Page:Popular Science Monthly Volume 11.djvu/587

Rh muscular contraction require time. Had these been considered, the solution would have been farther from the truth than it really was. Recent investigations of more precision have not, however, given results that differ widely from that obtained by Haller. The rate as given by Helmholtz, after many experiments, is about 111 feet per second. This is now generally accepted as the most accurate statement. Slight differences in results for movements from the brain, and for those proceeding to it, have been obtained in the investigations, but the rate is regarded by the best authorities as essentially the same for both movements. If there be a difference, it arises because the rate of voluntary impulses moving from the brain outward is the more rapid of the two.

A difference also has been found to exist in the rapidity with which we perceive impressions received through the eye, and those received through the ear. This at one time was supposed to be caused by a difference in the rate of transmission in the respective nerves. The better authority now is that the disparity is occasioned either by difference of length of the two nerves, or because the mind does not so readily distinguish impressions of one kind as those of the other.

If these data are correct, it requires at least one-twelfth of a second for us to perceive a sensation in our foot. A mosquito which selects our ankle as the field of his operations has nearly a sixth of a second in which to make his escape, for it requires at least one twelfth of a second for us to find out that the mosquito is there, and another twelfth of a second for us to make up our mind to declare war upon him and to initiate hostile action.

It will be observed that the rate of nervous transmission is comparatively slow. Electricity travels at the fate of many thousand miles a second, or, more accurately, 16,000,000 times as fast as nervous action. Light moves about two-thirds as fast as electricity. If we examine movements which are comparatively sluggish, we find that a cannon-ball, when fired, moves about 900 feet a second, or nearly ten times as fast as nervous energy. A railroad-train speeding along at sixty miles an hour would be moving at about the same rate as an ordinary nervous stimulus, though in a contest the stimulus would probably win.

By further experiment it has been determined that the time required for the excitation of the muscular fibres for such acts as the supposed pressing of the button, this being the factor last in order of the entire six, is about one-hundredth of a second.

It is also declared with some precision, both by analogy to this last act and by experiment, that the time required for the reception of the impression by the sensitive membrane, which is the first and a corresponding factor of the complete physiological time, is also about one-hundredth of a second.