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are divided, and the portion of dura mater (tentorium) which projects between the hemispheres and the cerebellum is cut through at its edges, the brain comes readily out.
It is best examined fresh. If numbers of brains have to be prepared and kept, I have found it a good plan to put them first in a solution of chloride of zinc, just dense enough at first to float them
FIG. 34. The human brain from below, with its nerves numbered, after Henle. I, olfactory; II, optic; III, oculo-motorius; IV, trochlearis; V, trifacial; VI, abducens oculi; VII, facial; VIII, auditory; IX, glosso-pharyngeal; X, pneumogastric; XI, spinal accessory; XII, hypoglossal; ncI, first cervical, etc.
and to leave them for a fortnight or less. This softens the pia mater, which can then be removed in large shreds, after which it is enough to place them in quite weak alcohol to preserve them indefinitely, tough, elastic, and in their natural shape, though bleached to a uniform white color. Before immersion in the chloride all the more superficial adhesions of the parts must be broken through, to bring
the fluid into contact with a maximum of surface. If the brain is used fresh, the pia mater had better be removed carefully in most places with the forceps, scalpel, and scissors. Over the grooves between the cerebellum and hemispheres, and between the cerebellum and medulla oblongata, thin cobwebby moist transparent vestiges of the arachnoid membrane will be found.
The subdivisions may now be examined in due order. For the convolutions, blood-vessels, and nerves the more special books must be consulted.
First, looked at from above, with the deep longitudinal fissure between them, the hemispheres are seen partly overlapping the intricately wrinkled cerebellum, which juts out behind, and covers in turn almost all the medulla oblongata. Drawing the hemispheres apart, the brilliant white corpus callosum is revealed, some half an inch below their surface. There is no median partition in the cerebellum, but a median elevation instead.
Looking at the brain from below, one still sees the longitudinal fissure in the median line in front, and on either side of it the olfactory lobes, much larger than in man; the optic tracts and commissure or · chiasma'; the infundibulum cut through just behind them; and behind that the single corpus albicans or mamillare, whose function is unknown and which is double in man. Next the crura appear, converging upon the pons as if carrying fibres back from either side. The pons itself succeeds, much less prominent than in man; and finally behind it comes the medulla oblongata, broad and flat and relatively large. The pons looks like a sort of collar uniting the two halves of the cerebellum, and surrounding the medulla, whose fibres by the time they have emerged anteriorly from beneath the collar have divided into the two crura. The inner relations are, however, somewhat less simple than what this description may suggest.
Now turn forward the cerebellum; pull out the vascular choroid plexuses of the pia, which fill the fourth ventricle; and bring the upper surface of the medulla oblongata into view. The fourth ventricle is a triangular depression terminating in a posterior point called the calamus scriptorius. (Here a very fine probe may pass into the central canal of the spinal cord.) The lateral boundary of the ventricle on either side is formed by the restiform body or column, which runs into the cerebellum, forming its inferior or posterior peduncle on that side. Including the calamus scriptorius by their divergence, the posterior columns of the spinal cord continue into the medulla as the fasciculi graciles. These are at first separated from the broad restiform bodies by a slight groove. But this disappears anteriorly, and the 'slender' and 'ropelike' strands soon become outwardly indistin‹ guishable.
Turn next to the ventral surface of the medulla, and note the anterior pyramids, two roundish cords, one on either side of the slight median groove. The pyramids are crossed and closed over anteriorly by the pons Varolii, a broad transverse band which surrounds them like a collar, and runs up into the cerebellum on either side, forming its middle peduncles. The pons has a slight median depression and its
FIG. 35.-Fourth ventricle, etc. (Henle). III, third ventricle; IV, fourth ver tricle; P, anterior, middle, and posterior peduncles of cerebellum cut through; Cr, restiform body; Fg, funiculus gracilis; Cq, corpora quadrigemina.
posterior edge is formed by the trapezium on either side. The trapezium consists of fibres which, instead of surrounding the pyramid, seem to start from alongside of it. It is not visible in man. The olivary bodies are small eminences on the medulla lying just laterally of the pyramids and below the trapezium.
Now cut through the peduncles of the cerebellum, close to their entrance into that organ. They give one surface of section on each side, though they receive contributions from three directions. The
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 vitæ 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 sickle. shaped, 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
FIG. 36.-Horizontal section of human brain just above the thalami.-Ccl, corpus callosum in section; Cs, corpus striatum; Sl, 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