SIGNS OF ACTIVITY IN MUSCLE AND NERVE 431 



just as tetanus is, by action currents which signify a discontinuous excitation 

 (Loven). But it is worthy of note that the rhythm of these action currents 

 in voluntary contractions, and others produced under the influence of the 

 central nervous system, is only about half as rapid as the frequency of stimu- 

 lation necessary to produce a complete tetanus. And yet the voluntary con- 

 traction as ordinarily recorded is quite continuous. This must be due to the 

 fact that the single impulses sent out to the muscles from the central organs 

 to produce a voluntary contraction last longer than the ordinary instantaneous 

 stimuli (Loven), and that the separate twitches are therefore more readily 

 fused. We know, indeed, that a " time stimulus " (page 423) is longer drawn 

 out than a momentary stimulus and that it is therefore better adapted to 

 produce summation with a low frequency of stimulation. 



The trembling of the muscles which accompanies a strained effort to over- 

 come some great resistance or an attempt to hold a muscle contracted voluntarily 

 to its utmost, are generally regarded as expressions of the individual impulses 

 discharged from the central nervous system. The regulation of the innervating 

 mechanisms would seem in these cases to be disturbed in some way so as to 

 affect the fusion of the separate contractions. It has been shown that the num- 

 ber of such oscillations per second varies in man from seven or eight to twelve 

 or thirteen (Loven, v. Kries, Schafer). The greatest muscular efforts are made, 

 it appears, with a frequency of ten to twelve impulses per second. 



4. SIGNS OF ACTIVITY IN MUSCLE AND NERVE 



A. ELECTRICAL PHENOMENA 



1. Action Current. The general law of the electrical variation known 

 as the action current, which makes its appearance when nerve or muscle is 

 active, has already been given on page 48. In view of its great importance 



B uit 



FIG. 168. Schema illustrating a rheotome experiment. 



for the general physiology of muscles and nerves, however, we must discuss 

 it here somewhat more in detail. 



In order to study time relations of the action current, one can use either 

 the capillary electrometer whose excursions can be recorded by the photographic 

 method, or the repeating rheotome of Bernstein. 



Suppose we have an electrical variation of the form represented in Fig. 168. 

 The galvanometer is too slow to reproduce this form correctly. But if we 

 arrange the experiment so that a definite portion of each variation of the cur- 

 rent e. g., that included between a^ and Z^ in Fig. 168 affects the galvanometer, 

 and this is repeated many times, from the excursion of the galvanometer we can 

 learn the extent of the electrical variation during this portion. If now we can 

 determine in the same way the excursion of the galvanometer for the other 



