186 COMPARATIVE PHYSIOLOGY. 



the contraction begins, which,- as shown by the time marking, 



occupies in this case — , or about ^V of a second. In the tracing 



the upward curve indicates that the contraction is at first rela- 

 tively slow, then more rapid, and again slower, till a brief sta- 



a h 



Fig. 174.— Muscle-curve obtained by the peTidulum myograph (Foster). Read from 

 left to right. The latent period is indicated by the space between a and 6, the 

 length of which is measured by the waves of a tuning-fork, making one hundred 

 and eighty double vibrations in a second; and in like manner the duration of the 

 other phases of the contraction may be estimated. 



tionary period is reached, when the muscle gradually but rap- 

 idly returns to its previous condition, passing through the same 

 phases as during contraction proper. In other words, there is 

 a period of rising and of falling energy, or of contraction and 

 relaxation. 4. A period during which invisible changes, as 

 will be explained later, are going on, answering to those in the 

 nerve that cause the molecular commotion in muscle which 

 precedes the visible contraction — the latent period, or the period 

 of latent stimulation. 



The facts may be briefly stated as follows : The stimulation 

 of a muscle either directly or through its nerve causes contrac- 

 tion, followed by relaxation, both of which are preceded by a 

 latent period, during which no visible but highly important 

 molecular changes are taking place. The whole change of events 

 is of the briefest duration, and is termed a muscle contraction. 

 The tracing shows that the latent period occupied rather more 

 than T^ second, the period of contraction proper about j^ttj and 

 of relaxation ^^t second, so that the whole is usually begun and 

 ended within jV second ; yet, as will be learned later, many 

 chemical and electrical phenomena, the concomitants of vital 

 change, are to be observed. 



In the case just considered it was assumed that the muscle 



