The grey matter is chiefly arranged in a layer so as to surround the cavity (Fig. 104), this layer being deeper on either side of the descending portion of the cavity than elsewhere. For the rest the grey matter is much interspersed among the white matter, except at the circumference of the section, which is entirely formed of white matter. It contains a large number of small cells, of which the nuclei are alone visible; in the parts mentioned where the layer is most marked these cells are arranged in oblique rows, between which pass fine bundles of medullated fibres (Fig. 104, between u and h′). A group of large cells (Fig. 104 u) is found on either side of the middle line and under the floor of the cavity; the cells are about 0.032 mm. long, and 0.016 mm. broad; the oculo-motor nerve may be traced to this group, which is the oculo-motor nucleus. A small commissure of decussating fibres connects the nuclei of opposite sides (Köppen).

The substance of the roof of the optic lobes (Fig. 104 x) is arranged in very distinct layers: above is a layer free from cells; the fibres of this layer are extremely fine and delicate, and have not been accurately traced; under this is a nuclear layer; a second layer of fine fibres follows, which is succeeded by a second layer of nuclei in a granular matrix, and lastly epithelium (Stieda). (Reissner describes three nuclear layers in Bufo variabilis, and this is also the case in R. temporaria, G. H.; according to Köppen, the number is variable.) In the anterior portion of the roof a distinct bundle of fibres may be made out; externally they bend downwards, and can be traced as far as the crura cerebri.

Fibres corresponding with the commissural and arched fibres of the medulla oblongata are continued into the hinder portion of the crura cerebri, the change from pars commissuralis to crura cerebri being very gradual. The longitudinal white fibres are much increased in number in the crura cerebri, and a portion of them can be traced to the nucleus magnus.

A peculiar irregular group of large cells (Fig. 104 w) is found where the roof meets the crura cerebri; these vary in diameter from 0.024 to O.040 mm., and their processes are very indistinct.

The fibres of the optic tracts arise, according to Köppen, from two different origins: the one lies on the hindermost part of the optic lobe; from this point the fibres curve downwards and forwards to form longitudinal fibres; this root Köppen names the ventral ascending root, it can be traced through the entire length of the organ. The second root arises in the tectum opticum near the longitudinal fissure; it is smaller than the foregoing, and has been named the dorsal ascending optic root. The fibres of these two roots unite anteriorly near the posterior commissure, at which point they receive additional fibres (Köppen). Bellonci traces a large proportion of the fibres of the optic tract to the nucleus magnus, which pair of nuclei, as already stated, he regards as the posterior pair of corpora quadrigemina of higher animals.

The pars peduncularis is the continuation of the pars commissuralis underneath the optic lobes; a gelatinous mass lying in the median plane and containing numerous isolated nuclei (Ganglion interpedunculare) divides it into two lateral halves. The longitudinal fibres are ungrouped posteriorly, but arranged in rounded strands in the middle, especially dorsally; anteriorly the grouped arrangement is lost and the number of fibres diminished.

The pigment of this region has, in a transverse section, an outline which has something the form of a lyre; commencing on either side of the median line, and underneath the deepest portion of the cavity, the pigment line passes, first, directly outwards; then suddenly turns upwards and slightly outwards parallel with the wall of the deeper part of the cavity; it then curves outwards to pass below the cavity of the optic lobe, where it divides, one portion passing outwards, the other between the Sylvian aqueduct and the ventricle of the optic lobe.

d. The Thalamencephalon (Huxley), (Lobus ventriculi tertii, Stieda; Thalami optici, Reissner; Thalamus opticus s. Lobus ventriculi tertii, Stannius; Ganglien der Haemisphaeren, Carus).

α. External form. From above (Fig. 98 Tho) the thalamencephalon is seen as a lozenge-shaped mass lying in front of the optic lobes, and behind and between the diverging posterior ends of the cerebral hemispheres; it is covered by a thick vascular membrane, the choroid plexus, through which passes the pedicle of the pineal body (Glandula pinealis). On removing the choroid plexus a small aperture is seen in the roof of the thalamencephalon, connecting the hollow pedicle of the pineal gland with the third ventricle. The ventricle appears as a narrow slit in the median line, its walls being formed by the optic thalami. By pressing aside the cerebral hemispheres the posterior commissure (Commissura posterior) may be seen lying quite in front and deep in the cleft of the ventricle. Immediately behind the pedicle of the pineal body is a slight but well-marked depression (Fig. 98 G), the origin of which has not been investigated (Wiedersheim).

The choroid plexus is continued forwards between the cerebral hemispheres (Figs. 98, 103 Ad) for some distance, and terminates in a fine thread of connective-tissue.

