hrain. As the fore-brain expands, it covers the inter-brain, the under surface of the cerebral hemispheres of the former l>eing closely applied to the roof of the latter. As a consetjucnce, the pia mater on the under surface of the fore

Fig. i:iO.—itra<n of A^tm of ihreemnnihs, enlarged. Tbc outer wall of the light

hcmlipheru hu been lemoveil ; LH, left bemlHpliere ; Ca, part of corpiu ■Irialum; FS, site of fossa of Kylviiis; I', vascular fold of pia mater which has been InvagInalcd ihniuRh the mesial wall of the hemisphere: Mb, miil-broln; C.ceiebellum; Jf , medulla oblongala,

brain is brought into contact with and adheres to the pia covering the roof of the inter-brain. Thus the thin epithelial roof of the inter-brain becomes closely united with the two layers of the pia luater to form the velnm iutarpodtam or tela choroidea anterior or superior of adult anatomy. Obviously, the edges of the velum interpositum rest upon the optic thalami, and its piamatral layers are continued into the cavities of the lateral ventricles (Fig. 150). The space occupied by the velum is designated the transyerse fissure of the brain, and it is often stated that the pia mater is pushed in from behind, between the optic thalami and the cerebral hemispheres. As will be seen from the above description^ however, its development really begins in front.

The pineal gland or conarium develops from the back part of the roof of the inter-brain at its point of junction with the mid-brain (Fig. 144). This body is found in all vertebrate animals except the amphioxus, but its form varies greatly in difierent groups. In all cases it begins as a small pouch-like evagination from the roof of the inter-brain, the diverticulum being directed forward. In the human brain alone the structure is subsequently directed backward, so that it conies to occupy a position just over the corpora quadrigemina. This peculiarity of location is due probably to the greater development of the human corpus callosum, by whicli the conarium is crowded backward.

In selachians (sharks and dog-fish), the enlarged vesicular end of the diverticulum, which is lined with ciliated columnar cells, lies outside the cranial capsule and is connected with the inter-bruin by the lonij: hollow stalk which perforates the roof of the (M-aiiiuni. In many reptiles, the conarium is more liighly specialized. In the chameleon, for example, the peripli(;ral extremity has the form of a small closed vesicle which lies outside the roof of the cranium and which is covered by a trans|)aroiit pat(*li of skin. The stalk in this case is ])artly a solid cord and ]>artly a hollow canal, which latter oj)ens into the cavity of the inter-brain. The solid portion lies within a foramen in the pjirietal bone, the parietal foramen. A farther modification of the conarium is jiresented in lizards, blind- worms, and some other reptiles. In these the vesicle underji^oes a marked specializjition, its peripheral wall being so nicxlilled as to become trans))arent and to resemble the crystalline lens of the eye, while the opposite deeper wall comes to consist of several layers of cells — some of which become piirmente<l — ainl ac(juire«* a striking resemblance to the retina. The stalk of the body, which perforates the roof of the skull and is attacheil to the roof of the interbrain, bears a certain likeness to the optic nerve, being solid and composed of fibers and elongated cells. The presence of the transparent epidermal plate which covers the vesicle serves to complete the similarity of this particular type of pineal body to the eye of vertebrate animals. It is for this reason that it is often designated the pineal or parietal eye and that it has been looked upon as a third or unpaired organ of vision.

In man and other mammals and in birds the pineal diverticulum does not reach the degree of development that is attained in certain of the Reptilia. The evagination from the roof of the inter-brain begins in the sixth week in the human embryo. The peripheral end of the process enlarges somewhat and small masses of cells project from it into the surrounding mesodermic tissue. These cellular outgrowths, giving off secondary projections, become converted into small closed follicles lined with columnar ciliated cells. The follicles in the case of mammals very soon become solid or nearly so by the accumulation of cells in their interior. Solid concretions of calcareous matter, the so-called brain-sand (acervulus cerebri) are found in the follicles in the adult. By these alterations the pineal body of birds and mammals acquires a structure resembling that of a glandular organ. Since it is onlv the end of the diverticulum that becomes thus altered, the remaining part constitutes the relatively slender stalk of the pineal body, the stalk being solid at maturity except at its point of attachment to the inter-brain, where a portion of the cavity persists as the pineal recess of the third ventricle.

The pineal body of man and the higher vertebrates is therefore a rudimentary structure and is the representative of an organ that is much more highly developed in some of the lower members of the same series. Its true significance is still a matter of conjecture. Although resembling the eye in its structure, and although regarded by some on that account as primitively an organ of vision, it is considered probable by others that in its most highly developed condition it is an organ of heat perception.

The floor of the inter-brain vesicle presents several interesting

iiu'tainorphosos. Tlui anterior ])art of the floor n?mains quite thill an<l Ix'coines the lamina cinerea of th<^ niatinv hraiu (Fig. 111). Iiuiiiediately posterior to this region, the floor of the vesiele poiK^hes out, this evagination developing into a slender IhIm', (he inftmdibuluin. Behind the [XHut of origin of the ini'undilMihiin a sc>eond protnheranee indicrates the beginning nf* (lie tuber cinereum. By subse(|uent altenitions, the tuber eiurreiini enlarmnir in ('ircuniferenee so as to include the point ol'ori<::in of the infundibnlnm, the base of attachment III" the infnn(lil)nluin eonies to be the center of the tuber riniTruni, so that the cavity of the former is a continuation of thr eavilv of the latter. the end of the infundibulum limiMies tiie posterior lobe of tiie pituitary body or hsrpoMliynlH ( Vi\r>, 144 and 140). Posterior to the tulxjr cinertiiiiii a small evagination of the floor of the vesicle .ippiMi'i an<l berimes divide<l in the early part of the fourth iiiniilh iiiln two lateral halves bv a median furrow. The Iwii bllh' bodies thus forme<l become, after further developiiH lit. ihr corpora albicantia.

