Differences in structure characterize extended sections of the gray matter. In front of the fissure of Rolando, pyramidal cells predominate. In the parietal and temporal lobes, small cells are more numerous than larger

ones.

The White Substance of the Hemispheres:-This is nerve-fibers. These are of three kinds:

(1) Commissural fibers. These connect similar parts of the two hemispheres. The principal mass bears the name of corpus callosum and is a broad band situate at the bottom of the longitudinal fissure, the individual fibers of which spread out to all the convolutions of the frontal, the parietal, and the central lobes. Next in importance is the anterior commissure, which connects those convolutions of the two hemispheres which lie below the fissure of Sylvius. It is through the agency of these commissural fibers that the two hemispheres are enabled to act in unison. As long as each hemisphere is in a healthful condition, and these connecting fibers are unimpaired, there is unity of action in the two associated halves.

(2) The fibers of association. These connect the convolutions of the same hemisphere. Of these, some unite adjoining convolutions; some, passing under two or three adjacent convolutions, connect those somewhat remote from each other; some run from one side of the hemisphere to the other, putting the most distant parts into immediate communication with each other.

(3) The medullary fibres. These connect the hemispheres with the medulla oblongata, and consequently with the spinal cord, and through it with the gray part of the organism.

Functions of the Hemispheres:-These, as has been shown negatively, are sensation, volition, and ideation,

for removal of the hemispheres destroys consciousness, voluntary power, and memory, leaving an animal a mere automaton, the movements which resemble those prompted by conscious sensation, intelligent adaptation, and volitional control, being regarded as simply reflex and having no connection whatever with mind, but having their origin in ganglia which are capable of being stimulated to unconscious activity.

On the other hand, as has been seen, the hemispheres are not to be regarded as directly connected with the maintenance of physical life. In quadrupeds large portions of them can be removed without impairing the vital functions. In fishes, reptiles, birds, and even in some quadrupeds, they may be entirely destroyed without causing death. In man, they may suffer extensive injury without destroying life. The cases are numerous which prove conclusively that the hemispheres are not indispensable to a continuance of the functions of animal-life.

It being thus rendered more than probable that the cerebrum, which is not indispensable to continued physical life, is the seat of intelligence in general, the reader may enter upon an investigation of the more direct evidence bearing upon this generally accepted theory. This will be presented in the succeeding chapter.

CHAPTER XX.

THE CEREBRUM THE ESPECIAL ORGAN OF MIND.

IN proof of this proposition, attention is called to the following considerations:

1. Other things being equal, the size of the hemispheres invariably determines the degree of mental power. In idiots the circumference of the head above the ears is uniformly small, sometimes only 12 or 13 inches. The average well-developed head is 22 inches in circumference. The heads of savages are smaller than those of persons possessing average intelligence in civilized nations; consequently, as we might expect, their intellectual powers are feebler. The brain-capacity of the negro-race averages 82 cubic inches; that of the AngloSaxon, 100. The average weight of the negro brain is 46.9 ounces; that of the English and German is 52. The intellectual vigor of each race, and its success in the struggle of life, are commensurate with the size of the brain possessed.

2. The greater the mental strength and the more numerous the faculties in active exercise, the larger, as a rule, is the cerebrum. Thus, it is more fully developed in insects than in worms; more fully in birds than in reptiles; more fully in monkeys than in elephants, in proportion to the size of the body; more fully in man than in any other animal. Indeed, its development in the

several species of animals, and as well also in different individuals, is the measure of mental power possessed. In primitive races it was small. There is, it is true, a difference in the quality of the cerebral matter, as well as in the quantity. Still, whatever the quality may be, in all cases of marked difference in intellectual vigor, there is, as a rule, a perceptible difference in the size of the hemispheres as compared with the weight of the animal. Each species, and each individual in each species, possesses more mental power the larger the hemispheres are, other things remaining substantially unchanged. In animals possessing little intelligence the cerebrum is small, and its convolutions are few, if not entirely wanting. The posterior lobes are almost peculiar to man. The monkey has them, but they are small. *

Increased strength in certain faculties is also well known to be accompanied by an unusual development of certain sections of the hemispheres. A well-developed forehead indicates the possession of good reasoning powers. An unusually large development in any definite extended portion of the cerebrum indicates the possession of unusual faculties of a particular kind; and the possession of unusual powers leads us to expect an unusual development in some portion of the cerebrum.

3. If a portion of the skull is removed, the hemispheres are found, on examination, to be continually agitated

* Some animals, particularly insects, have what Dr. Carpenter denominates "unconscious cerebration." Ants, we are told, build houses, make diving bells, bore galleries, construct vaults, and erect bridges. They line their houses with tapestry, clean them, air them, and close them by ingeniously constructed doors. They prepare ware-rooms, devise traps, hunt, rob, and plunder. They have social laws, a common language, division of labor, and gradation of rank. They recognize those belonging to the same community, maintain armies, go to battle, send out scouts, post sentinels, carry off prisoners, keep slaves, and tend domestic animais.

during the time the mind is actively engaged, the agitation being proportioned to the degree of mental excitement. They are viewed as physical organs which are called into exercise during the activity of the mind. Unless this theory is accepted, it is difficult to resist the temptation to regard mentality as identical with physical changes, so intimate are the relations subsisting between psychical and physical states. These countless molecular vibrations, and these numberless physical changes have been viewed under three aspects: (1) As efficient agents in the evolution of thought; (2) As identical with mental activities; (3) As instrumental agents in the production of ideas, the mind, an immaterial force, being regarded as the true and only efficient cause. The first regards mental states,-sensations, perceptions, ideas, and volitions--as effects of a definite series of changes in a material substance, the changes originating in the substance itself, as a result of the operation of purely physical forces which are properties of brain-matter. What these sensations, perceptions, and ideas really are, it makes no effort to determine. It contents itself with pronouncing them effects of changes in matter. The second theory views mental operations as identical with these physical changes and molecular vibrations. Sensation is a change in a definite portion of brain-tissue. Volition is a change in the gray matter of the cortex. Thoughts are vibrations of matter. Imaginations are vanishing atoms wreathed into fantastic forms. Conscience, and its commendations and reproaches, are states of unstable matter. Memory is aggregated particles of matter stored away for future The third of the three aspects, under which these changes may be viewed, regards them as effects of the activity of mental force, the brain being the organ of mind.

use.