The preparation was made with the brain in situ (hardened). The skull cap and membranes having been removed, the brain was sliced down to the level of the corpus callosum. The left lateral ventricle was then opened by cutting away its roof, and the island exposed by slicing away the opercula. The drawing is made from a photograph.

I.R., insula Reilii (the line points to the middle of the three gyri breves); s.c., sulcus centralis insula; g.l., gyrus longus insulæ ; c. c., corpus callosum; n. L., nerves of Lancisi; str.t., stria tecta ; f.mi., forceps minor; f.ma., forceps major; c.a., cornu anterius of ventricle; c.p., cornu posterius ; c.i., entrance to cornu inferius; f. M., foramen Monroi; s. M., sulcus leading backwards to the foramen Monroi; c.str., corpus striatum; th.opt., thalamus opticus, anterior tubercle; pl.ch., plexus choroides; f, fornix; f', its anterior pillar; h, posterior end of hippocampus major; tri., trigonum ventriculi ; calcar, calcar avis.

grey matter is the nucleus caudatus of the corpus striatum (c. str.) It is pear-shaped, with the larger end projecting into the floor of the anterior horn, and the smaller tapering posterior end (tail) extending to the origin of the descending horn, and then curving downwards and forwards along the dorsal part of this horn, in which it forms a long, narrow, rounded eminence. The posterior horn (figs. 88, 89, c.p.), which is curved around the parieto-occipital fissure, its concavity therefore being directed inwards, passes, as before said, into the occipital lobe. It is roofed by the fibres of the corpus callosum which are passing to the temporal and occipital lobes; part of these fibres also form its lateral wall. At the upper part of its mesial wall there is a bundle of fibres (forceps major) sweeping round from the splenium of the callosum to enter the occipital lobe. This produces a projection into the cavity of the horn, which is known as the bulb of the posterior horn. Below it is another curved eminence, which extends from the base of the horn backwards for a variable distance, to end in a pointed extremity: this is termed, from its resemblance to a cock's spur, the ergot (Morand), or calcar avis (also hippocampus minor). corresponds with the calcarine fissure on the mesial surface of the hemisphere.

It

The hippocampus minor is not peculiar to the human brain as was at one time thought, but is found in the brains of quadrumana. In the human subject the posterior horn varies greatly in size, and the hippocampus minor is still more variable in its development, being sometimes scarcely to be recognised, and at others proportionally large. It is usually most developed where the posterior horn is longest; but the length of the posterior horn, and prominence of the hippocampus minor, are by no means in proportion to the dimensions of the hemisphere.

The middle, lateral, or descending horn (fig. 89) is directed at first backwards and outwards from the posterior end of the body of the ventricle; it then passes downwards and forwards with a bold sweep (at about the plane of the parallel sulcus), being finally curved inwards, and ending about an inch from the apex of the temporal lobe, where it abuts against the amygdala. The dorso-lateral boundary (roof) is formed by the fibres of the callosum, which are arching over it into the temporal lobe, forming the so-called tapetum. The tail of the nucleus caudatus and the tænia semicircularis are prolonged into the roof. Extending along the floor of the horn is an eminence known as the hippocampus major or cornu Ammonis (fig. 89, h), which becomes enlarged as it descends, and being notched, its edge here presents a certain resemblance to an animal's paw (pes hippocampi). This eminence corresponds with the hippocampal fissure externally, which thus indents the floor of the cornu, and the grey matter at the bottom of the fissure being separated from the cavity of the ventricle by a thin layer of white substance (covered with ependyma), shews through and gives a bluish-white appearance to the hippocampus. Mesial to the hippocampus is the white band known as the fimbria (fig. 89, fi), prolonged from the posterior pillar of the fornix; and over the fimbria, at the angle which the floor of the horn here forms with its roof, the choroid plexus projects into the cavity from the external pia mater; the plexus is, however, covered by the epithelial lining of the cavity which extends over it in passing from roof to floor. If the pia mater is pulled away it drags along with it this covering layer of epithelium, and the cornu is made to open on the mesial surface of the brain. A cleft-like opening is thus produced, which has been called the inferior fissure of the cerebrum, being the lower part of the so-called transverse fissure, which follows the plane of the velum interpositum (see p. 184) over the thalamus and third ventricle, and emerges over the corpora quadrigemina. It is along this fissure that the choroid plexuses of the pia mater are invaginated into the ventricles, covered by the ventricular epithelium, which is pushed in before them. It is only when the choroid plexuses are dragged away that an actual fissure is formed in the hemisphere-wall between the fornix and fimbria on the one side and the optic thalamus on the other; but with the