I hr hypophysis or pituitary body briefly referred to above iii|iiiir: iimrr <'.\tende(l consideration because of its morlihuiti^firid liiipnriancc. The posterior lobe of this body is the il.ii^fiil nid of the infiindihulnm, which is an evagination of I hi iImiii 111' I he inlcr-brain. The cells in the lower end of I hi iiitiiiiihbiihnu specialize into nerve-cells, and ncrvelilii I < .il:ii drv<'h»p. In some lower vertebrates these eleiiii 111 < .111* i-i'iiiiiird throu(rhout lii'<\ but in man and the hi;'. hi I l\pi- niiiiiial> the distinctively nervous character of ihi II- 111 • I- -ooii lost, and the cavity of this part of the ihiiiiiilihiihiiii iill'ris oblitenition. The bmnched ])igmentiilh •iiiir(iiiir-i nM'oiTiii/jihh' in the j)osterior K)b(» of the hiiiiiaii piiiiitiir\ body an* the only remnant of tiie early III I \ »• ii'lU.

I hi' Hiitttiior lobn of the hypophysis is essentially different III Hiii'.iii ii^ wi'll MM in structure from tin* ]>ost<»rior h»be. It i- piodiii-nl b> nil cviiixination from the pcisterior wall of the piiiiiili\c phar\n\,l»ut from that region of the ])harvnx which i- anterior to the |»haryngcal membrane and which therefore bcloii^> to the primitiv(^ mouth-cavity (Fig. GO, p. l;ilj. The out-pocketing of the pharyngeal wall begins in the fourth week, shortly after the rupture of the pharyngeal membrane. The little pouch is the pocket of Bathke. The pouch grows upward and backward toward the floor of the inter-brain and meets the end of the infundibulum. As the pliaryngeal diverticulum lengthens, its stalk becomes a slender duct, which for some time retains its connection with the pharynx. As the membranous base of the skull becomes cartilaginous, the duct begins to atrophy, and finally entirely disappears. In selachians, however, it is retained permanently, establishing thus a connection between the hypophysis and the pharyngeal cavity. AVith the disappearance of the duct the enlarged extremity of the diverticulum becomes a closed vesicle lying now within the cavity of the brain-case, in contact with the end of the infundibulum. From the wall of the vesicle numerous little tubular projections grow out into the enveloping mesodermic tissue, and these, by detachment from the parent vesicle, become closed tubes or follicles. The entire structure becomes converted in this manner into a mass of closed follicles held together by connective tissue, after which event this mass acquires intimate union with the infundibular lobe.

Thus the pituitary body consists of two genetically distinct parts, the anterior lobe being derived from the ectoderm of the primitive pharyngeal or buccal cavity, and the posterior lobe from the ectoderm of the central nervous svstem. The posterior lobe, developing as it does as an evagination from the floor of the inter-brain, is to be regarded as a small outlying lobe of the brain.

AVHiat remains of the cavity of the inter-brain, after its walls have thus developed into the several structures described, is the third ventricle of the adult brain, and the aperture of communication with the secondary fore-brain vesicles becomes the foramen of Monro. Since the lateral walls become the massive optic thalami, while the dorsal and ventral walls give rise to much thinner structures, the cavity of the vesicle is encroached up(m to a greater extent on the sides than from above and below, and hence the form of the third ventricle in the mature condition is that of a narrow vertical fissure between the thalami.

The Metamorphosis of the Fore-brain Vesicle

The secondary lore-brain vesicle gives rise to the telencephalon, which includes the cerebral hemispheres and the structures belonging directly to them. As above indicated, this vesicle grows from the anterior wall of the primary forebntin vesicle as a diverticulum which is at first single, but which sfK)n becomes divided into two lateral halves by the formation of a cleft in the median plane (Fig. 147, /6). This cleft or interpallial fissure is the early representative of the longitudinal fissure of the adult cerebrum. The two vesicles remain attached at their bases or stalks with the parent vesicle and communicate by a common orifice with its cavity. The vesicles of tiie secondarj' fore-brain grow in an upward and backward direction as well as laterally, and their develo])ment is so much more raj)id than that of the other vesicles that they soon spread over them and partially hide them from view. It is for this reason that the mass resulting from the fore-bniin vesicles, except their basal ganglia, is known in comparative anatomy as tiie pallium or mantle (Fig. 144).

The relative rate of growth of the cerebral hemispheres is such that in the third month th(»y completely overlie the inter-bniin and bv the sixth month thev have extended so far back as to hide the corpora rjuadrigcniina.

The mesodermic tissue surrounding the developing brain becomes ditlerentiated into the three brain-membranes, which penetrate into the fissure and thereforc invest the vesicles on their mesial surfaces as well as elsewhere. The invaginating layers of the dura mater constitute the ])rimitive falx cerebri.

The metamorphosis of this pair of sacs into the cerebral hemis])heres is broujrht about by three important processes : first, the multiplication of the cells whicli compose its walls to form the masses <»f nerve-cells and fibers of the hemispheres ; second, the formation of folds in the wall whereby are pr<Khiced the fissures which divide the hemispheres into lobes and convolutions ; and third, the development of adhesions within certain areas between the mesial walls of the two vesicles, by which the system of commissures of the hemispheres is produced.

The walls of the cerebral vesicles are at first very thin, consisting merely of several layers of spindle-shaped cells. By the rapid multiplication of these cells, the walls are thick•ened and the cavity of the vesicle is gradually encroached upon until the mature condition of the brain is attained, when the cavity is relatively very much smaller than in the fetus and constitutes the ventricle of the hemisphere or the lateral ventricle. The nerve-cells develop processes or polar prolongations, of which the most conspicuous, the axis-cylinder processes, lengthen out to form the axis cylinders of nerve-fibers. The fibers thus formed are directed away from the surface and make up the white medullary matter of the hemispheres, while the more superficially placed layers of cells constitute the gray matter of the cortex of the brain.

In addition to the cortical or superficial gray matter there are masses of gray matter within the hemisphere, the basal ganglia, which are likewise collections of nerve-cells. Witliin a limited area on the lateral wall of each cerebral vesicle, near the lower margin, the cells undergo excessive proliferation resulting in the production of a large ganglionic mass, the corpus striatum, and of two smaller aggregations of cells, the claustrum and the nucleus amygdala. These basal ganglia are in reality an infolded part of the cortex.

