What the cerebrum is when fully developed in man has already been shown; what it is in the fishy stage of development, when it is the smaller portion of the brain, may be understood by a dissection given in Serres “Anatomie Comparée du Cerveau,” representing the brain of the codfish dissected or opened from above.
When animal life reaches a high development, the functions which are diffused become concentrated into special organs. Intelligence or psychic life is concentrated in the cerebrum, and entirely removed from the spinal cord. The physiological energy apart from the psychic, is concentrated in the cerebellum, and thus the intermediate psycho-physiological organ, the optic lobes or quadrigemina, being no longer important, dwindles to become the smallest part of the brain.
Explanation.—In the codfish, roach, and flounder, II is the cerebellum, n the optic lobes, in front of which is the cerebrum, from which the olfactory nerve extends forward. Behind the cerebellum is the superior end of the spinal cord. The letter c is placed on the restiform bodies or posterior part of the medulla oblongata of the cod. The engravings show the upper surfaces of the brains, as we look down upon them.
If the reader will look at the sketch of the brains of the codfish, flounder, and roach, as figured by Spurzheim, he will see in each a very small cerebrum, a larger cerebellum, and still larger middle brain or optic lobes. This is the model on which the human brain is first developed, when in the second month it becomes possible to study it with the microscope. It presents to view in the third month three vesicles of soft neurine, the one which is to form the cerebellum being larger than that which is to become the cerebrum.
These are three brains of different grades, formed alike on the same vesicular plan. The resemblance of the optic lobes to the cerebrum is very striking, and when we open them we see what corresponds to the lateral ventricles of the cerebrum, with a structure at the bottom corresponding in position and character with the inferior ganglion of the cerebrum. The subdivision of function is similar to that of the cerebrum, the anterior portion of these lobes being of an intellectual, perceptive character, and the posterior the seat of the impulses. This has been demonstrated also in the experiments of vivisectors, in which the irritation of the posterior part has produced a vocal utterance or bark. Spurzheim gives a view of the brain of the pike with an optic lobe partially opened, to show the ventricle.
The cerebellum or physiological brain is formed on the same general plan, having its energetic or forcible functions in the posterior inferior regions, and its more sensitive functions located anteriorly.
In the embryo of twelve weeks a great advance has taken place; the optic lobes or quadrigemina are still large, but the cerebrum is larger than all the remainder. Still, it has not yet developed what might be called frontal and occipital lobes. The basis of the middle lobe, which is the most physiological portion of the cerebrum, being devoted to the sensibility, appetites, and muscular impulses, is that which first presents itself, being the first outgrowth from the great inferior ganglion or summit of the spinal system.
