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believed to be absent from the nerve fibres in the brain and spinal cord, as well as at the peripheral terminations of many nerves. The medullary sheath is a fatty and albuminous substance, which refracts the light strongly. Not unfrequently it collects into little ball-like masses, and sometimes causes irregular bulgings on the fibre, and produces a knotted, varicose appearance; at other times it becomes granular, and makes the fibre opaque. By gentle pressure it can be squeezed out of the broken end of a fibre. The axial cylinder is a pale, grey, cylindriform band, usually about one-third or one-fourth the diameter of the fibre, which possesses more tenacity than the medullary sheath, and not unfrequently, as in Fig. 56, 2, projects for some distance beyond the broken end of a fibre. Max Schultze showed that it is not homogeneous, but exhibits a very delicate longitudinal fibrillation, and at the ends of the nerves these primitive fibrillæ may separate from each other. Although from its great delicacy the axial cylinder cannot be seen in the living fibre of a cerebro-spinal nerve, yet there are many reasons for regarding it as a structure existing in the living nerve, and not the product of a post mortem change. It is the part of a fibre which first appears in the course of development, the medullary sheath and primitive membrane being secondary investing structures, superadded as development proceeds. It forms not unfrequently the only constituent of a nerve fibre at its central and peripheral terminations, and is therefore the part of the fibre which is anatomically continuous with the nerve cell, or with the peripheral end-organ. As it is the sole constituent of many nerve fibres at their terminations, and of all nerve fibres in the earlier stage of development, and as it forms the medium of connection between them and the structures in which they terminate, it is obviously of primary importance, both anatomically and physiologically, and is believed to be the part of the fibre directly concerned in the conduction of impulses; whilst the investing structures serve the purpose of insulating materials. Lister and Turner pointed out, in 1859, that essential differences in chemical composition existed between the axial cylinder and the medullary sheath; the former being unaffected by chromic acid, though the latter is rendered opaque and brown, and concentrically striated under its influence; while, on the other hand, the axial cylinder is stained red by an ammoniacal solution of carmine with great facility, although the medullary sheath is unaffected by it. They further showed that these differences in the mode of action of chromic acid and carmine might advantageously be employed in the demonstration of the structure of nerve fibres. Ranke has subsequently stated that the axial cylinder possesses an acid, and the medullary sheath an alkaline reaction.

Medullated nerve fibres vary materially in diameter in different parts of the nervous system. In the brain, for instance, they are sometimes as fine as the th inch; whilst, in the distributory nerves, fibres of th of an inch in diameter may be seen; though it should be stated that, even in the nerves of distribution, fibres of great minuteness are often placed in the same bundle with those of the largest size. Nerve fibres do not branch in their course, but only at their central or peripheral terminations, and much more frequently at the latter than the former.

Non-medullated Nerve Fibres.-These fibres, which are characterised by the absence of a medullary sheath, are chiefly found in the sympathetic nervous system, but they occur also in the cerebro-spinal system. The fibres of the olfactory nerve are non-medullated, so also are the peripheral terminations of the cerebro-spinal nerves, and indeed all nerve fibres in the first stage of their development. In Petromyzon it has been stated that all the nerve fibres are distinguished by the absence of a medullary sheath.

This form of nerve fibre consists of pale grey, translucent, flattened bands, theth toth inch in diameter. They usually appear as if homogeneous or faintly granular; but Schultze showed that, when carefully examined, they present a delicate fibrillated appearance, like that seen in the axial cylinder of a medullated nerve; so that, like that cylinder, they are supposed to be composed of multitudes of extremely delicate primitive fibrilla imbedded in a finely granulated material. Sometimes these fibres consist solely of this fibrillated material, at other times they are invested by a sheath similar to the primitive membrane of a medullated fibre. Nuclei are also found both in the substance of the fibre and in relation with the primitive membrane. The FIG. 57. presence of multitudes of fibres in the sympathetic nervous system, formed either entirely, or almost entirely, of a material precisely similar in structure to the axial cylinder of a medullated fibre, and by which the proper function of the fibre can alone, therefore, be exercised, is, of course, an additional argument to those previously advanced, in favour of the existence of the axial cylinder as a normal constituent of the fibre, and of its functional importance.

