Page:Popular Science Monthly Volume 2.djvu/378

362 weight, hung on this lever, serves to give the muscle the required tension. Every thing is so arranged that, at the instant the current is closed, a shock is produced either directly in the muscle, or in a given point of a nerve which is isolated for a length of about an inch, and still adheres by one end to the muscle which it is to stimulate. Under the influence of this excitement, the muscle contracts, stirs the lever, and breaks the electric circuit in which it was a part. The duration of circulation of the current is indicated by the magnetized needle. It is found, then, that contraction occurs later when the nerve is excited than when the muscle is excited directly; the difference discloses the speed of transmission of the nervous agent, which is found equal to very nearly 80 feet a second. Helmholtz has ascertained, moreover, that, in every case, contraction follows the electric shock only after an interval of time equal to $1/100$ of a second, which he calls the time of latent stimulus. The muscular fibres do not, therefore, instantly obey the spur of electricity. Thus the waters of the sea rise under the influence of lunar attraction only after the planet is long past the meridian.

After these beautiful experiments, which revealed for the first time the knowledge of the way in which a stimulus is transmitted along the nerves, Helmholtz devised another method, permitting the analysis of the phenomenon in its minutest details. In this, also, the contraction of the muscle lifts a light lever, but the lever carries a point which leaves a white mark on a revolving cylinder covered with lamp-black. A peculiar arrangement causes the same point to mark the instant of production of the stimulus, and, from that instant to the moment of the muscular contraction, the point traces a straight line in the lamp-black. When it is afterward lifted by the tension of the muscle, it draws a curve which at once represents to sight, by its appearance, all the different phases of the movement of contraction. By this method, Helmholtz discovered that the speed of the nerve-current was a fraction over 83 feet. He proved, moreover, that the tension of the muscles gradually increases from the first moment of movement, that it reaches a maximum after about $5/100$ of a second, and diminishes again until the muscle returns to its natural state.

This second instrument of Helmholtz received the name of a myographe. It has been perfected or rather modified by several physiologists. The great difficulty was, to measure precisely the time corresponding to the different points of the tracing drawn by the point on the cylinder. Helmholtz communicated motion to the cylinder of his apparatus by a clock-work arrangement which pointed out to the eye the length of its revolution. For this method, the use of the diapason has been advantageously substituted. Dr. Marey, in his course of medical physiology, employed for this purpose a diapason which made 500 simple vibrations every second; those vibrations noted themselves on the cylinder alongside the curve traced by the extremity of the muscle; it was sufficient to count the number of vibrations