Page:Popular Science Monthly Volume 84.djvu/586

582 whole series of processes—i. e., the response of the living system to stimulation. First we must note that since in any special case the response as a whole is constant, all of its single component stages or separate processes must also be constant, both in their character and their interconnections. There must therefore be some one constant initial process which is directly caused by the external event or stimulus, and upon which the others automatically and inevitably follow. This initial process thus constitutes the critical or activating event in the physiological sequence. It alone is directly dependent on the stimulus; the others are dependent upon it. What is remarkable is that it should be produced by such a variety of different agents. The problem first to be considered may therefore be put somewhat as follows: What is the nature of the initial change produced in the irritable living tissue by the action of the external agent, and how does it happen that it can be caused by such diverse agencies? This problem has evident relations to a wide group of physiological and psychological problems; thus the question of the basis of the "specific energies" of the special sensory apparatus belongs here. In this case also the response—the conscious affective state or sensation—is distinctive and its quality independent (within certain limits) of the character of the stimulus. This is in fact characteristic of all cases of stimulation. How this can be possible I shall now attempt to indicate.

Let us take the case of the simplest of the irritable tissues of higher animals, one in which the excitation-process occurs in a highly characteristic form, but unaccompanied by highly specialized physiological effects like contraction or secretion. Such a tissue is nerve. What are the essential features in the response of this tissue to stimulation? It is first to be noted that the process set up by the external stimulus is self-propagating. The disturbance, whatever its nature, which originates at the point of stimulus is of such a kind that it imparts a stimulus to the adjoining regions of the nerve beyond the original point of stimulus; these on becoming active stimulate the next stretch of nerve, and in this way the state of excitation passes along the entire nerve to its termination. Evidently there is an active change of some kind, forming an essential component of the local nerve process, that acts as stimulus to adjoining regions. Now there is no mechanical change in a nerve as the impulse passes, little or no production of heat, apparently a slight physical or chemical change involving a loss of carbon dioxide; but none of these is in itself sufficient to act as stimulus. There is, however, another definite physical change which has this power: namely, the electrical variation—the bioelectric process or action-current—which always accompanies the activity of a nerve or indeed of any other irritable tissue.