-Nonmedullated

nerve fibres from the sympathetic sys

tem.

Nerve Cells.-Nerve cells constitute an important division of the nervous tissue. They are the characteristic structures in the nerve centres, are susceptible to impressions, or nervous impulses, and are the texture in which the molecular changes occur that produce or disengage the special form of energy named nerve energy, the evolution of which is the distinctive mark of a nerve centre. The central extremities of the nerve fibres lie in relation to, and are often directly continous with, the nerve cells. It was at one time thought that nerve cells were globular in form; but it is now generally understood that, though the body of the cell is not unfrequently globular, two or more processes or poles project from it, and are continuous with its substance. Nerve cells are distinctly nucleated; the nuclei are usually large, and contain one, and often two nucleoli. The cell substance is granular, and not unfrequently brown or yellow pigment is collected around the nucleus. A cell wall is sometimes apparently present, though at others it cannot be demonstrated. The nerve cells in the grey matter of the brain and spinal cord are imbedded in the neuroglia. In the smaller nerve centres, as the sympathetic ganglia and the ganglia on the posterior roots of the spinal nerves, the nerve cells are surrounded by a capsule of connective tissue. Fräntzel, Kölliker, and others, have described this capsule as lined by an endothelium formed of flattened cells; and it should be stated that Ranvier has described a similar endothelium in relation to the connective tissue investment of the cerebro-spinal nerves. It is not improbable that these endothelial cells form the walls of delicate capillary rootlets of the lymphatic vascular system.

Nerve cells from which two poles or processes proceed are called bipolar. Charac teristic specimens of these cells, as was first pointed out by Robin and R. Wagner, may be recognised without difficulty in the ganglia on the posterior roots of the spinal nerves of fishes, and it is probable that FIG. similar cells exist in the corresponding centres in other vertebrates. These cells tinuous with it, from the spinal ganglion usually possess a globular body, though of a skate. sometimes it may be elongated; and from opposite points

58. - Bipolar

nerve cell, with two nerve fibres con

sp

AC

of the surface of the body a strong process is given off, | multi-polar nerve cells of the brain and spinal cord as which is directly continued into a nerve fibre. The axial forming an excessively cylinder of the fibre is continuous with the cell substance, minute network, from and Schultze has shown that both exhibit a delicate fibril- which minute medullated lated structure. The medullary sheath and the primitive nerve fibres arise; and F. membrane are also usually continued from the fibre over Boll conceives that a simithe nerve cell. Hence these bipolar cells seem to be, as lar arrangement occurs in Schultze expressed it, nucleated enlargements of the axial the cells of the cerebellum. cylinder. One, at least, of the processes of a multipolar nerve cell does not branch, but becomes directly continuous with a nerve fibre, and has been named the axial - cylinder process. This process was first recognised by Deiters in the cells of the spinal cord; but Hadlich and Koschennikoff have since described the central process of the cells of the cerebellum as continuous with a medullated nerve fibre; and the latter observer has pointed out, that from the base of a pyramidal nerve cell in a cerebral convolution a process may be traced directly into a nerve fibre. Hence it would appear that the multipolar nerve cells may have two modes of union with nerve fibres-one directly through the passage of the non-branched axial-cylinder process into a fibre, the other through the origin of fibres from the minute network in which the branched protoplasm processes terminate. The branched processes of adjacent nerve cells may also blend with each other, so as to form an anastomosing cell network, though these anastomoses are, in all probability, not so frequent as was at one time supposed. Schultze has pointed out that not only the protoplasm substance of the body of a multipolar nerve cell, but both the non-branched and branched processes, possess a fibrillated structure similar to that described by him in the axial cylinder of the nerve fibres.