Inasmuch as the cortical matter develops more rapidly, as regards superficial extent, than does the medullary substance, the cortex becomes thrown into folds, forming thus the convolutions and fissures of the hemispheres.

Some of the fissures of the brain are produced by an infolding of the entire thickness of the vesicle-wall so that their presence is indicated by corresponding projections in the walls of the ventricles. Such fissures are distinguished as total fissures. Included in this category are the fissure of Sylvius, which is represented in the wall of the lateral ventricle by the corpus striatum; the calcarine fissure, the dentate fissure, and the collateral fissure, which are responsible respectively for the calcar avis, the hippocampus major, and the collateral eminence of the lateral ventricle ; and the gnst transrerse flseure of the brain, the infolded wall in thia case being very thin and consisting merely of the layer of epithelium which covers the choroid plexus.

The flssnre of Sylvins is the earliest fissure formed and one of the most imjmrlant. At an early period in the history of the secondary fore-brain, there is a region in the lower part of the lateral wall of the vesicle where expansion is loss rapid than elsewhere, this area, as it were, remaining fixed. As the vesicle-wall innnediatoly surrounding thb

8))ot etmtiniies to expaml, n dimpling of the wall is produced, (he depression bcinfj (li'sii;imtc(l the fossa of Sylvius (Fig. 152, S'V The jKirt of the vcsiclo-wall In-hind the fos.'ia advances forwanl and downward to form the future temporal lobe, and thus till- fiwHJi ronu's ti» Ih' siirroiindwl hy a convolution having the form of an incfiniplctt' rinfr, i>|M'n in front — the ring lobe. The llcmr of tlii^ fossi undcinocs very eonsidorable thiekeninj; to form ilie basal ganglia — that is, the corpus striatum, the amygdaloid niielens, and lln" ehuistnim. These structures, most conspicuously tli<' i-orpns striafmn, cniToiioh ujKin the cjivily of iho vesiili', the nucleus caudatus of the

corpus Btriatum bulging intu the floor anJ outer wall of the adult lateral ventricle.

The cortical matter of the floor of the fossa of Sylvius, beiug circumscribed by a groove or sulcus, constitutes tbe

bi. wllh right half of fore-braJn. Dved: lb, CBvilf of inter-brHln; hy, stle of b^p. : Mbr. mid-brain roof; mv, nilil-braliL cuvily ; C wrvbi-lliiin : M. medulla

central lobe or island of Reil, which is subsequently brokea up, by seiMjndnry fissures, into from five to seven email convolutions.

By the extension of the fossa of Sylvius backward, and by the increased gi^owth of the vesicle-wall above and below it, the fossil is converted into the flssnre of Sylvius (Fig. 156, B)y and the island of Iteil is hidden from view. Subsequently the ascending and anterior limbs are added to the chief or horizontal part of the Kssure.

The anterior part of the ring lol>e corresponds with the future frontal lobe, the ]K>sterior part represents the parietal lobe while the lower part of the ring becomes the temporal lobe. A backward extension of the ring lobe produces the occipital lobe.

The cavity of the vesicle is mcKlifiiMl in form and extent eoincidentally with the formation of the corpus striatum and the alterations in the ring lobe. Just as the ring lobe partially encircles the fossa of Sylvius, so does the cavity of i\w. ventricle partially encircle the corpus striatum. An anterior prolongation of the cavity extends into the com|>l<»tc(I frontal lobe as the anterior comu of the ventricle, and iin (»xteusion downward and forward into the apex of the temporal lobe constitutes the descending comu, while the posterior horn is ^nulually protruded into the occipital lobe as ihr latter dcvc](>])s. From the earliest stage, therefore, until I he eoiii])lete(l condition is attained, the cavity of the ventrieli' eoiilonns in a general way to the shape of the henii■'?ph«Te. The a])ertiires of (Mummiuieation between the vesirli-j (>r thi* cerebrum and the eavitv of the inter-brain are the lithr Y shaped foramen commune anterius or the foramen of M«»iito.

The numial surfaces of the hemispheres are much modified ht ehintieirr by the (levelopnicnt here of two total fissures, (hr tiiruato flKHure and the choroid fissure. TIksc ap|)ear in Hie lillh week while the ve^i<*les are >till separate fnun each iiilnr ilnNMi III (heir Ntall\< of attaehnient to the inter-brain, pii«ii it» the development, th(M*efor(', of tile eorpiis callosuni .Old the Cnriiiv. The two lis<ni*e^ lie ejox' to<rother, pandlel Willi iiiili othri* an«l with the niarLrin of the riuir lobe, their r»»iir-.i' ront'nnniiiL: lo ihiit ot'ihe eaviiv t»t'the ventricle. Ik»'jiiiiiMiv ne;ir the anlrrioi* evtreniitv of the brain, ahove the

level of the corpus striatum, they pass backward and then downward and afterward forward to terminate near the anterior extremity of the temporal lobe, thus incompletely encircling the striate body.

The arcuate flssnre is the more peripherally placed of the two. Its anterior portion lies just above the region throughout which adhesions subsequently develop between the two hemispheres, or in other words, above the position of the future corpus callosum (Fig. 154, a./.). This part of the arcu

Fig. 154.— Mesial surface of left fore-brain vesicle of brain shown in Fig. 148 (F6) : /.3/, foramen of Monro, or opening into inter-brain ; o/, arcuate fissure : chj, choroid fissure ; r," randbogen," corresponding to future corpus callosum and fornix; o^f, olfactory lobe.

ate fissure is the sulcuB of the corpus callosum of the mature brain. The posterior segment, that which belongs to the temporal lobe (not present at this stage), is the future bippocampal or dentate Assure. The hippocampal fissure is represented ujion the mesial wall of the descending horn of the lateral ventricle by the prominence known as the hippocampus major.

