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A MANUAL OF PHYSIOLOGY 



a change of potential, which can be measured by determining what 

 electromotive force is just required to balance it, and which may 

 actually exceed that of the current of rest. Thus, Sanderson and 

 Gotch obtained an average of 0*08 of a Daniell cell (the electromotive 

 force of the Daniell would be about a volt) as the electromotive force 

 of the action current due to a single indirect excitation of a vigorous 

 frog's gastrocnemius, and about 0*04 Daniell as that of the current of 

 rest. The electromotive force of the current of rest in rabbit's nerve 

 was found by du Bois-Reymond to be 0*026 ; Gotch and Horsley 

 found the average for the cat o - oi, and for the monkey only 0-005. 



That the fusion of the successive variations of a tetanized muscle y 

 as seen with the galvanometer, is only apparent has been shown by 

 means of the capillary electrometer. Even with a frequency of 

 stimulation far beyond what is necessary for complete tetanus, each 

 stimulus is answered by a movement of the meniscus (Figs. 203, 204). 

 In nerve, also, as has been recently demonstrated, each of two succes- 

 sive stimuli causes iis appropriate electrical change when they are 



separated by an interval longer 

 than a certain small fraction 

 of a second. The precise 

 interval at which the second 

 stimulus ceases to be effec- 

 tive depends on the tempera- 

 ture of the nerve, being 

 markedly increased by cold 

 (Gotch and Burch). 



Before Burdon Sanderson in- 

 troduced the capillary electro- 

 meter for the study of the 

 electrical phenomena of living 

 tissues, and Burch perfected a 

 method for the measurement 

 of the curves, the differential 

 rheofcnie, originally constructed 

 by Bernstein, was the most 

 valuable instrument we pos- 

 sessed for experiments on the 

 time relations of these pheno- 

 mena. By its aid, for instance, 



it was shown that me rate of propagation of the electrical change in. 

 muscle is the same as that of the mechanical change, and in nerve 

 the same as that of the nervous impulse. Quite recent observations. 

 on muscle made by the more accurate method of the capillary electro- 

 meter have confirmed those earlier results, the velocity of propaga- 

 tion of the diphasic variation along a fresh sartorius at 14 C. being 

 in one experiment 2-8 metres, in another at 18 C., 3-5 metres (Sander- 

 son). (Seep. 575.) 



The differential rheotome consists essentially of a stationary metal 

 ring, the whole or part of which is graduated, and of a portion which 

 can be made to revolve at a known rate. The latter carries two- 

 contacts : a, an obliquely-placed platinum wire which touches at 



FIG. 205. DIAGRAM OF DIFFERENTIAL 

 RHEOTOME. 