A remarkable modification of the bipolar nerve cell, carefully studied and described by Lionel Beale, is found in the sympathetic ganglia of the frog. The cells are pear-shaped, and from the narrow end of the pear two nerve fibres arise, one of which, called the straight fibre, forms, as it were, the stalk of the pear; whilst the other, or spiral fibre, winds spirally round the straight fibre, and then passes away from the cell in the opposite direction. Both fibres are nucleated, and at their origin consist, apparently, of axial cylinder substance only; but in their course they may acquire both a medullary sheath and a primitive membrane. The straight fibre passes into the interior of the cell substance, and Arnold and Courvoisier be- FIG. 59.- Pyriform lieve that they have traced it into the nerve cell St, nucleus; but the spiral fibre apparently Sp. spiral nerve arises nearer the periphery of the cell. The pyriform cells are invested by a distinct capsule of connective tissue. The nerve fibres of these pyriform cells, although they both arise close together from one end of the cell, represent its poles. Should one of the poles, either in this, or in the bipolar form of nerve cell described in the preceding paragraph, be from any cause removed or not developed, then the cell would be unipolar; and if both poles were absent it would be apolar.

ՏԵ

straight nerve fibre;

fibre; C, capsule of
connective tissue
around nerve cell.
(After Beale.)

In other localities, as in the sympathetic ganglia of man and many other vertebrates, and in the several subdivisions of the cerebro-spinal nervous axis, the nerve cells have more than two poles or processes projecting from them. Cells of this kind are called multipolar, and in many localities they present characteristic forms. In the grey matter of the spinal cord, more especially in its anterior horn, they give rise to numerous processes, and have a stellate or radiate form. In the grey matter on the surface of the convolutions of the cerebrum they are pyramidal in shape. The apex is directed to the surface of the convolution, the base towards the white matter. The processes arise from the base, apex, and sides of the pyramid. In the grey matter on the surface of the cerebellum the body of the cell is almost globular. From that aspect of the cell which is directed towards the white matter a slender central process arises; from the opposite or peripheral aspect of the cell two strong, many-branched processes extend for a considerable distance. In the human sympathetic ganglia, again, the stellate form of cell prevails, and the existence of a capsule of connective tissue around the individual cells can be recognised. The processes which arise from a multipolar nerve cell, as a rule, divide and subdivide as they pass away from the body of the cell, until at last they give rise to branches of extreme tenuity. These branching processes apparently consist exclusively of cell protoplasm, and have been called protoplasm processes. Gerlach has described the protoplasm processes of the

FIG. 60.-Multipolar cell
from human sympa
thetic ganglion,
capsule of connective

tissue.

C,

FIG. 61.-Multipolar cell from the grey matter non-branched axial-cylinder process directly of anterior cornu in the spinal cord. A C, continuous with a nerve fibre.

Peripheral End-Organs or End Bodies.-Nerve fibres at their peripheral extremities terminate in connection with peculiar structures, named end-bodies, terminal bodies, or peripheral end-organs, which are situated in the several organs of the body. The motor nerves end in the voluntary and involuntary muscles; the vaso-motor nerves end in the muscular coat of the blood-vessels; the sensory nerves end in the skin, mucous membranes, and organs of special sense; and it has been stated that secretory nerves terminate in connection with the ultimate cell elements of the secreting glands. These end-organs possess certain structural peculiarities, which are by no means uniform in the different parts, so that the end-body connected with the peripheral termination of a nerve is distinctive of the organ in which it is situated. It will be a matter of convenience to defer the consideration of the peripheral end-bodies in the skin, organs of special sense, coats of the blood-vessels, and the several glands, until these parts are described. In this place the mode of termination of the motor nerves in the voluntary and involuntary muscles, of the sensory nerves in the mucous membranes, and of the ending of the nerves in the remarkable bodies named Pacinian corpuscles, will alone be examined.