The choroid fissure or fissure of the choroid plexus, forming an incompI<?te ring within, and parallel with, that described by the arcuate fissure, encircles the corpus striatum more closely (Figs. 154, 155). It begins at the foramen of Monro, and its anterior part lies under the position of the body of the future fornix. It then sweeps around into the tem])oral lobe and terminates near the anterior part of the latter. The fissure of the chon)id plexus, like other total fissures, is an infoMing of the wall of the cerebral vesicle. It presents the l>eculiarity, however, that the infolded part of the wall is extremely thin, consisting of but a single layer of epithelial cells. The pia mater, which everywhere closely invests the surface of the bniin, is infolded with the vesicle-wall, the infolded part becoming very vascular and constituting the choroid plexus of the lateral ventricle. The choroid plexus, although within the limits of the ventricle, is excluded, strictly sjKjaking, from its cavity by the layer of epithelium which still covers it and which has been simply pushed before it into that cavity. Since the epithelial layer is very thin and easily ruptured, the choroid fissure is apjiarently an opening into the cavity of the ventricle through which the pia enters ; in the adult it is called the great transverse fissnre of the brain.

The calcarine Assure, another of the total fissures, develops in the latter part of the third month as a branch of the arcuate fissure. It bulges into the mesial wall of the posterior horn of the ventricle, i)r()ducing the elevation known as the calcar avis or hippocampus minor. Since the posterior horn of the ventricle is developed as an extension of the cavity into the backward prolongation of the ring lobe which forms the occipital lobe, the calcarine fissure necessarily is later in appearing than the fissures above described.

The parieto-occipital fissure is added in the fourth month as a branch of the calcarine, ellecting the definite demarcation between the parietal and occi])ital lobes.

The fissure of Rolando develo]is in the latter part of the fifth month in two ])arts. The two furrows are at first entirely se])arat('(l from each other by an intervening area of cortex. Subsecjuently this part of the cortex sinks l>eneath the surface, as it were, sinec it expands less rajndly than the adjacent regions, and in this way the upi>er and lower limbs of the fissure become continuous. The sunken cortical area is to Ix* found even in the adult brain as a deep anneetant gyrus embedded in the Kolandic fissure at the position of its superior genu. TIk^ development of the fissure of Kolando effects the division betw(HMi the fnmtal and jwirietal lobes.

The collateral fissure appears in the sixth month as a longitudinal infolding of the mesial wall of the hemisphere below and parallel with the hippocarapal fissure. Being a total fissure, its presence affects the wall of the cavity of the vesicle, producing the eminentia collateralis. At about the same time the calloso-marginal Assure . makes its appearance, and this is morphologically continuous, through the medium of the post-limbic sulcus, with the collateral fissure (Fig. 157). These three fissures constitute the peripheral boundary of a region of the mesial wall which is known in morphology as the falciform or limbic lobe.

The longitudinal Assure in the early stage of the growth of the cerebrum separates the two vesicles from each other except at the place where they are attached to the inter-brain ; here the two sacs are united by that part of their common anterior wall which is immediately in front of the apertures of communication with the inter-brain and which is called the lamina terminalis.

The development of adhesions between the mesial surfaces of the hemisphere vesicles throughout certain definite areas marks the beginning of the corpus callosnm and the fornix. The fusion of these areas begins in the third month in the region corresponding to the anterior pillars of the fornix, the septum lucidum and the genu of the corpus callosum ; in the fifth and sixth months adhesion occurs in the position of the body of the fornix and of the body and splenium of the corpus callosum.

Although the central white medullary matter of the cerebral hemisphere is covered almost universally by the cortical gray matter, there is a limited area of the mesial surface from which the gray matter is absent, leaving the white matter ex]>osed. The area of uncovered white matter has the form of a narrow band, which begins at the base of the hemisphere, in front of the opening into the inter-brain, extends upward along the anterior wall of the inter-brain, then passes backward along its roof and curves downward and outward behind, and then forward under it, to terminate at the front part of the temporal lobe. Thus this white band, which is known as the fimbria, and which represents the lower mesial edge of the hemisphere, almost encircles the inter-brain. The fimbria runs between the arcuate fit^sureand the fissure of the choroid plexus (Fig. 155, /). It holds such a close relation to the lat

Fig. IM.— Mu'sl«l Bnrftcc of left hcmlsphcp;, hmln of fi'tim of three months (ciilurRiid) : /.. fiirnii: r.r.. beginning of u>rpus cHlluiam; c.rl., partot vuriiui atrtstiiiii iirelilnB uruiind fmra of Sylvius ; a/., unttrinr. mill nj./i.. |HiitvrU'r parti of urrnauj Huiin? ; rhj., i-l]i>ri>id Usrun.', the coni'ui li.v lH-tH'i'i.'ti u'liii'li und the corpni klrlatnin ucComniudatLii the luUT-linin, which hna Ik'vii ivmiirtil. The Itiaare U ■icL'Upiiil by the pla luutuc.

tor fissure, Iwiiig placet! on its ]>erii)licr.il side, that it constitutes the e<lge of the apjtaroiit o|Htniiig into the cavity of the vesicle thi-ough wliiuh the piji niiiter, iKiiriiig bloofl- vessels, is reflctrted iuto the interior, and which, as pointed out above, is the tniiisverM' fissure of the Itniin. The <i|ieuing is only a])piir(^nt, however, since tlif wall is still iinlirokfii, although reduced to ii single layer of ejntltflinin. The pia mater, forming, with its blood-vessels, the cliDniiil plexus of tlic lateral ventri.-Ie, pushes the layer "f cpirhellnin before it, and althoM^di the |)lexns is s;ii«l to be within the eiivily of the ventricle, it is still covtirod by the layer of epitliclinm, the ependrma, whifh lines that oiivity.

Thi' part of (he (iuibria that ini mediately overlies the roof of the inter-hrnin Iieeonii's iiitinmtely uniteil, as noted alx)ve, with the eorri'sponiling jiarl of the titiibria of the other heniis|ihen>, these fused portions of the two limhri;e forming a flat tnangular sheet, the body of the fornix. the anterior and IKisti'i'ior portions of the fimbria, wich diverge from the moilian plane, represent res|Mitiv<'ly the anterior ami posterior limbs of the fiiniix.

