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The Cerebrum

hemispheres, brain, mid-brain, inter-brain and neurones

THE CEREBRUM The cerebrum with its great hemispheres is that part of the brain which especially characterizes man. In man only do the hemispheres reach such predominant development. Though they are mere outgrowths of the anterior brain-vesicle in the beginning, they completely overshadow all other parts of the brain by the seventh month of embryonic life, extending farther forward, backward and lateralward than any other part. Within the cerebrum lies the physical basis of all conscious mental function; it constitutes the central mechanism of thought and consciousness. The active, functionating elements of the cerebrum are the neurones, which constitute a little more than half its bulk. Every mental process, whether con scious or unconscious, is attended by a mysterious physico chemical process in certain neurones. That process consists of an increased blood supply, increased metabolism, altered chemical reaction, elevation of temperature, and permanent changes in the neurones that persist as records or memories. Those records are very gradually reduced in vividness by the normal nutritive changes, according to Ribot's law of regression; but they are never entirely eradicated except by degeneration or dissociation of the neurones.

Reference to the table given above shows that the cerebrum is made up of three parts: (r) The end-brain, which includes the cerebral hemispheres and their connecting links; (2) the inter-brain, comprising the thalami and their associated nuclei, which with the former constitutes the fore-brain; and (3) the mid-brain (Figs. 17, 18, and 33). The cerebrum is an ovoid mass, flattened inferiorly, which fills the vault of the cranium and rests, below, upon the floor of the cranial cavity in the anterior and middle fossx and upon the tentorium cerebelli over the posterior fossa (Fig. 2). It comprises seven-eighths of the

entire brain, weighing on the average from 1074-1204 gm. (38-43 oz.). Viewed from above, it is sufficiently round to suggest a sphere; and, being divided in the median line by the longitudinal fissure, the lateral halves are called hemispheres. The most anterior point is the _frontal pole, and the most posterior is the occipital pole (Fig. 26). In the floor of the longitudinal fissure of the cerebrum the corpus callosum can be seen joining the hemispheres together; and beneath it, con cealed from view, are the fornix and anterior commissure. Those are the connecting links, proper, of the hemispheres (Figs. 42,47 and 48). Inferior to them is found the inter-brain. The latter forms an additional union of the hemispheres, as may be seen by viewing the base of the brain. Just caudal to the inter-brain is the mid-brain which occupies the tentorial notch of the dura mater; and, situated in the median line, is so overhung by the cerebral hemispheres as to reveal only its an terior surface. It resembles the inter-brain in this respect. Inferiorly the mid-brain joins the rhombencephalon. Their plane of union cuts the isthmus (Fig. 56).

In studying the gross structures of the cerebrum it is most convenient to divide it into its earliest embryologic divisions, viz., the fore-brain and the mid-brain.