The under surface of the thalamencephalon (Fig. 102) is divided into two parts by the optic chiasma (Cho): the anterior portion (Lt) is the lamina terminalis (Substantia cinerea anterior); the posterior (Tu.c) the tuber cinereum. The lamina terminalis is bounded on either side by the cerebral hemispheres. The tuber cinereum (Figs. 102, 103 Tu.c) is a small median swelling immediately behind the optic chiasma, and caused by the depression of the floor of the third ventricle to form the infundibulum (Diverticulum infundibuli, Reissner).

The pituitary body (Hypophysis cerebri) is a flattened sac, placed behind the tuber cinereum and continuous with it by means of the infundibulum.

β. Internal structure. The aqueduct of Sylvius, after communicating with the ventricles of the optic lobes, again contracts (Fig. 105), but still remains somewhat larger than before. In the thalamencephalon the Sylvian aqueduct opens into the third ventricle, which gradually assumes the form of a vertical slit with the walls bulging slightly outwards in their upper parts. The thin roof of this ventricle, where complete, contains a band of transverse fibres. The floor is depressed both before and behind the part immediately above the optic chiasma, the posterior depression lying above the tuber cinereum, which here descends towards the infundibulum: a transverse section through this portion of the third ventricle has the form of a square standing on one angle, the superior angle being produced into the vertical slit of the general cavity. The anterior depression is formed by the general cavity being prolonged downwards and forwards to the lamina terminalis in the form of a narrow and shallow slit.

The white and grey matter of the thalamencephalon are only so far distinct in that the portion immediately surrounding the cavity is darker than the rest of the section. In the immediate neighbourhood of the cavity are many small cells and nuclei, which become scarcer further from the ventricle; they are arranged in rows, separated by a fibrillated matrix. On either side is a distinct bundle of longitudinal fibres, the ‘round bundle’ of Köppen, which come from the optic lobes but appear to arise from either the pars commissuralis or the medulla oblongata, and to receive additional fibres from the optic thalami; they pass forwards to the base of the cerebral hemispheres (Stieda). A second set of longitudinal fibres arises in the substance of the tuber cinereum and passes forwards to the hemispheres (strand of the Tuber-cinereum and Thalamus, Köppen); this band, together with the ‘round bundle,’ form a crus cerebri (Köppen).

The commissural fibres of the thalamencephalon are: (1) a commissura transversa Halleri in the posterior portion of the organ; (2) an optic commissure, consisting of fibres arising from the thalamencephalon (thalamencephalic root) and crossing the median line to join the optic tract of the opposite side; (3) a probable commissure between the optic nerves just in front of the chiasma opticorum; the existence of the latter is not yet proved beyond doubt; (4) the large commissure of transverse fibres found in the roof of the third ventricle; whether the fibres decussate or not is uncertain (Köppen).

The fibres of the large commissure of the roof are, in part, continued into the strand of the Tuber-cinereum above mentioned, and thus conducted to the posterior parts of the hemispheres (Köppen). A group of fibres (Meynert’s band, Köppen) is found in each lateral wall of the third ventricle; they pass from the region of the nucleus parvus downwards in a curved course almost parallel with the external border of the thalamencephalon.

A distinct group of cells may be noted in this section of the brain, an arciform or circular group (Nucleus parvus, Reissner; ganglion of the habenula, Köppen) of large spindle cells (average diameter 0.016 mm.), placed under the upper border close to the third ventricle; the group extends alongside the whole length of the ventricle. A second group lying in the middle and posterior parts beneath the ventricle, Köppen names the ‘ventral nucleus’ of the thalamencephalon.

The pigment in the posterior part of the thalamencephalon is arranged in a manner similar to that in the optic lobes and crura cerebri; anteriorly where the third ventricle is prolonged forwards and downwards the arrangement is different; the pigment lies in a curved line above the process of the ventricle, with its concavity directed downwards, each end bifurcating, in order that one branch may pass upwards, the other downwards.

The pineal body‍53 is a small vesicle placed underneath the skin above the fronto-parietal bones; in the embryo it is connected with the third ventricle by means of the pedicle‍54 already mentioned; the skin covering the body is always paler than the surrounding skin, and the usual cutaneous glands are absent in this part; the paler spot on the head may always be found, but is more distinct in Rana temporaria than in Rana esculenta. The structure on the roof of the third ventricle, which is usually known as the pineal body, is nothing more than a thickened portion of the choroid plexus, and consists of a group of convoluted vessels surrounded by pia mater, which is described by Wyman as being covered with ciliated epithelium (R. pipiens). The true pineal body is a small body with an outer connective-tissue capsule, derived from the pia mater; this encloses an irregular mass of epithelial cells; according to de Graaf a twig of the ramus supramaxillaris reaches it subcutaneously, and a blood-vessel accompanies the pedicle through the foramen parietale. According to Darkschewitsch, the pedicle contains medullated nerve-fibres derived directly from the brain.

