362 THE POPULAR SCIENCE MONTHLY. 



weight, hung on this lever, serves to give the muscle the required ten 

 sion. 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 T ^ 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 influ- 

 ence 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 oi 

 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 t 4q- of a second, and diminishes 

 again until the muscle returns to its natural state. 



This second instrument of Helmholtz received the name of a myo- 

 graphe. It has been perfected or rather modified by several physiolo- 

 gists. The great difficulty was, to measure precisely the time corre- 

 sponding 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 extrem- 

 ity of the muscle ; it was sufficient to count the number of vibrations in- 