After a nerve has entered a voluntary muscle it ramifies in the connective tissue, which lies between the fasciculi, and at the same time divides and subdivides into smaller branches. These branches interlace with each other and form plexuses, from which slender nervous twigs, often consisting of only a single medullated nerve fibre, proceed, which ramify in the connective tissue, separating the individual muscular fibres from each other. The single

nerve fibres in their turn branch, accompanied by a splitting of the axial cylinder, and these branches usually lose the medullated character. The mode of termination of these very delicate branches has been a subject of much dispute. Beale described them as forming a minute network, situated on the exterior of the sarcolemma, but in contact with it, and the fibres of this nervous network were distinctly nucleated. Other observers have, however, described peculiar bodies, called motorial end-plates, at the extremity of these nerves. These end-plates consist of a clump of richly nucleated protoplasm, somewhat oval or perhaps irregular in form, into which the axial cylinder of the nerve fibre penetrates. The exact position of these end-plates in relation to the muscular fibres is difficult to determine. Krause holds that they lie outside the sarcolemma, but adherent to it; whilst Kühne, Margo, and Rouget maintain that the end-plate lies within the sarcolemma, and that the nerve fibre has to pierce that membrane before it can enter the end-plate. After the axial cylinder has entered the end-plate it subdivides into very minute branches. Each muscular fibre has apparently only a single end-plate, and consequently only a single axial cylinder in connection with it.

In the non-striped muscles the nerves are distributed in the connective tissue which separates the fasciculi from each other. Here they form plexuses, which in some localities, as in the myenteric plexus of Auerbach in the muscular coat of the intestines, have collections of nerve cells, forming microscopic ganglia lying in them. From these plexuses fibres arise which subdivide into delicate nonmedullated fibres possessing nuclei. These delicate fibres form still finer plexuses, which in their turn give origin to minute fibres, which pass between the muscular fibre cells to form a still more minute intra-muscular network. Frankenhäuser maintains that the delicate nerve fibrils which arise from this terminal network penetrate the muscular fibre cells, enter the nucleus, and terminate in the nucleolus; but Arnold considers that, after having entered the nucleus, the fibril again gives off a filament, which passes out of the cell to join the intra-muscular plexus; the ending of the nerve, therefore, within the nucleus is only apparent, and is rather to be regarded as the nodal point of a fine intra-nuclear plexus.

The termination of the sensory nerves in the mucous membranes has been especially studied in the conjunctiva, the mucous membrane of the soft palate, and the glans of the penis and clitoris. In these parts Krause discovered oval or globular end-bodies, which consisted of a soft, homogeneous substance invested by a nucleated capsule of connective tissue. A nerve fibre pierces the capsule and terminates in the interior of the end-body, which forms a bulbous enlargement at the end of the nerve, and is called the end-bulb. After the nerve has entered the end-bulb, it may consist only of the axia' cylinder and terminate in a pointed extremity, or it may twist upon itself and form a coil within the end-bulb. When the structure of the skin is described, it will be seen that the ending of the nerves in the cutaneous papillæ bears a general resemblance to their termination in the end-bulbs of a mucous membrane.

But in certain of the mucous membranes delicate nerves have been traced into the layer of epithelium, situated on the free surface of the membrane. Petermöller described nerve fibres continuous with the nerves of the cornea passing into the layer of conjunctival epithelium on the front of the cornea. Klein recognised an intra-epithelial nervous network in the same locality. Chrschtschonovitsch traced non-medullated nerve fibres proceeding from a subepithelial network into the deeper epithelial layers of the vaginal mucous membrane, and similar nerve fibres have

been seen by Elin to end in the epithelial investment of the mucous membrane of the mouth.

are

Connected with the sensory nerves in some localities Pacinian the remarkable bodies named the Corpuscles of Pacini, corpuscles which were the first terminal organs discovered in connection with the peripheral distribution of the nerves. These corpuscles have been found attached to the nerves which pass to the skin of the fingers and toes, to the nerves which supply the skin of the neck and arm, to the intercostal nerves, to the nerves of the joints, to the nerves of the periosteum, to the nerves of the genital organs, and to the mesenteric nerves. In cats they are often extremely abundant both in the mesentery and omenta. A Pacinian