Noting the relation ->r tlu^ anterior part of the fimbria to till- a]>or(ur(Mif eonniinnii'ation between the inler-brain and the cereijnil vesicles, it becomes apparent that the anterior pillar of the fornix forms the anterior and n|»per Iwuiidarj' of

the foranifQ of Munro. When, further, one considers the relatiou uf the fimliria to the apparent oi>ening into the ventricle, through which the pia mater i« invaginated (the transverse fissure), it is explained why the edge of the fornix appears as a narrcjw white band, not only as viewed from within tile ventricular cavity, Imt tiho in a nit-Mal section of the brain (Fig. 156, C).

t^o. liiK.—Fctal tiriiln at thr \ieg\xm\ng of Ihe tiiihlh moiitli <MlliaIkovlm> : A.iiiperloi, B. Inderal, C, rnvslnl aiirfaru: K. tliaiiK of KoUndo: prf, ■■reecnlral fi«Buw; S|f, HylirlanHwure: inip, lntBrp»rletiil llHiiin.'; jhw, parfet(ww«lpil«l(i»aurB! pU, psratlel tiiuurv: eailra, calloiomirglnnl Buure: u'T, unoui : cale. culCBTine

Another important region of fusion of the opposed mesial surfaces of the hemispheres is that corresponding lo the future corpus calloBum Throughout this area the liemispheres closely unite with each otiier The line of fusion begins at the Imses of the vesicles, some little distance in front of the anterior parts of the fimbriie (Fig. 155, f-c). and after passing upward and luruird, curves horizontally backward kward I

in close relation with the fused portions of the fimbria?, now the body of the fornix. The atUiesiou begins at the anterior part in the third month, and atfects the regit)n of the body and gplenlum of the future corpus callosum in the fifth and sixth mouths. Fibers penetrate from one hemisphere to the , other throughout this zone of contact, intimately uniting the cerebral hemispheres. The corpus callosum is therefore & great commissure lietween the two halves of the cerebrum, and is necessarily composed of fibers having a transverse 1 direction.

While the back part of the corpus callosum lies over the body of the fornix and is in close contact with it, the front part of the body of the corpus collusum, as also its genu or | curve and its rostrum or ascending part are at some distance ( from the front parts of the fimbrite. In other words, while the j great longitudinal fissure extends at first to the bases of the cerebral vesicles, this fissure is made relatively loss deep by the adhesions which occur between the mesial walls and which result in the development of the corpus callosum ; and the space below the anterior part of the corpus callosum, between it and the anterior parts of the fimhriie (Fig. 1.56, C), is an isobtied part of the great lonrjitudinal fissure. This space is bounded on either side by thatjtart nf the wall of the corres- , ponding cerebral vesicle or hemisphere which is limited above | and in front liy the corpus collusum, and behind by the anterior part of the fimbria or anterior limb of the fornix. The ' space is the so-cniled fifth ventricle of the adult brain. The | circumscribed parts of the mesial walls of the hemisphei which form the lateral walls of the space, together constitute ' the Beptum lucidum. The jtarts of the hemisphere walls that ' become the septum lucidum do not participate iu the procesa ' of fusion mentioned above. Their surfaces are iu contact) [ however, and do not develop the typical gray cortical matter, I such OS appears elsewhere ujKin the surface of the cerebrum. J Cortical gray matter is produced here, but only iu radi- 1 mentary form.

From what has been said, it will be seen that the t layers of the septum lucidum are circumscritKid and opposed.

parts of the mesial walla of the hemispheres ; that the fifth ventricle is not a tnie ventricle but an isolated part of the longitudinal tiggure having no connection whatever witli the system of ventricular cavities ; and that this .so-called ventricle is not, like the true ventricles of the brain, lined with ependyma, but with atrophic gray cortical matter.

The limbic lobe has been referred to as that part of the mesial aurf'ace of the hemisphere which is circumscribed by the calloso- marginal fissure, the post-limbic sulcus, and the collateral fissure. It is limite<l centrally by the fissure of the corpus callosum and the hippocampal fissure, which are represented in the fetal brain by the single uninterrupted arcuate fissure. Hence the limbic lobe would include the gyrus fornicatus, the isthmus, and the gyrus uncinatua, which constitute morphologically a single ring-like convolution. Schwalbe, however, includes with this so-called limbic lobe all the surface of the mesial wall of the hemisphere included between the arcuate fissure and the fissure of the choroid plexua (Fig. 154), designating it the folciform lobe (Fig. 157).

The falciform lobo therefore consists of two ring-like convolutions, one within the other, the two l)eing separated from each other by the arcuate fissure (the adult callosal and dentaf« fissures) and being limited centrally by the fissure of the choruiii plexus (the great transverse fitisurc of the adult brain). While the outer of these concentric convolutions — the limbic lobe of Broca — develops into the fornicate or callosal, the isthmian, and the uncinate gyri, the inner ring differentiates but slightly, its cortical matter remaining atrophic. The atrophic condition of the cortex here is associated with those adhesions between the mesial walls of the hemispheres that result in the formation of the corpus callosum and the septum lucidum. By these adhesions the continuity of the inner concentric convolution is broken, and it is therefore represented, after the development of the corpus callosum, by the atrophic gray matter of the septum lucidum, by the gyrus dentatus, and by the lateral longitudinal striae on the free surface of the cor])us callosum, the latter being an atrophic or rudimentary convolution. Since the transverse fissure of the brain is the centric boundary of the ring, the fornix is also a part of the falciform lobe. To sum up, the falciform lobe includes the gyrus fornicatus, the isthmus, the gyrus uncinatus, the lateral longitudinal striae or taenia tectae of the corpus callosum, the gyrus dentatus, the lamina* of the septum lucidum and the fornix.

The olfactory lobe or rhinencephalon is an outgrowth from the vesicle of the cerebral hemisphere. Its development begins in the fifth week by the pouehing-out of the wall of the vesicle near the anterior part of its fioor (Figs. l-t7 and 149). This diverticulum, which contains a cavity contiinious with that (»f the vesicle, grows forward and so<ni l)ecomes somewhat clul)-sha]>ed. In the selachian.'^ (.sharks and dog-fish) the projection attains a great relative size, the olfactory lobes in these animals being one of the most eonsj)icuous ]uirts of the brain. In all mammals except man it is well developed, and in the horse its cavity persists throughout life. In man the cavitv soon becomes obliterated and the lobe itself in part aborts. The ])rotru(le(l })ortion, becoming more distinctly club-shaped, differentiates into the olfactory bulb and the olfactory tract, the ))osition of the original cavity being indicated by a more or less central mass of neuroglia conspicuous in cross-sections of those structures. The proximal portion (»f the olfactory lobe is represented in the adult human Urain by the gray matter of the anterior perforated lamina {or space), ami by the trigonuin ol facto riiiiii and the area of Broca, as well aa by the inner and outer roots of the olfactory tract (note olfactorj- lobe of dog's brain, t"ig. 156).