The pituitary body (Figs. 102, 103 Hy) when examined with a lens is seen to consist of two portions: an anterior, superior, and smaller white portion, and a larger, inferior, posterior, and reddish portion. The anterior portion has the form of a very small, flat disk, and is enclosed in a connective-tissue capsule which sends in larger and smaller processes. In either transverse or longitudinal section it is seen to be formed of two horizontal layers separated by a line of blood-vessels and connective-tissue. The upper layer consists of a granular and reticular matrix, containing many nuclei (averaging 0.006 to 0.010 mm. diameter), and divided into irregular rounded or polyhedral spaces by bands of tissue derived from the capsule. This layer is more vascular than the lower. The lower layer consists of a mass of clear, nucleated rounded or polyhedral cells (0.016 to 0.024 mm. in diameter; nuclei from 0.008 to O.012 mm. in diameter, Reissner), pierced by very fine connective-tissue septa derived from the capsule. The septa are, for the most part, vertical and longitudinal (Reissner), the blood-vessels are very few.

The posterior larger portion of the pituitary body (Fig. 106) is also compressed from above downwards, and in transverse section as an oval outline. It possesses an external thin connective-tissue capsule, which sends in fine processes to support a mass of convoluted tubes, between which course a few blood-vessels; these tubes possess an outer nucleated basement-membrane, and are lined with a single layer of more or less cylindrical epithelium, which entirely fills the tube; hence the tubes possess no lumen. The tubes are from 0.04 to 0.08 mm. in diameter; the cells are clear or granular, and possess distinct, rounded nuclei.

e. The Cerebral Hemispheres and Olfactory Lobes. The cerebral hemispheres (Lobi hemisphaerici, Stieda; Lobi cerebrales, Reissner; Centralmasse des Geruchssinns, Carus; Hemisphaeren des grossen Hirns, Tiedemann; Grosse Hemisphaeren, Schiess; Prosencephalon, Huxley). The olfactory lobes (Tubercula olfactoria, Stieda; Lobi olfactorii, Reissner; Riechkolben, Schiess; Rhinencephalon, Huxley).

α. External form (Figs. 98, 102, 103 Hc and L.ol). The two cerebral hemispheres form together the largest section of the brain; from above they are seen to be separated by a dorsal longitudinal fissure, which is here well marked: each hemisphere is an ovoid body with the smaller end directed forwards and continuous with the corresponding olfactory lobe; the posterior end forms one half of the anterior boundary of the thalamencephalon. The olfactory lobes are two elongated, rounded bodies directly continuous with the corresponding cerebral hemispheres, and likewise partially separated in the median line by a dorsal longitudinal fissure: at the point of union of the cerebral hemispheres and olfactory lobes is a faintly marked transverse depression.

On the ventral surface the parts are again marked off from one another by a corresponding ventral longitudinal fissure and a transverse groove; the two longitudinal fissures being continuous, anteriorly, between the olfactory lobes. The cerebral hemispheres appear to be more widely separated behind (Fig. 102) than is the case on the dorsal surface, and in the space so formed is the lamina terminalis (Lt). Seen from the side, the slight depression of the upper surface, between the cerebral hemispheres and the olfactory lobes, is seen to be continued downwards and slightly backwards to join the corresponding groove on the inferior surface.

The longitudinal fissures are shallow and do not meet, except at one point, at about the middle of the cerebral hemispheres (Fig. 105). The olfactory bulbs arise superficially (Figs. 102, 103 I′ and L.ol) from the whole length of the olfactory lobe, between the anterior extremity (I′) and the posterior (L.ol), where they are also attached to the cerebral hemispheres.

β. Internal structure. The cerebral hemispheres and olfactory lobes are hollow, the common cavity of each side being known as the lateral ventricle (Ventriculus lateralis); these ventricles communicate with each other, and with the third ventricle (Fig. 105). The narrow aperture by which the lateral ventricles communicate is known as the Foramen of Monro (MF); it communicates with the third ventricle posteriorly, and with the space between the cerebral hemispheres anteriorly, and thus forms a common cavity (Ventriculus communis loborum hemisphaericorum, Stieda).

