124 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



contractions of the different fibres, the combined action of which produces a 

 more or less regular continued contraction. Another view would be that con- 

 tracture might be produced under the influence of the changes caused by the 

 electric current, and a condition result similar to that which causes the pro- 

 longed contractions which are characteristic of poisoning with veratria, etc. 

 (see p. 128). 



(d) Normal Physiological Contractions. All normal physiological contrac- 

 tions of muscles are regarded as tetani. Even the shortest possible voluntary 

 or reflex movements are considered to be too long to be single contractions. 

 Inasmuch as we can artificially excite muscles to continuous contraction only 

 by means of a series of rapidly following stimuli, we find it hard to explain 

 continuous contractions on any other basis, and hence the view that the exci- 

 tation sent by the nerve-cells to muscles has always a rhythmic character, and 

 that the normal motor-nerve impulse is a discontinuous rather than continuous 

 form of excitation. The view is probably correct, but cannot be considered as 

 proved. The evidence in favor of it is as follows. 



Muscle-sounds, Tremors, etc. During voluntary muscular contractions the 

 muscle gives out a sound, which would imply that its finest particles were not 

 in a state of equilibrium, but vibrating. By delicate mechanisms it has been 

 possible to obtain records of voluntary and reflex contractions which showed 

 oscillations, although the contraction of the muscle appeared to the eye to be 

 continuous. If the surface of a muscle be exposed and be wet and glistening, 

 the light reflected from it during continued contractions is seen to flicker, as 

 if the surface were shaken by fine oscillations. The tired muscle passes from 

 apparently continuous contraction to one exhibiting tremors, and muscular 

 tremors are observed under a variety of pathological conditions. 



With these facts in mind, a number of observers have endeavored to dis- 

 cover the rate at which the muscle is normally stimulated. Experiments in 

 which muscles have been excited to incomplete tetanic contractions by induced 

 currents, interrupted at different rates, have shown that the muscle follows the 

 rate of excitation with a corresponding number of vibrations, and does not 

 show a rate of vibration peculiar to itself. Further, it has been ascertained 

 that the sound given out by a muscle excited to complete tetanus, i. e. an 

 apparently continuous contraction, corresponds to the rate at which it is ex- 

 cited. Apparently, any rate of oscillations detected in a muscle during normal 

 physiological excitation would be an indication of the rate of discharge of 

 impulses from the central nerve-cells. 



Wollaston was the first to observe that a muscle gives a low dull sound 

 when it is voluntarily contracted, and that this sound corresponds to a rate of 

 vibration of 36 to 40 per second. It may be heard with a stethoscope placed 

 over the contracting biceps muscle, for instance, or if, when all is still and the 

 ears are stopped, one vigorously contracts his masseter muscles. Helmholtz 

 placed vibrating reeds consisting of little strips of paper, etc., on the muscle, 

 and found that only those which had a rate of vibration of 18 to 20 per 

 second were thrown into oscillation when the muscle was voluntarily contracted. 



