bend named the genu (fig. 91). The inferior or reflected portion, which is named the rostrum, becomes gradually narrower as it descends, and is connected with the lamina cinerea by a thin white layer, the commissura baseos alba of Henle. It gives off also two bands of white substance, the peduncles of the corpus callosum, which, diverging from one another, run backwards across the posterior margin of the anterior perforated space on each side to the uncus of the temporal lobe, where they meet the outer olfactory roots. These peduncles traced upwards around the genu are found to be continued into the mesial longitudinal striæ.

Behind, the corpus callosum terminates in a free thickened border, named the splenium, but this term is sometimes restricted to the under part of this thickened border, which appears as if rolled round under the rest (splenium proper, Beevor).

Fig. 92.-VIEW OF THE CORPUS CALLOSUM FROM ABOVE (from Sappey after Foville).

The upper surface of the corpus callosum has been fully exposed by separating the cerebral hemispheres and throwing them to the side; the gyrus fornicatus has been partly detached, and the transverse fibres of the corpus callosum traced for some distance into the cerebral medullary substance.

1, the upper surface of the corpus callosum; 2, median furrow or raphe; 3, longitudinal striæ bounding the furrow; 4, swelling formed by the transverse bands as they pass into the cerebrum, arching over the side of the lateral ventricle; 5, anterior extremity or knee of the corpus callosum; 6, posterior extremity; 7, anterior, and 8, posterior fibres proceeding from the corpus callosum into the frontal and occipital lobes respectively; 9, margin of the swelling; 10, anterior part of the gyrus fornicatus; 11, fissure between the corpus callosum and this convolution opened out; outside 12, is the termination of the calloso-marginal fissure, and before 13 is the parieto-occipital fissure; 13, upper surface of the cerebellum.

The mesial part of the under surface of the corpus callosum is connected behind with the fornix, and in the rest of its length with the septum lucidum, a vertical partition between the two lateral ventricles, which is included in the anterior bend of the corpus callosum. On the sides the corpus callosum roofs in the body and I anterior horn of the lateral ventricles. The enlarged posterior part or splenium lies over the mesencephalon, with pia mater between. The transverse fibres of the corpus callosum pass in a radiating manner, interlacing with those of the internal capsule, through the medullary centre to the cortex of each hemisphere. Those in front which sweep round into the prefrontal region form the so-called forceps minor.

Those from the body and the upper part of the splenium which curve over the lateral ventricle form the tapetum, whilst a large mass of fibres from the splenium proper curves round into each occipital lobe and is known as the forceps major.

The septum lucidum (fig. 91, sept. luc.), a thin double partition, separates the anterior part of the two lateral ventricles. It is composed of two distinct laminæ, having an interval between them (ventricle of the septum) which contains fluid probably of the nature of lymph. It occupies the interval between the corpus callosum above and in front, and the anterior part of the fornix behind (fig. 91); and gradually tapers backwards to end over the middle of the third ventricle, from which it is separated by the fornix and velum interpositum. Antero-inferiorly it extends downwards in an angle which reaches the anterior commissure.

The laminæ of the septum are formed of a part of the mesial wall of the hemispheres which has remained free, while the surrounding parts have become united by the development of the corpus callosum above and in front and the fornix below and behind. The ventricle of the septum was therefore originally a part of the great longitudinal fissure, and has no connection with the internal ventricular cavity of the brain. Accordingly we find that it is not lined by ciliated epithelium, but each lamina consists of a thin layer of grey matter next to the cavity, and homologous with the grey matter of the cerebral cortex, and a thicker layer of white matter continuous below on either side with the general white matter of the hemisphere. Externally, next to the lateral ventricle, is a layer of ependyma, and covering this the epithelium which lines the lateral ventricle; internally is a layer of connective tissue, homologous with the pia mater.

The fornix (fig. 89, f) is an arched longitudinal white tract consisting of two lateral halves, which are separated from each other in front and behind, but in the intermediate part are joined together in the median plane. The two parts in front form the anterior pillars (columnæ fornicis); the middle conjoined part is named the body; and the hind parts, which are again separated from each other, form the posterior pillars (crura fornicis).

