92 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



different places. He found the average normal rate for man to be about 34 

 meters per second, a rate which is considerably quicker than that of our 

 fastest express trains, but a million times less than the rate at which an electric 

 current is transmitted along a wire. These determinations are still accepted 

 as approximately correct for human nerves, although they are found to vary 

 very considerably under different conditions, a high temperature and strong 

 irritation quickening the rate to 90 or more meters per second. Moreover, 

 considerable differences exist in nerves controlling different functions, even in 

 the same animal. Thus Chauveau gives the rate for the fibres of the vagus 

 nerve, which supply the rapidly contracting striated muscles of the larynx, as 

 66.7 meters per second ; and the rate for vagus fibres, controlling the slower 

 smooth muscles of the oesophagus, as 8.2 meters per second. The rate of 

 transmission in the uon-medullated nerves of invertebrates appears to be still 

 slower ; the nerve for the claw-muscles of the lobster conducts at a rate of 

 from 6 to 12 meters per second, according as the temperature is high or low 

 (Fredericq and Vandervelde). 



Rate in Sensory Nerves. We have no definite knowledge of the rate of 

 conduction in sensory nerves. The attempt has been made to measure.it, by 

 stimulating the sensory fibres of a nerve-trunk at two different points and 

 noting the difference in the time of the beginning of the resulting reflex acts ; 

 or, in experiments on men, the difference in the length of the reaction time 

 has been taken as an indication. By reaction time is meant the interval which 

 elapses between the moment that the irritant is applied and the signal which is 

 made by the subject as soon as he feels the sensation. Oehl found the mean 

 normal rate of conduction in the sensory nerves of men to be 36.6 meters per 

 second. 1 Dolley and Cattell, 2 by employing the reaction-time method, found 

 the rate for the sensory fibres of the median nerve of one of them to be 21.1 

 meters per second, and for the other 49.5 meters per second, while the posterior 

 tibial nerve gave rates, for one of them 31.1 meters, and for the other 64.9 

 meters. They attribute these wide variations in part to differences in the 

 effectiveness of the irritant at different parts of the skin, but chiefly to differ- 

 ences in the activity of the central nervous processes involved in the act. 



In spite of the great difficulty of getting definite measurements on men, 

 we may conclude from the work of these and other observers that the rate of 

 conduction in sensory fibres is about the same as in motor fibres ; in the case 

 of man about 35 meters per second. 



Influences which Alter the Rate and Strength of the Conduction Pro- 

 cess. (a) Effect of Death-processes. Normally, the rate of conduction in mus- 

 cle-fibres is so rapid that the whole muscle appears to contract at the same time ; 

 but there are certain conditions under which the transmission of the exciting 

 influence is very much slowed, or even altogether prevented, so that the stimu- 

 lation of a given part of the muscle results in a local swelling, or welt, limited 

 to the excited area. When a muscle is dying, the rate of conduction as well 



1 Oehl : Archives italiennes de Biologic, 1895, xxi., 3, p. 401. 



2 Psychological Review, New York and London, 1894, i. p. 159. 