In general terms, each ventricle may be said to be a semilunar cavity, prolonged backwards and forwards (Fig. 107 c); the outer wall is always more or less concave, while the inner varies according to the part of the hemisphere examined. At the anterior and posterior extremities the inner wall is convex and bulges into the cavity (Fig. 107); in the middle portion of the cavity the inner wall presents a longitudinal groove (Ventriculi lateralis cornu internum, Reissner), (Fig. 108 d), and consequently the inner wall has here two rounded masses, an upper and a lower (Corpus striatum, Wiedersheim), projecting into it. By tracing them backwards and forwards, the lower swelling is seen to increase at the expense of the upper, while at the same time the lateral grooves disappear; the outline of the cavity shown in Fig. 107 is then obtained. The roof of the ventricle is arched and broader than the floor, which, in the middle part, exists only as a vertical slit (Fig. 108), (Ventriculi lateralis cornu inferius, Reissner): towards the anterior and posterior extremities it widens and becomes shallower (Fig. 107).

The cerebral hemispheres and olfactory lobes are composed of a fine granular matrix, enclosing spindle-shaped, rounded, or pear-shaped nerve cells and nuclei, and containing very fine fibres. The cells (Fig. 109 b) are more numerous towards the ventricle, and somewhat sparse towards the superficial surface. The cells are of two chief sizes, the smaller and more numerous average 0.004 mm. to 0.008 mm. in diameter; they are found chiefly in the deeper portions of the section, but also form a very thin irregular layer beneath the pia mater: the larger cells have an average diameter of 0.010 mm. to 0.012 mm., and are placed towards the periphery, especially in the dorsal part of the inner wall.

In this irregular collection of cells the following centres have been described: (1) The nucleus, through which the corpus callosum passes (Köppen); (2) the lower internal or median cell-area (Osborne), situated above the foregoing nucleus in the posterior and middle portions of the hemispheres; (3) the upper internal cell-area (Osborne) is the area of large cells in the dorsal part of the inner wall; (4) the Corpus striatum (Osborne) is a mass of cells between the corpus callosum and the commissura anterior; Köppen doubts the correctness of Osborne’s opinion, and suggests that a group of cells found in the wall of the third ventricle in front of ‘Meynert’s band’ may perhaps be a corpus striatum.

A bundle of longitudinal medullated nerve-fibres, the ‘round bundle,’ is found on either side of the median line (Fig. 107 d), and near the lower border in the posterior portion of this region; these fibres can be traced from the posterior section of the thalamencephalon; they diminish in number as they course through the base of the cerebral hemispheres, and are ultimately lost in the lower anterior part of the outer walls. A second strand of longitudinal fibres is the continuation of that of the Tuber-cinereum above described, which can be followed to the outer wall of the posterior part of the ventricles, and to the anterior commissure.

The commissures are: (1) The corpus callosum, a large bundle of transverse fibres, seen best in a transverse section, at the point of junction between the lamina terminalis and the cerebral hemispheres, forming an arch over the roof of the anterior prolongation of the third ventricle. The fibres course to the inner and anterior parts of the hemispheres, and are situated chiefly behind the foramen Monroi. To this commissure must be added those fibres which unite the two olfactory lobes, and possibly the fibres (Commissura posterior) found in the roof of the third ventricle (Köppen). (2) The Commissura anterior (Stieda), a smaller set, found immediately under the floor of the common ventricle, forming in their course outwards a curve, with the concavity directed downwards. This commissure connects the ‘round bundles’ of opposite sides, and those fibres coursing with the ‘round bundles’ to the olfactory lobes constitute the pars olfactoria of the commissura anterior. To this commissure must also be added some fibres found on the ventral surface of the commissura anterior and connecting the two strands of the Tuber-cinereum; an unusually coarse strand of these fibres can be traced to the inner wall of the ventricle, and is termed the pars olfactoria interna by Osborne.

The general structure of the olfactory lobes resembles that of the hemispheres; the olfactory nerves arise each by two roots, an outer and inner. The outer root arises from the outer wall near the groove between the corresponding hemisphere and olfactory lobe; the inner or anterior root arises from the anterior surface of the olfactory lobe. Both roots have a peculiar method of origin from the extremely fine fibrillar network of the matrix (Nerve-fibre-conglomerate, Köppen), in which are rounded dark bodies known as ‘glomeruli;’ in the ‘glomeruli’ dark points and nuclei are seen, between larger and smaller bands of nerve-fibres. Köppen holds that all the sensory nerves of the brain arise in a similar manner.

A decussation takes place between the two inner roots of the olfactory nerves; possibly the external roots are connected by means of the commissura anterior.

