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posterior and middle portions we have seen the anterior peduncles pass forward to the corpora quadrigemina. The thin white layer of nerve-tissue between them and continuous with them is called the valve of Vieussens. It covers part of the canal from the fourth ventricle to the third. The cerebellum being removed, examine it, and cut sections to show the peculiar distribution of white and gray matter, forming an appearance called the arbor vita in the books.

Now bend up the posterior edge of the hemispheres, exposing the corpora quadrigemina (of which the anterior pair are dubbed the nates and the posterior the testes), and noticing the pineal gland, a small median organ situated just in front of them and probably, like the pituitary body, a vestige of something useful in premammalian times The rounded posterior edge of the corpus callosum is visible now passing from one hemisphere to the other. Turn it still farther up, letting the medulla, etc., hang down as much as possible and trace the under surface from this edge forward. It is broad behind but narrows forward, becoming continuous with the fornix. The anterior stem, so to speak, of this organ plunges down just in front of the optic thalami, which now appear with the fornix arching over them, and the median third ventricle between them. The margins of the fornix, as they pass backwards, diverge laterally farther than the margins of the corpus callosum, and under the name of corpora fimbriata are carried into the lateral ventricles, as will be seen again.

It takes a good topographical mind to understand these ventricles clearly, even when they are followed with eye and hand. A verbal description is absolutely useless. The essential thing to remember is that they are offshoots from the original cavity (now the third ventricle) of the anterior vesicle, and that a great split has occurred in the walls of the hemispheres so that they (the lateral ventricles) now communicate with the exterior along a cleft which appears sickleshaped, as it were, and folded in.

The student will probably examine the relations of the parts in various ways. But he will do well to begin in any case by cutting horizontal slices off the hemispheres almost down to the level of the corpus callosum, and examining the distribution of gray and white matter on the surfaces of section, any one of which is the so-called centrum ovale. Then let him cut down in a fore-and-aft direction along the edge of the corpus callosum, till he comes through' and draw the hemispherical margin of the cut outwards-he will see a space which is the ventricle, and which farther cutting along the side and removing of its hemisphere-roof will lay more bare. The most conspicuous object on its floor is the nucleus caudatus of the corpus striatum.

Cut the corpus callosum transversely through near its posterior edge and bend the anterior portion of it forwards and sideways. The rear edge (splenium) left in situ bends round and downwards and becomes

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FIO 36.-Horizontal section of human brain just above the thalami.-Cel, corpus callosum in section; Cs, corpus striatum; S, septum lucidum; Cf, columns of the fornix; Tho, optic thalami; Cn, pineal gland. (After Henle.)

continuous with the fornix. The anterior part is also continuous with the fornix, but more along the median line, where a thinnish membrane, the septum lucidum, triangular in shape, reaching from the one body to the other, practically forms a sort of partition between the

contiguous portion of the lateral ventricles on the two sides. Break through the septum if need be and expose the upper surface of the fornix, broad behind and narrow in front where its anterior pillars plunge down in front of the third ventricle (from a thickening in whose anterior walls they were originally formed), and finally penetrate the corpus albicans. Cut these pillars through and fold them back, exposing the thalamic portion of the brain, and noting the under surface of the fornix. Its diverging posterior pillars run backwards, downwards, and then forwards again, forming with their sharp edges the corpora fimbriata, which bound the cleft by which the ventricle lies open. The semi-cylindrical welts behind the corpora fimbriata and parallel thereto in the wall of the ventricle are the hippocampi. Imagine the fornix and corpus callosum shortened in the fore-and-aft direction to a transverse cord; imagine the hemispheres not having grown backwards and downwards round the thalamus; and the corpus fimbriatum on either side would then be the upper or anterior margin of a split in the wall of the hemispheric ventricle of which the lower and posterior margin would be the posterior border of the corpus striatum where it grows out of the thalamus.

The little notches just behind the anterior pillar of the fornix and between them and the thalami are the so-called foramina of Monro through which the plexus of vessels, etc., passes from the median to the lateral ventricles.

See the thick middle commissure joining the two thalami, just as the corpus callosum and fornix join the hemispheres. These are all embryological aftergrowths. Seek also the anterior commissure crossing just in front of the anterior pillars of the fornix, as well as the posterior commissure with its lateral prolongations along the thalami, just below the pineal gland.

On a median section, note the thinnish anterior wall of the third ventricle and its prolongation downwards into the infundibulum.

Turn up or cut off the rear end of one hemisphere so as to see clearly the optic tracts turning upwards towards the rear corner of the thalamus. The corpora geniculata to which they also go, distinct in man, are less so in the sheep. The lower ones are visible between the optic-tract band and the 'testes,' however.

The brain's principal parts are thus passed in review. A longitudinal section of the whole organ through the median line will be found most instructive (Fig. 37). The student should also (on a fresh brain, or one hardened in bichromate of potash or ammonia to save the contrast of color between white and gray matter) make transverse sections through the nates and crura, and through the

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FIG. 87.-Median section of human brain below the hemispheres. Th, thalamus; Cg, corpora quadrigemina; V, third ventricle; Com, middle commissure; columns of fornix; Inf, infundibulum; Op.n, optic nerve; Pit, pituitary body; Av, arbor vitæ. (After Obersteiner).

hemispheres just in front of the corpus albicans. The latter section shows on each side the nucleus lenticularis of the corpus striatum, and also the inner capsule (see Fig. 38, Nl, and Ic).

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FIG. 38.-Transverse section through right hemisphere (after Gegenbaur). Cc, corpus callosum; Pf, pillars of fornix; Ic, internal capsule; V, third ventricle; Nl, nucleus lenticularis.

When all is said and done, the fact remains that, for the beginner, the understanding of the brain's structure is not an easy thing. It must be gone over and forgotten and learned again many times before it is definitively assimilated by the mind. But patience and repetition, here as elsewhere, will bear their perfect fruit.

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