Psychology: Briefer Course

by William James

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.

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, 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 indistinguishable.

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 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.

Fig. 35.—Fourth ventricle, etc. (Henle). III, third ventricle; IV, fourth ventricle; P, anterior, middle, and posterior peduncles of cerebellum cut through; Cr, restiform body; Fg, funiculus gracilis; Cq, corpora quadrigemina.