Very little pigment exists in the cerebral hemispheres or olfactory lobes, the greater portion is found in the upper part of the inner walls of the cerebral hemispheres.

The epithelium of the ventricles of the brain, like that of the central canal of the spinal cord, consists of conical cells with their bases directed towards the cavity, and their apices directed peripherally and prolonged into distinct processes (Figs. 104, 109 a). In such situations, as the choroid plexuses, where nervous tissue is absent and the cavity is completed by pia mater alone, the epithelial cells are flattened. Everywhere else it is ciliated‍55 and possesses distinct round nuclei which are as broad as the cells themselves. The epithelium is somewhat irregularly and sparsely pigmented; the ventral parts of the central canal of the spinal cord, of the fourth ventricle, and of the Sylvian aqueduct are always more pigmented than the dorsal parts.

PLAN OF THE ARRANGEMENT OF THE NERVE-FIBRES OF THE CENTRAL NERVOUS SYSTEM, ACCORDING TO KÖPPEN.

C. The Coverings of the Brain and Spinal Cord.

1. The Pia mater closely invests the whole of the brain and spinal cord; it may, in fact, be regarded as the flattened, outermost layer of the connective-tissue material which supports the nervous elements; fine processes (Fig. 109 c) pass radially from its inner surface to join the general connective-tissue matrix or neuroglia. It is usually pigmented, those portions covering the optic lobes and spinal cord being especially deeply pigmented: on the cerebral hemispheres it contains very little pigment; at times, indeed, in R. temporaria, pigment is absent from this part.

The pia mater is continued on to the choroid plexuses and pituitary body, and on to the pineal body by means of the pedicle; the membrane is very vascular, and forms, especially for the cerebral hemispheres and the olfactory lobes, a very important source for the direct supply of blood-vessels.

2. The Dura mater, or lining membrane of the cranial cavity and vertebral canal, is a connective-tissue membrane containing many very much branched, pigmented cells. This membrane is not so deeply pigmented as the pia mater, except in that portion covering the cerebral hemispheres and the olfactory lobes, which is much darker than the corresponding portion of the pia mater.

3. The arachnoid membrane is the layer of endothelial cells covering the inner surface of the dura mater: by means of the blood-vessels and nerves, etc., it is continued on to the pia mater of the spinal cord and brain, which it in like manner closely invests. Masses of calcareous crystals are found between the epineurium and the arachnoid (Wiedersheim) on each spinal nerve at its exit from the intervertebral foramen, also on the trigeminal nerves in the cranium. Additional smaller and more irregular masses are found on the dorsal part of the dura mater of the spinal cord.

D. The Vessels of the Brain and Spinal Cord. (Fig. 110.)

The arteries of these organs are derived from the internal carotids and the arteriae vertebrales. As soon as the internal carotid arteries reach the cranial cavity each divides into two branches, an anterior (Ramus anterior, Schöbl‍56) and a posterior (Ramus posterior, Schöbl); the anterior branches course forwards on the lower part of the surface of the brain as far as the anterior portion of the thalamencephalon, where they again divide; one division, arteria lobi hemisphaerici inferior externa (Schöbl), continues forwards along the outer surface of the cerebral hemisphere and of the olfactory lobe as far as the olfactory bulb, where it may still be distinctly seen; it gives off branches to the adjacent parts in its course forwards. The vessels of the two sides communicate with each other by means of delicate transverse vessels (Arteriae communicantes anteriores). The second division, arteria lobi hemisphaerici superior interna (Schöbl), of the ramus anterior courses on the thalamencephalon to the dorsal surface, gives a large branch to the choroid plexus of the fourth ventricle, and runs forwards in the dorsal longitudinal fissure; it supplies vessels to all the neighbouring parts.

The rami posteriores converge as they course backwards, and ultimately unite to form an arteria basilaris, which is continued in the median line of the under surface of the spinal cord as the arteria spinalis anterior. The ramus posterior gives off, in its course, many small vessels to the neighbouring parts, and two larger vessels on either side, one of which, the arteria lobi optici (Schöbl), is distributed to the optic lobes, while the other passes to the pituitary body.

