Animal life resides in the nervous system alone, and its character is proportioned to the development thereof, of which the brain is the principal mass. A subordinate portion of the general life, however, is in the nervous system of the body, and in proportion as the brain declines in development the relative amount of psychic energy in the body is greater. Thus the body of the alligator after decapitation is capable of sensation and voluntary acts, such as pushing away an offending body with its foot. The character of the life in the body is explained by physiology and sarcognomy. Its universal presence is due to the universal diffusion of the nervous system, of which the accompanying figure, showing the location of the spinal cord and spinal nerves, will give a proper conception. In this figure the spinal cord, with its thirty pairs of nerves, eight cervical at the neck, twelve dorsal in the back, five lumbar in the loins, and five or six in the sacrum (between the hips), is seen descending from the base of the brain below the cerebellum (which is rather too large in engraving), and proceeding throughout the body until lost in fine ramifications which the microscope can scarcely trace, but which quickly inform us if they are touched or disturbed.
It cannot properly be said that the spinal cord proceeds from the brain, nor on the other hand that the brain proceeds from the spinal cord, for they originate simultaneously in a soft, jelly-like condition in which the microscope cannot detect the latent structure, not as they are in the adult, but as they are in the fœtus in which they first appear, with a structure similar to that of the lowest class of vertebrate animals, the fishes.
From this embryonic condition, in which there is very little resemblance to the adult brain, its progress has been carefully traced by many observers, but chiefly by Tiedemann, through all the stages of life before birth into the soft, infantile form of the human brain. Some knowledge of this embryonic growth is necessary to a correct understanding of the adult brain, its essential plan, its growth, and the correct estimate of its development.
I have not found in our anatomical works what I consider a satisfactory exposition of this subject. Beginning as a student with Spurzheim’s anatomy of the brain, which ought to have been the clearest and most complete of all, I found it so obscure and unsatisfactory that until I had made many dissections I had no very clear understanding. I have never found any pleasure in the writings of Spurzheim. In more recent authors the anatomical details are very abundant indeed, and sufficient to tax the memory heavily, but without that system and philosophy which appeal to the understanding and make our conceptions satisfactory, as I hope to make them to my readers, who must have very incorrect conceptions of the plan of the brain, if they have relied upon the writings of Mr. Combe and his successors of the phrenological school, none of whom, so far as I am aware, have really understood cerebral anatomy.
Let us approach the subject by taking an exterior and general view, then by tracing the embryonic growth of the brain, and the interior connections of its fibres, until we are fully prepared to judge of its development as it lies in the skull, and to understand the relation of each organ to all other portions. Then we can study its functions with a clear understanding of the relations of the organs to each other, which is the material basis of psychic science, and with full confidence in our ability to judge and compare living heads and skulls of man and animals.
Let us take an exterior view by removing one half of the skull from the right side of the head. This enables us to see that the front portion of the brain rests above the sockets of the eyes, coming down in the centre as low as the root of the nose, but a little higher exteriorly. When we touch the forehead just over the root of the nose, our finger touches the lowest level of the front lobe, the seat of the intellect; but when we touch the external angle of the brow on the same level, we touch a process of bone, and our finger is fully half an inch below the level of the brain.
In the posterior view we see that below the great mass of brain which is called the cerebrum there lies a smaller body, shaped much like a small turnip, called the cerebellum or little brain, separated from the cerebrum by a firm, horizontal membrane called the tentorium (covering the cerebellum), on which the cerebrum rests.