Fig.— Buc of dogabialn: al., o\lar\otj lube: a.ji,»., rcxiin corrcipandiiiK M r perforaled spa<-c. which la Incluiled in the ulllictorv l<ibc ; /.S.. IliiBurt nT i; a.h., hlppooampal ((>tus. deTelopvd to a Rn-iitor rtpRree than In human 1., leellonal siirCice of oltactoty lobe: m.. olltictury sulcus.

Because of the relation of the place of evagination of the olfactory lobe to the fossa of Sylvius, it happens that a |>art of this lobe, the anterior perforated lamina, is i^ititatcd at the commeneeincnt of the fissure of Sylvius and that it is in eoiithmity with Iroth extremities of the ring lobe; hence, the olfactory lobe Is connected with both extremities of the falciform lobe. To express it in the language of human anatomy, the outer or lateral root of the olfactory tract is connected with the gyiuB uncinatUB, while the inner or mesial root'may be traced to the fore part of the gyrus fomicatns.

After what has been said, the reader need scarcely be reminded that the olfactory bulb and tract, often erroneously referred to as the olfactory nerAe, are parts of a lobe of the

iirain, a lobe which in man is rudimentary but which in all o(h(M* mammals is well developed.

Tabulated Rudiment of ihe Derivatives of the Brain-Besides,

Hkain VKtiK'l.KM. AfliT tiralu

Ililidbrnln vuhIcIo.

Mill tiruin VVblcie.

IlltlT

hritiii

Floor.

Medulla oblongata.

PonH Varolii.

Roof.

Tela choroidea inferior.

Lateral walls.

Inferior peduncles of cerebellum.

Hiiroiidary fi in* bruin Vfnirlt*.

Peduncles of cerebrum. Posterior perforated space.

(N)rpora albicantlii. Tuber cintTcum, infundlbulum, and imrt of hypojihvHis. Uptic ehiiiHm.

AntiTlor |K'rfonittMl lamina. CorpUM 8triiitum. Island of \W\\. Olfai'tory lolic.

Po8terit)r medullary velum. Cerebellum. Anteri(»r medullary velum.

Posterior part of toKnientum.

Corpora quadriuemina. I^amina quadrigemina.

Pineal l>ody. Posterior commissure. Epithelium of velum interpositum.

Middle and superior peduncles of cerebellum.

Brachia. Internal geniculate bodies.

Optic thalami.

Convolutions of cerebral hemisplieres. Corpus rallosum. Fornix. iSeptum lucidum.

Cavitt.

Fourth ventricle.

Aqueduct of Sylvius.

Third ventricle.

Lateral ventricles.

The Development of the Peripheral Nervous System

The development of the pcri])heral nervous system is still iiiV(»lvr<l ill some ilejxroe of obscurity. In general terms it nmy Iw .slal4*(l that tin* ])eri])lieral nervous apparatus is derive<l as an e\t<>nsion of the central cerehro-spinal axis.

In the ease of the spinal nerves, e^'ich nerve-trunk is com* poseil of hoth motor aiul sen.<orv fihers, the former being in continuity with the spinal cord tlirough the medium of the anterior (»r motor roots, and the latter through the posterior or sensory roots, ea<'h sensory root ))o.^.sessing a ganglion. The cranial nerves exhibit a less n^giilar composition. While the trigeminal nerve, for e.xam|>le, arises by two roots, after the manner of a spinal nerve, some others correspcmd in relative {N)sition and in mod(» of d<»velopinent to the ventral or motor r(M>ts of th(» spinal nerves, and still others are equivalent to the Hi»nsory spinal roots.

The deTelopment of the sensoir nerve-Sbers ib d^jH-udent upon and is preceiicd liy tliiit of the ganglia of the posterior roots of the tspinal nervi'.=, and nf several ganglia of the head rt'giiin which are related tu the development of certain of the cranial nerves. Hence the consideration of the genesis of

Fig. IM.

oritr Rnbl). The

primitive ergments vn atlLt conneL'Icd wilh Ihe reiaiiluluK porllnn genn-ltiTer. At the regiuu of tmnnltlon lliere ie to be ii

muHclc-plato ut the primiLlvc x gemi-Uyer ; pmi. i»ri«Ul, imi6, vlwx-ral mldillB Ujrpr. 8, crotMCClion through a Ilunl emhryo (nflvr Sagvuii'hl) : m, spinal cori] : tpa. lower thickened part of (he nuurul ridge: ipp'. Its Dpptr allcnuated part, which Ii conllnuoiu with the Toorof the Deural tube : ui. primitive aegmunt,

the ganglia niiist precede the account of the growth of the sensory nerve-fibers.

The oriKin of the ganglia is connected with the early history of the evoliilion nf the neural tuhe. Just after the sides of the medullary plate {vide p. 279) have united with each other to form the neural tube, there appear two ridges of cells between the tube and the epidermis, one on each side of the raphe or line of union of the sides of the tube. Thene ridges are the neural crests (Fig. 159). They first appear in the region of the hind-brain and advance from this point both headward and tailward. The ganglia develop from these neural crests. The cells of the neural crest are usually described as growing out from the neural tube, though according to His it is probable that they originate singly from the ectoderm.

Tlic mass of cells composing the neural crest grows out\vard and then ventrad along the Avail of the neural tube, and very soon undergoes segmentation into a number of cellmasses which are the rudimentary ganglia. In the spinal region the number of segments corresponds to the number of future spinal nerves. In the head region there are four segments. These latter, the cephalic ganglia, will be referred to sul)se(iuently.