The branches of these vessels form a network in the pia mater, from which the nervous tissue is supplied; they also send numerous branches directly into the brain and cord, and these have a similar arrangement for all parts of the brain with the exception of the cerebral hemispheres and olfactory lobes; more or less vertical branches arise from the posterior parts of the upper borders of the rami anteriores, from the whole of the rami posteriores, and from the arteria basilaris; these course upwards from their place of origin into the corresponding part of the brain, give off a few branches in the white matter, and then branch freely, and at somewhat sharp angles, in the grey matter. In the pars commissuralis a large branch may be traced from the ramus posterior on either side upwards into the cerebellum almost to its upper border. The vessels of the cerebral hemispheres and olfactory lobes seem to possess no other definite arrangement than that described above. The grey matter seems, on the whole, to be more vascular than the white; the vessels of the latter are chiefly arranged radially to the surface, and run in courses which are more or less straight; the vessels of the grey matter are more irregular and sinuous.

The Arteria spinalis anterior courses along the whole length of the spinal cord, giving off lateral branches and communicating with rami spinales (branches of the Arteria vertebralis). These branches form a plexus in the pia mater, from which vessels pass at irregular points into the cord; one set of small vessels, described by Reissner, pass in a straight course from the superior longitudinal sinus towards the substantia reticularis, where they divide. Other branches pass directly from the arteria spinalis anterior, through the ventral longitudinal fissure, and there divide; the twigs as a rule avoiding the septum medium and passing in greater part towards the ventral horns. The vessels in the white matter are for the most part radial and straight, while in the more vascular grey matter they are irregular and more sinuous in their course.

The choroid plexus of the third ventricle (Plexus choroideus ventriculi tertii, Reissner) lies, as already described, on the roof of the third ventricle; it is somewhat triangular in form (Plexus venosus triangularis, Schöbl), and is evidently a prolongation of the pia mater, with an increased supply of vessels. It receives, at its anterior angle, veins from the cerebral hemispheres and the adjacent parts. The under surface is covered with a layer of ciliated pavement epithelium. At the posterior angles of the plexus the veins communicate on either side (Fig. 110) with the anterior lateral prolongation of the vena spinalis superior, and with the internal jugular vein. The small body usually named the pineal body, and placed on the roof of the ventricle, is only a small thickened portion of the plexus, and consists of a group of convoluted blood-vessels. This plexus sends a considerable bunch of vessels into the upper part of the third ventricle. The choroid plexus of the fourth ventricle (Plexus choroideus ventriculi quarti, Reissner) is a triangular membrane, slightly attached to the borders of the fourth ventricle. Its upper surface is flat and bounded at either side by a large vein, the vena spinalis superior; anteriorly it is bounded by the cerebellum. The under surface is not flat; in the middle line is a slight furrow corresponding to the position of a median vessel, which may, when injected, be seen from the dorsal surface; from it a number of short vessels pass outwards and slightly backwards to the outer border. The connective-tissue between these vessels is pushed down into the cavity; in this manner one obtains a double row of flattened villous-looking bodies, which frequently have their tips pigmented. The under surface of the plexus is clothed with flattened, ciliated epithelium.

On either side of the pituitary body is another small venous plexus (Plexus lateralis, Schöbl); it communicates above with the posterior angle of the choroid plexus of the third ventricle; externally, with the internal jugular vein; and internally with its fellow of the opposite side by more or less irregular transverse vessels, which, together with a median vein from the fissure between the cerebral hemispheres, form an irregular circulus venosus around the pituitary body. These plexuses receive blood from the lower part of the cerebral hemispheres, the thalamencephalon, the optic lobes, and from the pituitary body. The veins of the spinal cord open into the dorsal, vena spinalis posterior (Fig. 110 spp), which bifurcates at the posterior angle of the fourth ventricle, one division passing to either side and forming, as already described, the outer border of the choroid plexus; anteriorly it opens into the internal jugular vein (Fig. 110 ji).

The blood-vessels, both arteries and veins, are irregularly pigmented, both on the surface and in the interior of the central nervous system; those of the cerebral hemispheres and olfactory lobes have very little or no pigment.

DESCRIPTION OF THE FIGURES ON PLATE I.

II. THE PERIPHERAL NERVOUS SYSTEM.

E. The Cranial Nerves.

(To facilitate reference the original arrangement of this part has been altered.)

[There are ten pairs of cranial nerves in the frog, which are numbered in order from before backwards. The mode of origin of these nerves, and their deeper relations with the respective parts of the brain to which they belong, have already been described.]

1. The olfactory nerve (N. olfactorius), [(Figs. 97, 98, 102, 103, 111 I) runs a very short course only a few lines in length, and escapes from the cranium by an opening in the cartilage of the sphenethmoid into the nasal cavity, where it divides into two branches, each of which breaks up into a brush of filaments, to be distributed in the olfactory mucous membrane. The nerve contains no white fibres]. (See organ of smell, p. 385.)

