528 NERVOUS SYSTEM. 



each curve representing ^5- of a second. If a lever be attached to a muscle 

 and be so arranged as to indicate upon the paper, moving at the same rate, 

 the instant when contraction takes place, it is evident that the interval be- 

 tween two contractions produced by stimulating the nerve at different points 

 in its course may be accurately measured ; and if the length of the nerve be- 

 tween the two points of stimulation be known, the difference in time will 

 represent the rate of nervous conduction. In experiments upon frogs, the 

 leg is prepared by cutting away the muscles and bone of the thigh, leaving 

 the nerve attached. The lever is then applied to the muscles of the leg, and 

 the nerve is stimulated successively at two points, the distance between them 

 being measured. 



Employing the myograph of Marey, Baxt, in the laboratory of Helmholtz, 

 succeeded in measuring the rate of nervous conduction in the human sub- 

 ject. In these experiments, the swelling of the muscle during contraction 

 was limited by enclosing the arm in a plaster-mould, and the contraction 

 was observed through a small opening. By then exciting the contraction by 

 stimulating the radial nerve successively at different distances from the mus- 

 cle, the estimate was made. The rate in the human subject was thus esti- 

 mated at one hundred and eleven feet (33 -9 metres) per second. 



The method used in determining the rate of conduction in motor nerves 

 an estimation of the difference in time of the passage of a stimulus applied 

 to a nerve at two points situated at a known distance from each other has 

 been applied to the conduction of sensations. Hirsch made the first attempt 

 to solve this question, in 1861. He employed the delicate chronometric in- 

 struments used in astronomy and noted the difference in time between the 

 appreciation of an impression made upon a part of the body far removed 

 from the brain, as the toe, and an impression made upon the cheek. This 

 process admitted of a rough estimate of about one hundred and eleven feet 

 (33 - 9 metres) per second as the rate of sensory conduction. 



It is not necessary to describe fully the complicated apparatus by means 

 of which the most recent estimates of the rate of nervous conduction have 

 been made. The general results of the observations of Helmholtz, Marey, 

 Baxt, Schleske and of many others nearly correspond with the estimates just 

 given, and they show that the rate is about the same for motor and sensory 

 nerves. This rate is modified by various conditions. It is diminished in 

 the anelectrotonic and increased in the catelectrotonic condition of nerves. 

 In the frog Helmholtz observed that the rate was very much reduced by 

 cold, at 32 Fahr. (0 C.) being not more than one-tenth as rapid as at 60 

 or 70 Fahr. (15-5 or 21-11 C.). 



The rate of transmission of impulses and impressions through the spinal 

 cord has been investigated by calculating the distances between nerves as 

 they are given off at different points and measuring the time required for 

 the appreciation of certain impressions and the beginning of certain move- 

 ments (Burkhardt). While these observations are not absolutely exact, their 

 general results are of considerable physiological interest. According to Burk- 

 hardt, the rate of motor conduction in the cord is about one-third of the nor- 