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corpuscle can be seen by the naked eye, and looks like a minute grain from th toth inch long. It is elliptical in form, and may either be sessile or attached to the nerve stem by a slender stalk. Examined microscopically, it is seen to consist of numerous layers of connective tissue concentrically arranged, which form its capsule, and surround a central core. Numerous connective tissue corpuscles may be seen in the concentric layers, and Hoyer has recently shown that an endothelial-like appearance exists on the inner surface of the corpuscle. Entering one pole of the corpuscle is a nerve fibre which extends along the axial core for a considerable distance, and usually termi

Development of nerve ssue.

nates in a slight bulbous enlargement. The nerve fibre parts with its perineurial sheath after it enters the Pacinian corpuscle; and as it lies in the core it loses its medullary substance, so that its terminal part consists only of the axial cylinder. Sometimes the nerve fibre divides into two branches within the corpuscle. Capillary blood-vessels are distributed to the concentric layers of the Pacinian corpuscle.

The mode of origin of the nervous tissue in the course of development of the embryo is still involved in some obscurity. It is, however, believed that the nerve cells are derived from the embryo cells, which multiply, and the young cells then grow and assume characteristically granular and finely fibrillated contents. Processes or poles then appear at the periphery of the cells, which, according to the observations of Beale, connect adjacent cells together. As the growth of the part goes on, the cells are more widely separated from each other, and the anastomosing processes in consequence become considerably elongated, and form the axial cylinder of the nerve fibre. In the course of time the medullary sheath and the primitive membrane may form around this axial cylinder so as to insulate it. The exact mode of formation of the medullary sheath is not properly understood; but it is believed that the primitive membrane, and the perineurial connective tissue, are derived from those surrounding embryonic cells which differentiate into connective tissue. Of the two originally contiguous cells from which the nerve fibre is, as it were, spun out, one, as Hensen conceived, may form a cell in a nerve centre, the other may differentiate into a peripheral end-organ. In the tail of the tadpole the formation and growth of nerve fibres have been studied by Kölliker and others, and it has been seen that the terminal part of a fibre may have fusiform or tri-radiate cells connected with it, the processes of which cells gradually differentiate into nerve fibres. The young cerebro-spinal nerve fibres are distinctly nucleated, and correspond in appearance and structural characters to the non-medullated nerve fibres of the adult. If in a young or adult person a nerve be cut across, its conducting power is destroyed; but after a time it reunites, and its function is restored. The part of the nerve which lies between the place of section and its peripheral extremity, undergoes, as Waller pointed out, degenerative changes. To how great an extent the degeneration affects the various constituents of each fibre, it is difficult to determine; for whilst some experiments would seem to show that only the medullary sheath broke up into granular particles and was absorbed, in others both it and the axial cylinder disappeared. In process of time, however, these parts may be reproduced, and the nerve then recovers its functional activity.

DESCRIPTIVE ANATOMY OF THE CEREBRO-SPINAL
NERVOUS SYSTEM.

In this section the anatomy of the Brain and Spinal Cord, and of the numerous distributory Nerves which arise from them, will be described. The brain and spinal cord are the largest and most important of all the nerve centres. They occupy the cranial cavity and spinal canal, and are continuous with each other through the foramen magnum in the occipital bone. As the arrangement of the structures which compose the brain and spinal cord is extremely complex, and as the names applied to the several parts are numerous and often very arbitrary, it may be well, before commencing a detailed description, to make a few general observations on their mode of development.

