Being Well-Born: An Introduction to Eugenics

by Michael F. Guyer

Neuron Theory.—According to the prevailing modern conception the entire nervous system is made up of a series of units called neurons. Each neuron is a single cell with all its processes. The latter consists typically of short branching processes on the one hand, known as dendrites, and of a single process on the other, known as the axon, which extends from the cell to become a nerve fiber (Fig. 32, [p. 209]). The various neurons, with possibly a few exceptions, are not anatomically continuous but contiguous. They communicate with one another apparently by contact only. The axon of each neuron ends in an elaborate series of fine branchings which lie in contact with the dendrites of another neuron, or in some cases with the body of the other cell (Fig. 32, [p. 209]). Thus the nervous impulse passes from one neuron to the other at these points of contact. An impulse is supposed to travel normally only in one direction through a neuron, the dendrites being the receiving and the axon the discharging terminals. There are various types of neurons. Some, particularly within the brain, have their main processes so provided with branches and brushes that they may come into physiological connection with a number of other neurons.

Fig. 32

A—Diagram to illustrate neurons and their method of connection; a, axon; d, dendrite; s, synapse. To simplify the diagram the medullary sheathes of such fibers as would have them have been omitted. The arrows indicate the direction in which the impulse travels. The lower series shows diagrammatically how from the same neuron in the cortex two subordinate neurons may be affected, the one excited to cause contraction of a certain group of muscle fibers, the other inhibited so that the antagonistic fibers may relax and thus not hinder the movement of a given part.

B—Section of a region of the cerebral cortex (after Cajal). The cells have been blackened with chrome-silver and are much less highly magnified than the diagrams in A. The numerals refer to certain characteristic layers of the cortex in this region.

Establishment of Pathways Through the Nervous System.—It is believed that more or less resistance to transmission of stimuli prevails at the point of contact (synapse) between two neurons but that this resistance is lessened by repetition of conduction. The frequent traversing of a given pathway by similar impulses finally results in an automatic occurrence of the transmission, or, in other words, the action becomes habitual. Education consists largely in establishing such routes through the nervous tissue. Because of the greater plasticity of the neural mechanism in youth it is easier to open up and fix pathways of conduction than in later years. Moreover the earlier established lines of conduction become the more permanent.

Characteristic Arrangements of Nerve Cells Are as Subject to Inheritance as Other Structures of the Body.—That the main features of the nervous system are inherited becomes obvious when we see that each kind of animal has its own distinctive numbers, arrangements and proportions of the various neural units. In man, for example, there are certain characteristics, types and groupings of nerve-cells which are reproduced generation after generation with remarkable fidelity. This means that in so far as these represent the mental make-up of the individual, his mentality is continuously linked with others which have gone before. The new-born child has all the nerve-cells in its brain that it will ever have but the ultimate linkages of the finer connectives between them, or at least the pathways of travel, remain in large measure to be made.

As we have already seen, the cerebral cortex is the seat of the chief mental faculties of man or at least of the highest of these. Professor Lloyd Morgan, one of our greatest authorities on comparative psychology, is inclined to believe that the instincts are located in the subcortical material. In any event, the inheritance of mental ability resolves itself into the inheritance of a certain cerebral mechanism.

Different Parts of the Cortex Yield Different Reactions.—The cerebral cortex, however, is not functionally homogeneous throughout. Certain regions have been shown to be motor, others sensory, and moreover, these regions are apparently further specialized so that a given one of them is associated with a specific type of sensory or motor response, not merely with responses in general. Thus by injuring one of the sensory areas we might destroy vision but not other sensations, or by stimulating one of the motor centers we would get a response in a corresponding motor organ but not in all such organs. Likewise, it is probable that still different areas, the so-called “association areas,” relatively of much greater development in man than in any other animal, are the regions in which various perceptions and conceptions are synthesized and formed into organized knowledge. Here also are engendered the volitions which when flashed through the motor centers become expressed in activity or behavior.

It seems highly probable that just as the sensory and motor areas differ in kind from one another, so we must suppose there are qualitative differences in various parts of the association areas so that the different parts give different reactions in consciousness; that is, each special mental ability of the individual is more or less centered in a special part of the cortex. And just as there may be variations in other structures of the organism so there may be variations in these areas. The “gifted” person in some one direction, whether it be in mathematics, music, painting, or what not, is on this hypothesis one who has that particular area of his brain which forms the basis for the talent in question more highly developed than it is in the average individual. And since such talents are handed down to descendants, this can only mean that a similar grouping of the neurons in the region in question has occurred.