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We are now to study the means whereby the neurons are made to control and coördinate the different parts of the body and bring about the necessary adjustment of the body to its surroundings. This work of the neurons naturally has some relation to their properties.
Sensations are necessary for intelligent and purposeful action and for acquiring all kinds of knowledge. To enable the stimuli to act to the best advantage in starting the impulses, special devices, called sense organs, are employed. These receive the terminations of the neurons, and by their special structure enable the most delicate stimuli to start impulses.
*The Neurons, or Nerve Cells.*—While a hasty examination of the nerve skeleton is sufficient to show the connection of the nervous system with all parts of the body, no amount of study of its gross structures reveals the nature of its connections or suggests its method of operation.
These connect with neurons that in turn connect with blood vessels and with them act reflexively, when the heart is likely to be overstrained, to cause a dilation of the blood vessels. This lessens the pressure which the heart must exert to empty itself of blood. These fibers serve, in this way, as a kind of safety valve for the heart.
The central thread in the axon is the axis cylinder. The neurons are in all respects cells. They differ widely, however, from all the other cells of the body and are, in some respects, the most remarkable of all cells. They are characterized by minute extensions, or prolongations, which in some instances extend to great distances.
*Properties of Neurons.*—The work of the neurons seems to depend mainly upon two properties—the property of irritability and the property of conductivity. It has the same meaning here. The neurons, however, respond more readily to stimuli than do the muscles and are therefore more irritable.
The ventral-root divisions, of the fibers of mon-axonic neurons, the cell-bodies of which are in the gray matter of the cord. The first convey impulses to the cord and are called afferent neurons; the second convey impulses from the cord and are known as efferent neurons. A division of this pathway reaches the brain.
The whole psychological switch system may have been brought into disorder by such abnormal setting of certain parts, but the connection of each resulting accident with the primary emotional disturbances does not contradict the fact that all the causes lie entirely in disturbances of the central paths. It is a change in the neurons and their connections.
Though the neurons in certain parts of the body differ greatly in form and size from those in other parts of the body, most of them may be included in one or the other of two classes, known as mon-axonic neurons and di-axonic neurons. The cell-body has in itself the form of a complete cell and was at one time so described.
Any agency, such as heat or pressure, which, by acting on the neurons of the body, is able to produce a sensation, may be called a sensation stimulus. It has perhaps already been observed that the stimuli that lead to voluntary action, as well as those that produce reflex action of the muscles, cause sensations at the same time.
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