Fig. 89.-VIEW FROM ABOVE AND THE SIDE OF THE WHOLE LEFT LATERAL VENTRICLE.
Natural size. (E. A. S. and G. D. T.)

The insula has been sliced away This is a further dissection of the preparation shown in fig. 88. and the middle or descending cornu, c.i., exposed. Within this are seen the following parts: fi, fimbria, continued from the fornix; h, the hippocampus major; coll., the eminentia collateralis. The other lettering as in fig. 88.

plexus in situ it is a deep sulcus, i.e., an invagination only, of the thin hemispherewall (here formed of the ventricular epithelium alone). It is known as the choroidal fissure, and appears at an early period of embryonic development.

Merkel and Mierzejewsky have described an actual cleft in the pia mater along the descending horn which effects a communication between the ventricle and the subarachnoid space, analogous to the foramen of Magendie and the lateral apertures in the fourth ventricle. This observation has not hitherto been confirmed.

Fig. 90.-Two VIEWS OF A PLASTER CAST OF THE CAVITIES OF THE CEREBRAL VENTRICLES.

A, from above; B, from the side. The projections into the cavities of the structures which bound the ventricles are seen as impressions upon the cast.

At the junction of the descending with the posterior cornu, and occupying the angle between the hippocampus major and the calcar, is a triangular space, the floor of which is mainly occupied by a smooth, somewhat raised surface, which extends backwards into the posterior cornu, and is often continued downwards into a longitudinal eminence which passes for some distance in the inferior horn alongside

of and lateral to the hippocampus major. This eminence corresponds with the collateral fissure externally, and is known as the eminentia collateralis or pes accessorius (fig. 89, coll.); this term is also by some authors used to include the smooth eminence at the junction of the cornua. The triangular space at this junction is termed by Schwalbe trigonum ventriculi.

The shape of the ventricles is best shown in a cast of the cavities, which can be made in plaster of Paris. The appearance of such a cast, viewed respectively from above and from the side, is given in the accompanying figure (fig. 90, A, B).

The corpus callosum or great commissure (trabs cerebri) (figs. 91, 92, 102), a white structure, about 10 centimeters long, which connects the two hemispheres throughout nearly half their length, approaches closer to the front than the back of

Fig. 91.-PORTION OF A MEDIAN SECTION OF THE BRAIN, SHOWING THE CORPUS CALLOSUM, THIRD VENTRICLE, AQUEDUCT AND FOURTH VENTRICLE, PONS, CEREBELLUM, ETC. (G. D. T.) 3.

the hemispheres. It is about 20 mm. in width behind, and somewhat narrower in front. Its thickness is greater at the ends than in the middle, and is greatest behind (8 mm.). It is arched from before backwards, its lower surface being concave and its upper surface convex. Its upper surface is distinctly marked by transverse furrows, which indicate the direction of the greater number of its fibres. It is also marked in the middle by a longitudinal furrow (raphe), which is bounded by two white tracts, placed close to each other, named the mesial longitudinal stric or nerves of Lancisi (fig. 88). On each side, near the margin, are seen other longitudinal lines (grey or lateral longitudinal stric). The lateral striæ lie within a fissure (callosal fissure) which separates the upper surface of the commissure from the gyrus fornicatus (by which convolution they are entirely covered in hence the name, taniæ tecta, which is often applied to them). Both the mesial and lateral longitudinal striæ are traceable, when well developed, into the gyrus dentatus posteriorly. The morphological value of these structures will be afterwards referred to (p. 155).

In front, the corpus callosum is reflected downwards and backwards, forming a