2. The optic nerve (N. opticus), (Figs. 97, 102, 103, 111 II) [arises, as already described, by the optic tract, and joins with its fellow at the optic chiasma or commissure, where part of the fibres pass over to the opposite side (according to Michel all the fibres cross). Each optic nerve then courses outwards, piercing the cartilage of the cranium and so reaching the eyeball]. (See organ of sight, p. 408.)

3. The motor oculi (N. oculomotorius, Oculo-motor, Motor communis), (Figs. 102, 111, 112, 113 III). From its origin it courses outwards and forwards, perforating with a slight obliquity the cartilaginous wall of the cranium, just in front of the Gasserian ganglion (Fig. 116 III); on reaching the orbit it divides into two branches, between the Levator bulbi and the Rectus internus and inferior. One branch, Ramus superior, runs over and parallel to the Ramos ophthalmicus Trig. and enters the under surface of the Rectus superior; the second, lower branch, Ramus inferior, supplies filaments to the Rectus internus and inferior and to the Obliquus inferior. Apparently it exchanges fibres with the ophthalmic division of the trigeminal nerve.

[That portion of the motor-oculi between its branch to the Rectus superior and to that to the Rectus internus contains a number of nerve cells; according to Schwalbe (l. c., p. 235 and Pt. XII, Fig. 4) these cells are arranged in four groups or ganglia; the second group forms a very slight prominence, and is covered by a very fine layer of nerve fibres, it also gives off some extremely fine nerves to the eyeball; this is perhaps a ciliary ganglion (Ganglion ciliare, Schwalbe). Beard, however, names it ‘mesocephalic ganglion.’ The other three groups are simply clusters of cells between the fibres of the nerve.]

4. The pathetic or trochlear nerve (N. trochlearis, patheticus), (Figs. 102, 103, and 112 IV). From its origin it courses forwards with and then crosses the motor-oculi; it perforates very obliquely the cartilaginous lateral wall of the cranium, in front of the motor oculi but above and very slightly behind the optic foramen (Fig. 116); it runs parallel to and in company with the Ramus opthalmicus trigemini, and appears to exchange a few fibres with it, which, however, according to de Watteville, Stannius, Cuvier, and Wyman, is not really the case. In this course it lies first to the inner, then to the outer side of the ophthalmic, over which it passes to supply the Rectus superior. [The pathetic and ophthalmic nerves are sometimes enclosed in a common sheath (de Watteville).]

5. The trigeminal nerve (N. trigeminus), (Figs. 97, 102, 103, 111, 112, 113, and 114 V) is the largest of the cranial nerves in the frog; from its origin it runs outwards and forwards to the skull wall, and just before reaching this enters the large Gasserian ganglion. It then passes through the cranial wall immediately in front of the auditory capsule, and divides at once into two main branches (Fig. 111 Vg, etc.), the Ramus ophthalmicus and the Ramus maxillo-mandibularis.

The Gasserian ganglion is a large, yellow, oval ganglion placed in a depression in the outer wall of the cranium; it is covered by a ‘periganglionic gland,’ which is similar in structure to that of a spinal ‘periganglionic’ gland (see p. 180), the fibrous capsule being stronger. This ganglion receives three other nerves besides the trigeminal, viz. the sixth and seventh nerves, and branches of the sympathetic.

According to de Watteville, these nerves are arranged on the lower surface of the ganglion, so that the sympathetic is below, the sixth above it, then the seventh, while the fifth is uppermost; the sympathetic splits into several bundles which join the various branches of the ganglion; the sixth divides into two bundles, one of which usually joins the ophthalmic, while the other makes its exit independently; the seventh splits into two bundles, one of which leaves the ganglion as the hyomandibular branch, the other gives some fibres to the palatine branch of the trigeminal nerve, and is then continued as the palatine branch of the facial.]

a. The Ramus ophthalmicus (Ramus nasalis, Fischer; Ophthalmic or Orbital Branch, Wyman; Orbito-nasal or Ophthalmic Nerve, de Watteville), (Figs. 111 and 112 Va). After leaving the Gasserian ganglion the nerve is directed forwards parallel to the side of the cranium, between this and the eyeball. It lies beneath the Rectus superior, but above all the other muscles of the eyeball and the optic nerve. At the anterior end of the orbit it divides into two branches (e and f) which pass through apertures (Foramen pro ramo nasali) in the cartilage of the sphenethmoid, to reach the nasal cavity, where they lie between the cartilage and mucous membrane. In this position the nerves supply branches to the mucous membrane and then pierce the skull to appear on the surface (Figs. 111, 112 g,g,g, 114 Va), the skin of which they supply. The branches inosculate with each other and communicate with the anterior twigs of the Ramus maxillaris (Fig. 114 Vc) and with those of the Ramus palatinus (Fig. 113 Vb).

