572 METHOD OF ESTIMATING RAPIDITY OF A NERVE-IMPULSE. 



at N, the deflection of the needle is greater in the former than in the latter case. From the 

 difference, we calculate the time for the conduction of the impulse in the stretch of the nerve 

 between n and N. 



[2. A simpler method is that shown in the scheme, fig. 412. Use a pendulum 

 or spring myograph (fig. 323), and suspend in a suitable manner a frog's gastro- 

 cnemius (m), with a long portion of the sciatic nerve (N) dissected out, by fixing 

 the femur in a clamp (/), while the tendo Achilles is fixed to a lever, which 

 inscribes its movements on the smoked glass plate (P) of the myograph ; place the 

 key of the myograph (2) in the circuit with the battery (B), and the primary 

 circuit of the induction machine (I). To the secondary coil (II) attach two wires, 

 and connect them with a commutator without cross-bars (C). Connect the other 

 binding screws of the commutator with two pairs of wires, arranged so that one 

 pair can stimulate the nerve near the muscle (a), and the other at a distance from 

 it (6). When the glass plate flies from one side to the other, the tooth (3) on 

 its framework opens the key (2) in the primary circuit, and if the commutator be 

 in the position indicated, then the induced current will stimulate the nerve at a, 

 and a curve will be obtained on the glass plate. Rearrange the pendulum as before, 

 but turn the handle of the commutator, and allow the glass plate to fly again. 

 This time the induced current will stimulate the nerve at b, and a second con- 

 traction, a little later than the first one, will be obtained. Register the velocity of 

 the glass plate by means of a tuning-fork, and the curve obtained will be something 

 like fig. 413, although this curve was obtained on a cylinder travelling at a uniform 



Fig. 413. 

 1, curve obtained on stimulating a nerve (man) near the muscle ; 2, when the stimulus was 

 applied to the nerve at a distance from the muscle ; D, vibrations of a tuning-fork (250 

 per second). 



rate. The difference between the beginning of the a and b curves indicates the 

 time that the nerve-impulse took to travel from b to a. This time is measured by 

 the tuning-fork, and if the distance between the points a and b is known, then the 

 calculation is a simple one. Suppose the stretch of nerve between a and b to be 2 

 inches, and the time required by the impulse to travel from a to b to be T ^o~ second, 

 then we have the simple calculation 2 inches : 12 inches : : - l \ 1J " : -g^-", or 80 

 feet per second. In fig. 413 the experiment was made on man; the curve 1 was 

 obtained by stimulating the nerve near the muscle, and 2 when the nerve was 

 stimulated at a distance of 30 centimetres. The interval between the vertical lines 

 corresponds to To ^ second, i.e., the time required by the nerve-impulse to pass 

 along 30 centimetres of nerve, which is equal to a velocity of 30 metres (90 feet) 

 per second.] 



In man, v. Helmholtz and Baxt estimated the velocity of the impulse in the median nerve 

 by causing the muscles of the ball of the thumb to write off their contractions on a rapidly 

 revolving cylinder. [In this case the "pince myographique " of Marey may be used ( 708). 

 The ends of the pince are applied so as to embrace the ball of the thumb, so that when the 

 muscles contract, the increase in fhickness of the muscles expands the pince, which acts on a 

 Marey's tambour, by which the movement is transmitted to another tambour provided with a 

 writing-style, and inscribing its movements upon a rapidly moving surface, either rotatory 

 or swinging.] The nerve is stimulated at one time in the axilla and again at the wrist. Two 

 curves are obtained, which, of course, do not begin at the same time. The difference in time 

 between the beginning of the two curves is the time taken by the impulse to traverse the above- 



