204 PHYSIOLOGY CHAP. 



sensory nerves of man, but the resulting data are discordant and 

 unconvincing. The method consists in determining the reaction- 

 time to tactile sensations sent in at two points on the skin of the 

 arm, at different distances from the centres. As soon as the 

 subject perceived the sensation he pressed a button which marks 

 the moment of' reaction upon a revolving cylinder. It was 

 formerly assumed that the reaction-time for two approximately 

 identical sensations, evoked at two points of the skin at different 

 distances from the centres, differed only in proportion to the 

 different length of nerve through which the impulse has to 

 pass before reaching the centres. The discrepancy of results 

 obtained by various experinaentors, which ranges from 26 to more 

 than 100 m. per second, however, shows that the lost time at 

 the centres, where the afferent excitation is transformed into a 

 motor impulse passing down the efferent nerve, must vary con- 

 siderably, according to the site of stimulation, the state of fatigue 

 and degree of attention of the subject, with other less appreciable 

 conditions. It is probable, judging from other experiments to be 

 described later, that the rate of conductivity is the same in 

 sensory nerves as in motor. 



Considerable differences in rate of conductivity are found in 

 the lower animals, and even in different kinds of nerve in the 

 same animal. Fredericq and van de Velde found for the nerves 

 of the claw of the sea-crab a velocity varying from 6 to 12 metres 

 per second when the temperature varied between 19 and 20 C. 

 v. Uexkiill found variations of 0*4-1 m. per second for the nerves of 

 the mantle of Cephalopoda ; Chauveau found in the vagus fibres 

 that innervate the smooth muscle cells of the oesophagus of large 

 mammals a velocity averaging 8*2 m. per second, while in the 

 vagus fibres that innervate the striated muscles of the larynx it 

 averaged 66*7 m. per second. According to Chauveau* this rate 

 is not uniform for all parts of the nerve, but falls in the parts 

 nearest the muscle. 



From some of Gotch's work, again, it seems highly probable 

 that the rate of transmission of the motor impulse is much lower 

 in the terminal branches of the nerve than it is in the principal 

 trunks. In experiments on the electrical organ of Malapterurus, 

 in which a gigantic nerve-fibre terminates in a very free arborisa- 

 tion, he measured the difference of latent period obtained on 

 exciting the organ directly or through the nerve, and found that a 

 non-negligible fraction of time (0'003-0'005 per second) was lost 

 in the transmission of the impulse along the twigs of the nerve. 

 On repeating the experiments of Babuchin on the same nerve 

 (see p. 199) to see if the retrograde centripetal conduction of 

 the impulse proceeded at the same rate as the centrifugal, his 

 results led him to conclude that the velocity of conduction did not 

 alter with the ascending or descending direction of the impulse. 



