2O ESSENTIALS OF PHYSIOLOGY. 



is warmed it clots, the proteins being converted into myosin, though the 

 myosinogen passes through a transition stage as soluble myosin, which 

 clots at 40 C. ; in the frog's muscle soluble myosin is present as such 

 in the living muscle. The coagulation of muscle plasma can be 

 prevented by the removal of calcium salts, but there is no evidence 

 that it is brought about by a ferment. 



Resting muscle is very extensible and can be stretched by applying 

 a weight to it ; it is also feebly but perfectly elastic, and returns 

 completely and rapidly to its original length when the weight is 

 removed. 



THE CONTRACTION OF VOLUNTARY MUSCLE. 



In response to a stimulus a living muscle alters its form, becoming 

 shorter and thicker ; this change of form constitutes muscular 

 contraction, and can be very easily studied in the muscles of the 

 frog. For this purpose the gastrocriemius muscle and the sciatic 

 nerve which supplies it are generally used, and are known as a muscle- 

 nerve preparation. The fibres in the gastrocnemius do not run 

 regularly from end to end of the muscle, and when it is desirable to use 

 a muscle the fibres of which run approximately parallel to one another, 

 the sartorius may be chosen. The muscle may be made to contract 

 either by a mechanical stimulus such as a pinch, or by a chemical 

 stimulus, for instance the application of acid or ammonia, or by an 

 electrical current. On account of the ease with which its strength can 

 be graduated, the electrical current is the most convenient of these 

 artificial stimuli, and it may be applied either as a constant current or 

 in the form of single or repeated induction shocks. The normal 

 stimulus to muscular contraction during life is an impulse passing from 

 the central nervous system along the nerve which ends in the muscle 

 fibres. In a muscle-nerve preparation, the muscle contracts either 

 when it is stimulated directly or when the stimulus is applied to the 

 nerve. 



It was formerly supposed that even when the stimulus was applied 

 to the muscle itself the latter did not respond directly to the stimulus, 

 but contracted because this acted upon the nerve fibres running within 

 the muscle. There is no doubt, however, that muscle can be excited 

 to contract independently of impulses reaching it along nerve fibres 

 (independent irritability of muscle). Curare paralyses the endings of 

 nerve fibres in muscle, and when it is injected into an animal stimula- 

 tion of a motor nerve has no effect upon the muscle, whereas direct 

 stimulation of the muscle causes it to contract. 



The changes which take place in a contracting muscle are (1) a 