Development of the Cerebro-Spinal Nervous Axis.-The brain and spinal cord are developed in the cranio-spinal groove of the embryo, and appear originally as a thin band extending along the whole length of this groove. About the time when the walls of the groove meet posteriorly to complete the cranio-spinal cavity, the margins of this band become elevated, bend backwards, and meet,

so that the originally simple band becomes converted into a cylindri- Developform cerebro-spinal tube. In the walls of this tube the nervous ment of structures of the brain and spinal cord are formed, whilst the axis cerebroof the tube forms a central canal. In the part which becomes the spinal Spinal Cord the central canal persists as the central canal of the system. spinal cord, and around it a layer of ciliated cylindrical endothelium is developed. Outside this layer a mass of grey matter containing nerve cells is formed, which is subsequently divided into two lateral crescent-shaped masses. Outside the grey matter white matter is produced, which ultimately becomes arranged in the form of longitudinal columns of nerve fibres. With the formation and growth of these columns and of the internal grey matter, a longitudinal me mesial fissure appears on the anterior and another on the posterior surface of the cord, which gradually increase in depth until the cord is almost completely divided into two lateral halves. At the bottom of the anterior median fissure the nerve fibres of the anterior commissure are developed, and at the bottom of the posterior median the two halves of the cord together. fissure those of the posterior commissure. These commissures unite

The upper or cerebral end of the cerebro-spinal tube becomes the Encephalon, or Brain. At first the cerebral part of the tube is uniform in appearance with the spinal part, but it soon expands into three vesicular dilatations-the primary cerebral vesicles. These vesicles, named (from before backwards) anterior, middle, and posterior, are separated from each other by constrictions, and as the development progresses the vesicles bend on each other and on the upper end of the spinal cord. As each vesicle is an expanspace in its interior is continuous with the central canal of the sion of the cerebro-spinai tube, it is necessarily hollow, and the spinal cord. In the walls of the vesicles the nervous structures are produced, which form the several subdivisions of the encephalon. The posterior cerebral vesicle bends first forwards from the upper end of the spinal cord, and then backwards; the part which bends forward becomes the medulla oblongata; that which bends backward the cerebellum, whilst the pons is developed at the angle where these two parts are continuous with each other; the central hollow forms the central canal of the medulla oblongata and the dilated space called the fourth ventricle. In the medulla oblongata shallow anterior and posterior median furrows then appear continuous with those in the cord, and each lateral half differentiates into grey matter and into a longitudinal arrangement of nerve fibres continuous with the corresponding structures in the cord. A large proportion of these fibres are continued upwards through the pons as its longitudinal fibres. The cerebellum consists at first of a central lobe, and in the lower vertebrates its development docs not proceed beyond this stage; but in mammals, including man, a lateral lobe or hemisphere is superadded on each side, and with the growth of these lateral lobes numerous transverse fibres, which connect the two hemispheres together, are developed in the pons. The cerebellum is also connected below with the medulla oblongata by the pair of restiform bodies, or inferior peduncles, and above with the corpora quadrigemina by the pair of superior peduncles. The middle cerebral vesicle bends forwards from the posterior vesicle. In its roof the optic lobes are formed; in its floor the crura cerebri; whilst the central hollow becomes the aqueduct of Sylvius. At first the optic lobes form a single structure, but about the sixth month of embryo life a median furrow divides this structure into two lateral halves (the corpora bigemina), and in the stage; but in the seventh month of embryo life of the human fœtus lower vertebrates the development does not proceed beyond this each lateral half is subdivided into two by a transverse fissure, so that four bodies (the corpora quadrigemina) are produced. The crura cerebri form the two cerebral peduncles, which, diverging from each other, pass upwards to the hemisphere of the cerebrum. They consist almost entirely of nerve fibres continuous with the longitudinal fibres of the pons, a few of which go to the corpora quadrigemina, but the greater number ascend to the cerebrum.