The segmentation of the neural crest corresponds in the main witli the sogmcntation of the ])araxial plate of the inc.-odcrm, whereby the myotomes are ))ro(luced, and ench segment lies upon the inner side of :i myotome. The connection of the segments with the neural tnl)e beccmies reduced in each case to a slender strand, the point of continuity of which with the tube is shifted farther awav from the median line, as (level(>])nient proi^resses, to eorresptmd with tl)e dorsolateral position of tiie sensory nerve-roots in the mature condition.

theeells of the tranirlia ae<|nir(» eaeli an axis-evlinder process and a (h-ndrite or |)n>toj)la>iiiie process, beeoniing thus bijKjIar e<'lls. Tli«' axons or axis-eylinder |>i'oeesses make their way into the spinal e<)nl — in the ease of* the spinal gauiilia — eunstitnting ilui> the dorsal nerve-roots, and ])nrsne their eour>e within tiie eord as the e(>lnnins of (loll and Bnr<la('h. The (hndrites, eon>titntini:' llie distal ]>ortions of the dorsal roots, join tlir ventral r(K)i< (»ii the distal si<le of the iranulia and lM'<'onie liie sensory nerve-fibers of* the spinal nerves. Allhonirji these two proe«.*sses grow out from opposite sides of the cell, the further growth of the cell is such that both processes are connected with it by a common stalk, thus producing the cell with T-sha|)ed process so characteristic of the spinal ganglia. Thus the ganglia are made up of cells which are interpolated in the course of the sensory nerve-fibers, and these cells may be regarded as having mignited from the developing cerebrospinal axis, or, if the view of His be acc^ptwl, from the region of the e^ctoderm from which the tube originates, their connection with the axis l)eing maintained by the gradually lengthening out axis-cylinder process.

The development of the motor nerve-fibers differs from that of the sensory. These fibers, or at least, the axis cylinders of the fibers, are the elongated neurits of nerve-cells of the spinal cord and brain. The neuroblasts of the thickened neural tube, as they become fully differentiated nerve-cells, migrate from their central position into the mantel layer, or superficial stratum (Fig. 140). On the distal side of the nucleus of the cell, the protoplasm first becomes massed and then lengthens out to form an axis-cylinder processor neurit, which in all vertebrate animals grows out from the cerebrospinal axis to form the axis-cylinder of a motor nerve-fiber.

Although, in the wise of the spinal nerves, the motor and sensory fibers are separated from each other at their origin from the cord, they soon intermingle to constitute a spinal nerve-trunk. In certain lower types, as cyclostomes and amphioxus, the motor and the sensory fibers permanently pursue separate routes to their ]>eriphenil distribution.

The envelopes of the nerve-fiber are acquired at a relatively late period. The appearance of the neurilemma precedes that of the white substance of Schwann. The neurilemma is derived from the mesmlerm. The cells of the latter apply themselves to the nerve and, penetrating between the fibers, become arranged as an enveloping layer upon each axis cylinder, ultimately forming a complete sheath, the neurilemma. The persistent nuclei of these cells, scantily surrounded with protoplasm, constitute the nervecorpuscles of the neurilemma. The medulla, or white substance of Schwann, is formed at a considerably later period within the neurilemma. The ileixwit of the medullary sheath 1 varies as to time lor (Jifferent groups of SIhts — although the ] time is constant for each groti|j — unJ proceeds always in a direction away from the cell from which the fiber originates, or, differently expressed, in the direction in which the fiber eonveys impulses. Thus, in the spinal cord, groups of afferent fibers may be distinguished from those that are efferent hf observing the direction in wliich the medullary sheath develops — that is, whetiier tlio sheath appears first at the upper end | of the fiber or at the lower end.

The cranial nerve-flbers in their development follow in the \ main the same general principles that govern the growth of the iipina! nerves. That is to say, the motor fibers grow out 1 as extensions of the axis-cylinder processes of nerve-cells of i the cephalic jmrt of the neural tul)e and the sensory fibers \ proceed from the cells of outlying ganglia, or in the case of at least one nerve, the olfactory, from infolded and highly I specialized ceils of the ectoderm.

The cephalic ganglia, four in number, have been referred I to as resulting from tiie segmentation of the head-region of ] the neural crest. As previously stated, the neural crest I begins to grow first in the region of the hind-brain and j extends from this point both forward and backward, occupying a iwsition upon the roof or dorsal wall of the hind-brain. \ The part of the neural crest belonging to the head-region i then divides into the four masses or head-giinglia which are I designated respectively the first or tiigemmaJ, the second or ] acosticofadal, the third or glosBophanrngeal, and the fourth ; or vagal, ganglia.

The trigeminal ganglion, which is very large, becomes di- i vidwl into a smaller anterior (M>rtiou, the ophthalmic or ciliaiT ganglion, and a. larger posterior segment, the tiigemlsal j ganglion proper. These two become widely so|mrated durin^J llic pnigres.'i of development, since they constitute respeo-J lively the later ciliary and aasserian ganglia, the ciliary I ganglion belonging to the ophthalmic division of the fifth I nerve, while the trigeminal belongs to the sui^-rior maxillary J division and the sensory part of the inferior maxillary division of the fifth. Their nerve-cells give rW. to the sensory fibers of these trunks in the same manner that the cells of the spinal ganglia produce the sensory fibers of the spinal nerves.

The acusticofacial ganglion, afler its migration from its original position on the dorsum of the hind-brain, lies just in front of the otic vesicle. This ganglion subsequently divides into the facial and the acoustic ganglia. The facial ganglion, the geniculate ganglion or intumescentia ganglioform is of the facial nerve, situated in the facial canal of the temporal bone, although described as a ganglion upon a motor nerve, the facial, is, in reality, connected mainly with the pars intermedia, a bundle of sensory fibers issuing from the nucleus of origin of the glossopharyngeal nerve. It is equivalent therefore to a spinal ganglion.

