iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 205 



The influence of temperature on rate of conduction in nerve is 

 very apparent. Helmholtz, experimenting with the motor nerves I 

 of frog, found that their conductivity diminished considerably 

 on cooling, and increased on warming to 25 C. Gotch and 

 Macdonald made a careful research, exciting the same nerve at 

 regular intervals with minimal or nearly minimal stimuli. They 

 found that on cooling the nerve to 5 C. the muscular response 

 diminished or disappeared, while on warming it to 35 C. it was 

 increased and became maximal. So that cooling diminishes not 

 only the velocity of conduction, but the intensity of the effect trans- 

 mitted by the nerve as well ; heating produces the opposite effect. 



Helmholtz and Baxt further observed that both the rate of 

 conduction and the intensity of the effect transmitted vary 

 with alterations in the strength of the stimulus. This result, 

 obtained on the brachial nerve of man, was confirmed by 

 Vintschgau for the motor nerves of frog, and by Fick for the non- 

 medullated nerves of Anodonta. It was, however, always disputed 

 by Rosenthal and Lautenbach, and it is in any case doubtful 

 whether it applies to mechanical and chemical excitation as well 

 as to electrical stimuli. It should further be noted that the 

 shortened latent period obtained on stimulating the nerve with a 

 stronger induction current may be apparent only, which is due 

 to the fact that in this case the current spreads further and 

 stimulates points of the nerve which lie nearer to the muscle. 



We shall later discuss the alterations in the conductivity of 

 the nerve caused by electrotonus. 



Fr. W. Frohlich (1904), in studying the oxygen demand, and 

 the effects of narcosis on the frog's sciatic (infra], showed by the 

 myographic method that the rate of transmission of the nervous 

 impulse undergoes a local diminution during asphyxia and 

 narcosis in the part of the nerve affected, and that this became 

 more marked in proportion to the length of nerve involved. This 

 delay in conduction is perceptible even in a state of narcosis or 

 asphyxia in which conductivity seems by other methods to be 

 unaltered. 



According to Ch. Richet the experimental results arrived at 

 by the various authors as to the velocity of transmission of the 

 excitation or active state of the nerve may be summarised as 

 follows : 



(r/,) In the frog the mean velocity of the nervous vibration (as 

 he terms the active or excited state of the nerve) is from 20 to 

 26 m. per second. 



(&) In warm-blooded animals this velocity is 30-34 m. per 

 second. 



(c) It varies with a number of factors, particularly with the 

 temperature. 



It is not identical in every part of the nerve. 