The anterior cerebral vesicle bends downwards from the middle vesicle. The posterior part of this vesicle forms at first a simple hollow sac, but subsequently divides into the two optic thalami, one on each side of the central hollow, which hollow becomes the third ventricle. This ventricle is prolonged downwards into a funnel-shaped process, the infundibulum, which is connected with the pituitary body, or hypophysis cerebri, lodged in the pituitary fossa in the sphenoid bone, whilst posteriorly it is continuous with the aqueduct of Sylvius. In its upper and posterior wall the pineal body, or epiphysis cerebri, is developed, and from this body two white peduncles run forwards on the sides of the optic thalami. Immediately below these peduncles the transverse fibres of the posterior commissure are developed, which pass between the two optic thalami. The anterior wall of this ventricle is closed in by the lamina cinerea or lamina terminalis, and behind it are formed the transverse nerve fibres of the anterior commissure, and the vertical fibres of the anterior pillars of the fornix. These fornix fibres pass to the base of the brain, and form the corpora albicantia, prior to entering the optic thalami. The posterior part of the anterior vesicle gives off from each side a flask-shaped prolongation, the

primary optic vesicle. The stem of the prolongation, at first hollow, becomes solid, and forms the optic nerve and tract, whilst the expanded distal end forms the nervous elements of the retina. The antero-lateral part of the anterior cerebral vesicle is prolonged forward as two hollow processes, the hemisphere vesicles, which become the cerebral hemispheres, and are separated from each other by a median longitudinal fissure; whilst the hollow in the interior of each forms the lateral ventricle. In the floor of each hemisphere vesicle is developed a large grey mass, striated with bundles of nerve fibres, the corpus striatum, which lies immediately in front and to the outer side of the optic thalamus; a curved band, the tania semicircularis, is formed along the junction of the thalamus with the corpus striatum, and at the inner and anterior end of this band, immediately behind the anterior pillars of the fornix, the two lateral ventricles become continuous with each other and with the third ventricle through the foramen of Monro. The roof and side walls of each hemisphere vesicle form a grey expansion or mantle, which is at first smooth, but subsequently becomes divided into lobes and convolutions, separated from each other by fissures. A deep gap or fissure now appears on the inner wall of each hemisphere vesicle, and is bounded above by a longitudinal band of fibres, which, continuous anteriorly with the anterior pillar of the fornix, joins its fellow in the middle line to form the body of the fornix, and then again diverging from its fellow passes backwards, downwards, and forwards as the posterior pillar of the fornix or the tania hippocampi. A transverse arrangement of fibres then forms in each hemisphere vesicle, above the plane of the fornix, which, reaching the mesial plane, joins its fellow, connects the two hemispheres together, and forms the corpus callosum. In the hinder part this corpus rests upon the upper surface of the fornix, but more anteriorly it lies some distance above the fornix, and then bends down in front of it. Hence there is enclosed between the fornix and the antero-inferior part of the corpus callosum two thin layers of grey matter, one belonging to the inner surface of each hemisphere vesicle, and called the septum lucidum. Between these two layers is a narrow space, the fifth ventricle, which, unlike the other ventricles, is not derived from the cerebro-spinal tube, but is merely a portion of the longitudinal median fissure shut in by the development of the corpus callosum and fornix. Each hemisphere vesicle also gives off from its anterior part a hollow process, which expands in front into a bulbous dilatation, named the olfactory bulb, from which the nerves of smell arise, whilst the stalk of the bulb solidifies and forms the olfactory peduncle.

Owing to the great development of the mantle of the hemisphere vesicles in the human brain, and the size and complexity of the convolutions, these parts of the hemispheres grow forward so as to overlap the olfactory bulbs and peduncles, and backward, so as to conceal not only the corpora striata and optic thalami, but also the corpora quadrigemina, crura cerebri, cerebellum, pons, and medulla oblongata, so that when the human brain is looked at from above, none of these structures can be seen. It is only when the brain is turned over and its base exposed that the medulla, pons, cerebellum, and crura are visible; and before the corpora quadrigemina, optic thalami, and corpora striata can be exposed, portions of the hemisphere substance must be removed. The great growth of the hemisphere vesicle leads also to a great expansion of the central hollow or lateral ventricle, which is prolonged forwards, backwards, and downwards as the anterior, posterior, and descending cornua. In the descending cornu is a projection, the hippocamus major, along which the tænia hippocampi of the fornix runs; in the posterior cornu is a smaller eminence, the hippocampus minor; and at the junction of these two cornua is a third elevation, the eminentia collateralis.

