Page:Popular Science Monthly Volume 2.djvu/379

Rh parallel to a part of the tracing by the muscle, to arrive directly at the time corresponding to the tracing. Marey detected, by this method, degrees of speed in transmission varying from 30 to 61 feet.

Moreover, the nerve-current travels more slowly at low temperatures than at high ones. Dr. Munk discovered, besides, that the speed is not alike in the different parts of a nerve; in the motor nerves it seems to increase toward the point of attachment of the muscle. And, according to De Bézold, this speed decreases when the nerve is under the influence of an electric current.

The point was now to repeat these experiments on the human subject. It was found possible to conduct them in this manner: An electric current produces a slight sensation of pain on one point in the skin; the instant of action by the current is marked on the revolving cylinder of a chronoscope. As soon as the person experimented on feels the shock, he gives a signal by touching an electric key, and a second mark is produced on the same cylinder. Measuring the interval comprised between these two marks, we have the time that elapses between the two signals. This time, which is from one to two-tenths of a second, is made up of several parts; transmission of external impression to the brain, perception, reflection, transmission of the will to the fingers, muscular contraction, which is the result; but, if the stimulus is applied successively to different points on the skin, these delays are always the same, except that which is due to the transmission of sensations. If, for instance, a point on the great-toe is first excited, and afterward a point in the inguinal region, the difference in the delays remarked will represent the time employed by sensation in ascending from the foot to the middle of the body.

These experiments were first made in 1861 by Hirsch, director of the Neufchâtel Observatory, by means of an apparatus which it would take too long to describe here. The person experimented on touched an electric key with the right hand at the instant of feeling that slight pain, not unlike a pin-prick, which the knob of an induction apparatus produces on touching the skin. The knob was applied in succession to the cheek, then to the left hand, then last to the left foot. The time lost in the transmission of this stimulus from the point touched to the right hand was found equal, in the three cases respectively, to $11/100$ $14/100$ and $17/100$ of a second; it required, therefore, $3/100$ of a second for sensation to arrive from the left hand at the head, and $6/100$ for its passage from the foot to the head. Hirsch inferred from tins that the nerve-current travels over a distance of six feet and a fraction in $6/100$ of a second, or about 104 feet in a second. Dr. Schelske repeated these experiments in a more thorough way at the Utrecht Observatory. He arrived at 91 feet as the speed of transmission of sensation in the human body. The same experimenter proved that the passage takes place with the same rapidity in the spinal marrow as in the nerves. This result is the more remarkable, as the