The acoustic portion of the acusticofacial ganglion divides still further to become the ganglion on the vestibular part of the auditory nerve, and the ganglion spirale of the cochlear division of the auditory, which latter is situated in the spiral canal of the modiolus. It is considered probable that the lateral accessory auditory nucleus, which is connected with the cochlear fibers of the auditory nerve and lies on the outer side of the restiform body, is also a part of the acoustic ganglion. From the cells of the vestibular ganglion, which is situated in the internal meatus, centrifugal fibers develop to form the vestibular nerve, while other centripetally growing fibers become the ventral or mesial (vestibular) root of the auditory nerve. The cochlear ganglion in the same way gives rise to the cochlear branch of the nerve and to its dorsal or lateral root. Thus the auditory nerve and its ganglia correspond respectively to the sensory root of a spinal nerve and to a spinal ganglion.

The third cephalic ganglion becomes the ganglion of the glossopharyngeal nerve, undergoing segmentation to form the upper or jugular and the lower or petrous ganglia of this nerve, while the axis-cylinder processes of its cells lengthen out to become the sensory fibers.

The fourth cephalic ganglion similarly becomes the two ganglia of the pneumogastric nerve and gives rise to its sensory fibers.

From what has been said, it will be apparent that the cranial nerves develop in a far less regular manner than the spinal nerves, and that consequently their trunks consist in some cases of only sensory fibers, in other cases of only motor fibers, and in still others, of both varieties. Typically, each cranial nerve would have a dorsal sensory root with a ganglion, and two motor roots, one lateral and the other ventral. But by the suppression of one or two of these typical roots there will be produced a nerve, for example, representing only the ventral root, as the sixth and twelfth nerves, or a trunk containing sensory and lateral motor fibers, as the vagus, or a nerve consisting solely of sensory fibers, as the auditory.

By way of recapitulation the cranial nerves may be briefly considered seriatim :

First Pair. — The olfactory nerve-filaments grow centripetal ly from the olfactory epithelium of the nasal mucous membrane.

Second Pair. — The optic nerve is not a true nerve (see Chapter XVI.).

Third Pair. — The oculomotor nerve represents a persistent lateral motor root of the first head-segment (the ophthalmic division of the fifth nerve being the sensory root of the same segment).

Fourth Pair. — The trochlear nerve represents a lateral motor root and belongs to the second head-segment.

Fifth Pair. — The trifacial or trigeminal nerve, containing sensory and motor fibers, represents a persistent lateral motor root and a dorsal sensory root. The ophthalmic portion of the sensory root belongs to the first head-segment, while all the remaining fibers, with the fourth nerve, are assigned to the second segment.

Sixth Pair. — The abducens develops as a ventral motor root and belongs to the third and possibly to the fourth segments.

Seventh and Eighth Pairs. — The acusticofacialis nerve, or the facial and auditory nerves, develop as a single nerve with several roots. The auditory nerve and the sensory fibers the facial — that is, the pars intermedia — correspond to a dorsal sensory root, the division of the acusticofacial ganglion into the several ganglia of the auditory nerve and the geniculate ganglion of the facial accounting for the division of the root into the auditory trunk and the ]xirs intermedia. (The sen* sory tibors of the facial pass off through the chorda tympani to go to the tongue as special-sense fibers.) The motor fibers of the facial develop as a lateral motor root^ originating from c-ells in the ventral zone. These two nerves, with the sixth, belong to the third and possibly to the fourth head* segments.

Ninth Pair. — The glossopharyngeal nerve, made up largely of s(»ns()rv fibers?, re[)resents a dorsal sensory root and a lateral motor root, the fib(»rs of which latter grow out from cells in the dorsal ])art of the v(»ntral zone of His, the later micleus ambigmis. It belongs to the fifth head-segment.

Tenth Pair. — The vagus develops in the same manner as the glossoj)harvngeal.

Eleventh Pair. — The sj)inal acoessorv represents in part motor spinal roots and in [)art probably the lateral motor and dorsal scnsorv roots of the cranial nerves.

Twelfth Pair. — The liy[)ogIossal develops as the ventral motor roots of several segments, being identical in mode of origin with the anterior roots of the sjnnal nerves. This nerve and the vati^us b(*long to the head-sc»gments from the sixth to the tenth inclusive.

The Development of the Sympathetic System

There are two views as to the origin of the sympathetio system. One theory, based uj)on the investigjitions of Paterson, is that the gangliated cord of the sym[)athetic is differentiated from mesodermie cells, the eell-eord thus formed acquiring, secondarily, coinieetions with the spinal nerves, and ])res<Miting still hiter the enlargements which C4mstitute the ganglia.

The more genendly accept(»d vi(?w, based upon the researches of Rilfour and the later work of Onodi and His, is that the Bsrmpathetie ganglia develop as offshoots from the ventral extremities of the spinal ganglia. Each little mass, which has budded off from a spinal ganglion, moves somewhat toward the ventral surface of the body, its bond of union with the parent spinal ganglion being drawn out to a slender cord, the representative of the future ramus communicans. Each primitive sympathetic ganglion sends out two small processes, one growing tail ward from its lower extremity, and one in the opposite direction from its upper end, the approaching processes from each two adjacent ganglia meeting and uniting and thus secondarily establishing the connection between the different ganglia of one side of the body and forming the gangliated cord of the ssrmpathetic. From these ganglia migrating cells probably pass out to develop into the secondary ganglia of certain viscera, as His has shown to be the mode of origin of the ganglia of the heart.

The Carotid Body, the Coccygeal Body, the Organs of Zuckerkandl

In connection with the sympathetic system may be mentioned the " carotid gland," or glomus caroticus, or intercarotid ganglion, found at the bifurcation of the common carotid artery ; the coccygeal body or " gland," Luschka's ganglion, found at the lower extremity of the coccyx in relation with branches of the middle sacral artery; and the organs of Zuckerkandl, found in later fetal life and for a short time after birth at the origin of the inferior mesenteric artery.

These structures present features in common with each other in that they are made up of knots of blood-vessels intermingled with collections of cells, among which are numerous chroviaffine cells such as are found in the medulla of the adrenal body and in the sympathetic ganglia ; and in the furthor fact that they are penetrated by sympathetic nerve-fibers.

That the cells of the carotid body and of the organs of Zuckerkandl are derived from the adjacent sympathetic ganglia has been established, but whether these bodies are for that reason to be classed as nervous structures is as yet uncertain.

Book - A Text-book of Embryology 15

Contents