Immediately investing the spinal cord and encephalon a vascular membrane, the pia mater, is developed, processes from which dip into the fissures between the two halves of the cord and between the cerebral convolutions. A broad band, the velum interpositum, which possesses two marginal fringes, the choroid plexuses, is admitted into the lateral ventricle through the gap or fissure in the inner wall of each hemisphere vesicle. This fissure is bounded above by the arch-shaped fornix, with its tenia hippocampi. When the two hemispheres are in situ, and the two halves of the fornix are joined together to form the body of that structure, the fissure, with its contained velum interpositum, passes across the mesial plane from one hemisphere to the other, having the fornix and tæniæ for its roof, and the optic thalami and corpora quadrigemina for its floor;

it is known as the great transverse fissure of the cerebrum.

MEMBRANES OF BRAIN AND SPINAL CORD.-These nerve centres are invested by three membranes or meninges, which lie between them and the bones that form the walls of the cranial cavity and spinal canal. The membranes are named dura mater, arachnoid mater, and pia mater.

Dura mater.-The most external membrane, named dura from its firmness, consists of a cranial and a spinal subdivision. The cranial part is in contact with the inner

table of the cranial bones, and is adherent along the lines of the sutures and to the margins of the foramina, which transmit the nerves, more especially to the foramen magnum. It forms, therefore, for these bones an internal periosteum, and the meningeal arteries which ramify in it are the nutrient arteries of the inner table. As the growth of bone is more active in infancy and youth than in the adult, the adhesion between the dura mater and the cranial bones is greater in early life than at maturity. From the inner surface of the dura mater strong bands pass into the cranial cavity, and form partitions between certain of the subdivisions of the brain. A vertical longitudinal mesial band, named, from its sickle shape, falx cerebri, dips between the two hemispheres of the cerebrum. A smaller sickleshaped vertical mesial band, the falx cerebelli, attached to the internal occipital crest, passes between the two hemispheres of the cerebellum. A large band arches forward in the horizontal plane of the cavity, from the transverse groove in the occipital bone to the clinoid processes of the sphenoid, and is attached laterally to the upper border of the petrous part of each temporal bone. It separates the cerebrum from the cerebellum, and, as it forms a tentlike covering for the latter, is named tentorium cerebelli. Along certain lines the cranial dura mater splits into two layers, to form tubular passages for the transmission of venous blood. These passages are named the venous blood sinuses of the dura mater, and they are lodged in the grooves on the inner surface of the skull referred to in the description of the cranial bones. Opening into these sinuses are

[blocks in formation]

FIG. 63.-Dura mater and cranial sinuses. 1, Falx cerebri; 2, tentorium 3, 3, superior longitudinal sinus; 4, lateral sinus; 5, internal jugular vein; 6, occlpital sinus; 6', torcular Herophili; 7, infe or longitudinal sinus; 8, veins of Galen; 9 and 10, superior and inferior petr.sal sinus; 11, cavernous sinus; 12, circular sinus, which connects the two cavernous sinuses together; 13, ophthalmic vein, from 15, the eyeball; 14, crista galli of ethmoid bone.

numerous veins, which convey from the brain the blood that has been circulating through it; and two of these sinuses, called cavernous, which lie at the sides of the body of the sphenoid bone, receive the ophthalmic veins from the eyeballs situated in the orbital cavities. These blood sinuses pass usually from before backwards: a superior longitudinal along the upper border of the falx cerebri as far as the internal occipital protuberance; an inferior longitudinal along its lower border as far as the tentorium, where it joins the straight sinus, which passes back as far as the same protuberance. One or two small occipital sinuses, which lie in the falx cerebelli, also pass to join the straight and longitudinal sinuses opposite this protuberance; several currents of blood meet, therefore, at this spot, and as Herophilus supposed that a sort of whirlpool was formed in the blood, the name torcular Herophili has been used to express the meeting of these sinuses. From the torcular the blood is drained away by two large

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