232 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



termined the rate in man to be about 30 m. per second in both the 

 sensory and motor nerves. 



Since Helmholtz's work other more accurate and detailed investiga- 

 tions of the rate of nerve transmission have been made, but his figures 

 still stand as approximately correct. DoUey and Cattell ('94) found it 

 in man to vary in individuals from 21 to 49 ra, per second. In the 

 average man it is probably about 34 m. ])er second under normal con- 

 ditions, reaching as high as 90 m. at high temperature and in excep- 

 tional cases. 



The rate varies in different animals, and directly with the complexity 

 of the organism. In lower animals it is much slower. In the medul- 

 lated electric nerve of the torpedo it is from 8 to 20 m. per second, and 

 in Malapterurus, 28 m. per second. Moreover, it is now known that the 

 rate varies for diflferent nei'ves in the same animal, and for the same 

 nerve from day to day, or with changes in the physical conditions, 

 temperature, etc. 



There is, too, a relation not generally recognized between the rate of 

 transmission of the nervous impulse and the degree of insulation of the 

 nerve-fibres. In those nerves which are insulated by a well-developed 

 medullary sheath, the rate is most rapid. Chauveau ('78) carried out a 

 series of observations on the horse to determine the relative rate of 

 transmission in the motor nerves supplying the voluntary muscles of 

 the larynx, and those supplying the involuntary muscles of the oesoph- 

 agus. He found that in the non-medullated nerves to the involuntary 

 muscles the rate of transmission is at most 8 m. per second, or nearly four 

 times as slow as that of the medullated nerves to the voluntary muscles. 



The rate of transmission in the non-medullated nerves of inverte- 

 brates is still slower. In the nerve for the claw muscles of the lobster, 

 Fredericq et Vandevelde ('79, '81) found the rate to vary with the 

 temperature from 6 to 12 m. per second. Jenkins and Carlson (-.OS) 

 found the rate of transmission in the ventral nerve-cord of worms to 

 vary from 5 to 9 cm. per second in nemerteans, to from 54 to 694 cm. 

 in annelids. According to Fuclis ('94), the rate of nerve-transmission 

 in the non-medullated nerves of the cephalopod Eledone moschatus 

 during the winter is about one metre per second. Boruttau ('97) has 

 found it, during the summer, to be from 3.5 to 5.5 metres in both 

 Eledone and Octopus. In the nerves of the mantle of Eledone, the rate 

 was found by Uexkull ('94) to be from only 0.4 to 1 m. per second, and 

 in Anodon, according to Fick ('63), it is as low as one centimetre per 

 second. 