In its course through the orbit the Ramus ophthalmicus gives off:‍—

(1) Near the Gasserian ganglion one, two, or three small branches, which communicate with the trochlear nerve, and may then be traced to the sclerotic of the eyeball; some of the twigs enter near the optic nerve, others nearer the cornea. They are regarded as Ciliary nerves (Nervi ciliares).

(2) A large branch, the palatine nerve (Ramus palatinus), (Figs. 111 and 113 Vb, b). This nerve, after receiving the palatine branch of the facial, descends on the inner border of the Levator bulbi to the mucous membrane of the mouth, to which it gives numerous filaments; it then runs parallel and near to the median line, lying on the base of the skull; and near the front of the orbit it divides into three branches. The smallest (a) supplies the Harderian gland (HD); the second (b) continues in the course of the original nerve, pierces the vomer, and divides, giving filaments to the mucous membrane in its whole course, while the terminal twigs supply the intermaxillary gland (Gl) and the surrounding structures. One of these (†) ascends on the anterior border of the sphenethmoid to the intermaxillary (internasal, Born) space to supply the numerous glands and to inosculate by one or two twigs with the nasal branch (tr) of the trigeminal. The third branch of the palatine nerve (c) curves directly outwards, just behind the palatine bone, to the inner surface of the maxillary bone; it then courses backwards, pierces the fibrous plate (db) between the eyeball and the pterygoid bone, and ultimately unites with superior maxillary division of the trigeminal nerve (Vc′). Just before piercing the fibrous plate it gives off a tolerably large nerve (d) to the mucous membrane, and in its whole course it gives off twigs to the mucous membrane and surrounding structures; many of these twigs inosculate with other terminal twigs of the trigeminal.

[This nerve has been minutely described by Stirling and Macdonald; these observers find that the branches form a very fine and close plexus in the mucous membrane, etc.; fibres were traced to blood-vessels, glands, etc., and ganglion cells were found scattered along the nerves. The fibres are both medullated and non-medullated, and many of the cells are described as ‘spiral cells’ (see page 201) such as those described by Arnold in the sympathetic system of the frog.]

b. The Ramus maxillo-mandibularis (Ramus maxillaris, Ecker and Hoffmann; Supra-maxillary, Humphry; Upper Maxillary Branch, Wyman), (Figs. 111, 112, and 113 Vc). This nerve is the largest division of the trigeminal; it runs directly outwards behind the eyeball, in front of the auditory capsule, and between the temporal and pterygoid muscles. After a very short course it divides into the maxillary and mandibular nerves. The nerve gives off before its division a few fine filaments (i,i) to the hinder half of the upper eyelid and to the surrounding skin, also a branch (h) [not correctly drawn in Fig. 111] which divides to supply twigs to the temporal and pterygoid muscles.

(1) The Ramus maxillaris (Ramus supramaxillaris, Ecker; Ramus maxillaris superior, Hoffmann; Upper Maxillary Branch, Wyman; Supramaxillary, Humphry and de Watteville), (Figs. 111, 112, 113, and 114 Vc) runs outwards and then forwards, between the eyeball and the outer wall of the orbit, to the margin of the upper jaw (K); a large portion terminates in a number of small branches for the supply of the skin of the lower eyelid, the upper lip, and of the parts between the tympanic membrane and the external nares. Some of these twigs inosculate with twigs of the palatine and ophthalmic nerves. The second portion of the nerve unites with the palatine nerve, as already described.

(2) The Ramus mandibularis (Ramus maxillaris inferior, Hoffmann; Mandibular or Lower Jaw Branch, Wyman; Inframaxillary, Humphry and de Watteville), (Figs. 111, 112, 113, 114, and 115 Vd, us). As far as the outer margin of the eyeball, this nerve runs parallel to and behind the Ramus maxillaris, in which course it supplies branches to the temporal and pterygoid muscles; it then curves backwards, outwards, and downwards to the under surface of the squamosal bone, where it supplies a twig to the Depressor maxillae, and then perforates the Masseter: in this manner it reaches the outer surface of the mandible, just behind the insertion of the temporal muscle; it then courses forwards, under the skin, to the Symphysis menti. In this course the nerve gives off numerous branches to the skin and surrounding parts, it also supplies the mylo-hyoid and submental muscles; one branch (Figs. 114 and 115 us) is larger than the rest, and supplies the under surface of the floor of the mouth and lower lip.