The body of the fornix is triangular in shape, being broad and flattened behind, where it is adherent to the under surface of the callosum, and narrower in front, where it is attached to the septum lucidum. Its lateral edges are free and are seen projecting into the lateral ventricies just above the choroid plexuses. Its under surface rests upon the velum interpositum, which separates it from the optic thalami and the third ventricle (fig. 94).

Beevor distinguishes (in the monkey) two sets of fibres in the body of the fornix. Of these, the mesial set, when traced backwards, appear to turn dorsally and pierce the corpus callosum just in front of the splenium; above the corpus callosum they may perhaps join the posterior fibres of the cingulum (see p. 158), but their actual destination has not been certainly followed out.

The anterior pillars (fig. 82, Cf), cylindrical in form, can be traced downwards, slightly separated from each other, in front of the foramina of Monro, of which they form the anterior boundaries, and through the grey matter on the sides of the third ventricle, curving backwards to the corpora albicantia. There each column turns upon itself, making a twisted loop which forms the white portion of the corpus albicans, and from this it appears to be continued, as the bundle of Vicqd'Azyr, upwards into the anterior nucleus of the optic thalamus (fig. 93). Each pillar is connected near the foramen of Monro with the stria pinealis (tænia fornicis), and with the tænia semicircularis, and it gives fibres to the septum lucidum (for other connections of the anterior pillar, see p. 158).

According to the experiments of Gudden and Forel the fibres of the anterior

VOL. III.

K

pillars find a termination in the grey matter of the corpora albicantia, and are not directly continuous, as in dissected preparations they seem to be, with the bundle of Vicq-d'Azyr.

The posterior pillars or crura of the fornix (fig. 94, 13) are the diverging posterior prolongations of the two flat lateral bands composing the body. At first

th.m

Fig. 93. SKETCH SHOWING THE ORIGIN AND CON

NECTIONS OF THE ANTERIOR PILLARS OF THE

FORNIX. (Schwalbe.)

A median section has been made through the third ventricle, and the superficial grey matter removed at its anterior and lower part as far as a. th.s, upper part of the thalamus; th.m, its mesial surface between the two is the medullary stria, leading from the pineal gland and trigonum habenulæ, tr.h, to the anterior pillar of the fornix, c.f; f bundle of Vicq-d'Azyr; c.m, corpus mamillare; i.f, fibres of the inferior peduncle of the thalamus diverging in its substance; co.a, anterior commis

sure; co.m, middle commissure; ch, chiasma; i, infundibulum; r.p, stalk of pineal body; qu, corpora quadrigemina; aq, aqueduct.

they adhere to the under surface of the corpus callosum, then, curving outwards and downwards over the pulvinar of the optic thalamus, each crus enters the descending horn of the lateral ventricle, where some of its fibres are distributed on the surface of

Fig. 94.-VIEW OF THE UPPER SUR

FACE OF THE VELUM INTERPOSITUM,
CHOROID PLEXUSES, AND CORPORA
STRIATA. (From Sappey after Vicq-
d'Azyr.)

1, fore part of the tela choroidea or velum interpositum; 2, 2, choroid plexus; 3, 3, left vein of Galen partly covered by the right; 4, anterior pillars of the fornix divided in front of the foramen of Monro; on either side are seen small veins from the front of the corpus callosum and the septum lucidum; 5, vein of the corpus striatum ; 6, convoluted marginal vein of the choroid plexus ; 7, vein rising from the thalamus opticus and corpus striatum; 8, vein proceeding from the inferior cornu and hippocampus major; 9, one from the posterior cornu; 11, fornix divided near its middle and turned backwards; 12, lyra; 13, posterior pillar of the fornix; 14, the splenium of the corpus callosum.

the great hippocampus and the remainder are prolonged as the narrow band of white matter known as the tænia hippocampi or fimbria (fig. 89, fi).

On examining the under surface of the conjoined posterior parts of the fornix and corpus callosum, the splenium of the latter with its thickened edge and the diverging halves of the fornix appear to enclose between them a triangular structure, marked with transverse, longitudinal, and oblique lines. To this part the term lyra has been applied

(fig. 94, 12). This contains a few fibres which pass across from one posterior pillar of the fornix into the other, and are considered to form a commissural connexion between the two hippocampal regions of opposite sides. This connexion is denied by Beevor (in the monkey).

