The Story of the Mind

by James Mark Baldwin

Fig. 2.—s c mt = reflex circuit; s c sp mp c mt = voluntary circuit.

Going upward in the nervous system, we next find a certain group of bodies within the gross mass of the brain, certain centres lying between the hemispheres above and the medulla and spinal cord below, and in direct connection by nervous tracts with both of these. The technical names of the more important of these organs are these: the "corpora striata," or striped bodies, of which there are two, the "optic thalami," also two in number, and the "cerebellum" or little brain, situated behind. These make up what is called the "second level" in the system. They seem to be especially concerned with the life of sensation. When the centres lying above them, the hemispheres, are removed, the animal is still able to see, hear, etc., and still able to carry out his well-knit habits of action in response to what he sees and hears. But that is about all. A bird treated thus, for example, these second-level centres being still intact while the hemispheres are removed, retains his normal appearance, being quite able to stand upon his feet, to fly, walk, etc. His reflexes are also unimpaired and his inner physiological processes; but it soon becomes noticeable that his mental operations are limited very largely to sensations. He sees his food as usual, but does not remember its use, and makes no attempt to eat it. He sees other birds, but does not respond to their advances. He seems to have forgotten all his education, to have lost all the meanings of things, to have practically no intelligence. A dog in this condition no longer fears the whip, no longer responds to his name, no longer steals food. On the side of his conduct we find that all the actions which he had learned by training now disappear; the trick dog loses all his tricks. What was called Apperception in the earlier chapter seems to have been taken away with the hemispheres.

Coming to the "first level," the highest of all, both in anatomical position and in the character of the functions over which it presides, we see at once what extraordinary importance it has. It comprises the cortex of the hemispheres, which taken together are called the cerebrum. It consists of the parts which we supposed cut out of the pigeon and dog just mentioned; and when we remember what these animals lose by its removal, we see what the normal animal or man owes to the integrity of this organ. It is above all the organ of mind. If we had to say that the mind as such is located anywhere, we should say in the gray matter of the cortex of the hemispheres of the brain. For although, as we saw, animals without this organ can still see and hear and feel, yet we also saw that they could do little else and could learn to do nothing more. All the higher operations of mind come back only when we think of the animal as having normal brain hemispheres.

Further, we find this organ in some degree duplicating the function of the second-level centres, for fibres go out from these intermediate masses to certain areas of the hemispheres, which reproduce locally the senses of hearing, sight, etc. By these fibres the functions of the senses are "projected" out to the surface of the brain, and the term "projection fibres" is applied to the nerves which make these connections. The hemispheres are not content even with the most important of all functions—the strictly intelligent—but they are jealous, so to speak, of the simple sensations which the central brain masses are capable of awaking. And in the very highest animals, probably only monkeys and man, we find that the hemispheres have gone so far with their jealousy as to usurp the function of sensation. This is seen in the singular fact that with a monkey or man the removal of the cortical centres makes the animal permanently blind or deaf, as the case may be, while in the lower animals such removal does not have this result, so long as the "second-level" organs are unimpaired. The brain paths of the functions of the second and first levels taken together constitute the so-called "voluntary circuit" (see Fig. 2).

In addition to this general demarcation of functions as higher and lower—first, second, and third level—in their anatomical seat, many interesting discoveries have been made in the localization of the simpler functions in the cortex itself. The accompanying figures (Figs. 3 and 4) will show the principle centres which have been determined; and it is not necessary to dwell upon additional details which are still under discussion. The areas marked out are in general the same on both hemispheres, and that is to say that most of the centres are duplicated. The speech centres, however, are on one side only. And in certain cases the nervous fibres which connect the cortex with the body-organs cross below the brain to the opposite side of the body. This is always true in cases of muscular movement; the movements of the right side of the body are controlled by the left hemisphere, and vice versa. The stimulations coming in from the body to the brain generally travel on the same side, although in certain cases parallel impulses are also sent over to the other hemisphere as well. For example, the very important optic nerve, which is necessary to vision, comes from each eye separately in a large bunch of fibres, and divides at the base of the brain, so that each eye sends impulses directly to the visual centres of both hemispheres.

Fig. 3.—Outer surface of left hemisphere of the brain (modified from Exner): a, fissure of Rolando; b, fissure of Sylvius.