The tænia semicircularis is a band of white matter which lies in the groove separating the ventricular surface of the nucleus caudatus from that of the optic thalamus. It extends from the extremity of the descending horn of the lateral ventricle, where it passes into the nucleus amygdala, to the anterior part of the body of the ventricle, as far as the foramen of Monro, having a curved course corresponding with that of the ventricle. Its anterior connexions are somewhat obscure, but, according to Schwalbe and Testut, it divides anteriorly into two parts, one of which is continued into the anterior pillar of the fornix, and the other passes over and in front of the anterior commissure to become lost in the grey matter which

THROUGH THE BRAIN AND
SKULL MADE WHILST FROZEN.

(Key and Retzius.)

c, c, corpus callosum; below its middle part the septum lucidum, and below that again the fornix; LV, lateral ventricle; th, thalamus; between the two thalami the third ventricle is seen; below the thalamus is the substantia innominata; str, lenticular nucleus of the corpus striatum ; c, caudate nucleus of the same; between th and str is the internal capsule; outside str is the thin grey band of the claustrum, and outside this again the island of Reil at the bottom of the Sylvian fissure; n, a, nucleus amygdalæ ; immediately within this is the optic tract seen in section; p, pituitary body; B, body of the sphenoid bone; sa, subarachnoid space; v, villi of the arachnoid.

intervenes between the septum lucidum and the anterior end of the nucleus caudatus: some fibres perhaps penetrating into that nucleus. According to Foville, each extremity is ultimately traceable into the anterior perforated space.

The corpora striata (ganglia of the cerebral hemispheres), situated in front and to the outer side of the optic thalami, are two large ovoid masses of grey matter, the greater part of each of which is embedded in the white substance of the hemisphere (extraventricular portion), whilst a part becomes visible in the body and anterior horn of the lateral ventricle (intraventricular portion). The corpus striatum derives its name from the streaked appearance it presents, especially in its anterior part, when cut into, an appearance which is due to the passage through it of bundles of white fibres of the internal capsule to the frontal lobe of the hemisphere.

The nucleus caudatus (intraventricular portion of the corpus striatum) (fig. 82, Cs.; fig. 88, c.str.), is of pyriform shape, its larger end being turned forwards and appearing in the floor and partly in the lateral wall of the anterior horn. This enlarged anterior extremity projects into the white matter of the frontal lobe, where it is bounded by the fibres which pass from the genu of the callosum. Its narrow end is prolonged backwards and outwards along the body of the ventricle and into the roof of the descending horn, passing nearly to the extremity of the latter. The caudate nucleus is separated from the thalamus by the tænia semicircularis. Where it lies in the lateral ventricle (superior surface) the nucleus caudatus is covered by a

layer of ependyma and over this by the ciliated ventricular epithelium. The ganglion is itself composed of a reddish grey substance like that of the outer zone of the lenticular nucleus (see below). On the deeper side, that turned towards the internal capsule, the nucleus receives from this a number of bundles of medullated fibres which are traceable from the crusta. According to Wernicke it is doubtful if any of these pass on to the white matter of the hemispheres, nor do they come from the

Fig. 96.-VIEW FROM BEHIND OF A FRONTAL SECTION OF THE HEMISPHERES PASSING THROUGH THE GENU OF THE CORPUS CALLOSUM. (E. A. S.) From a photograph.

g.c.c., genu corporis callosi; c.a., cornu anterius of lateral ventricle bounded laterally by the grey mass of the corpus striatum; cl., claustrum; i. R., insula; Sy., Sylvian fissure; c-m.s., sulcus callosomarginalis; s.f.s., sulcus frontalis superior; s.f.i., sulcus precentralis; s.olf., sulcus olfactorius ; fr.orb., orbital surface of frontal lobe; tr.olf., tractus olfactorius in section.

The section passes a little more anteriorly through the left hemisphere than through the right. crusta directly, but only after traversing the medullary laminae of the lenticular nucleus (see below).

The nerve-cells of the nucleus caudatus are multipolar, and, in the adult, pigmented. Some are moderately large, but most small, belonging to Golgi's second type, with short axis-cylinder process (Marchi). Their nerve-processes pass in various directions, some into the internal capsule. In addition, peculiar spheroidal cells containing two or more nuclei were described by Henle.

The nucleus lenticularis (extraventricular portion of the corpus striatum) is separated in the greater part of its extent from the intraventricular part by a layer