MUSCLE 



217 



first. Evidently a refractory period follows stimulation so that 

 the muscle refuses to respond again until it has nearly finished 

 relaxing. It is almost impossible to produce a tetanus (p. 2 28). 



7. Propagation of Contraction. — When a contraction is set 

 up in any part of a sheet of visceral muscle it passes over the 

 rest as a wave. The importance of this will be considered later. 



The physiology of Heart Muscle, which closely resembles 

 visceral muscle, will be dealt with when describing the action 

 of the heart (p. 423). 



B. SKELETAL MUSCLE. 

 1. Direct Stimulation of Muscle. 



Skeletal muscle is directly under the control of the 

 central nervous system. It remains at rest indefinitely until 



Fig. 107. — Curare Experiment, to show sciatic nerves exposed to curare, but 

 nerve endings protected on the left side ; while on the right side the 

 curare is allowed to reach the nerve endings in the muscle. 



stimulated to contract, usually by changes in the nerves, 

 produced by changes in the central nervous system. 



Can skeletal muscle be made to contract without the 

 intervention of nerves — can it be directly stimulated ? 



(1) To answer this, some means of throwing the nerves 

 out of action may be resorted to. If curare, a South 

 American arrow poison, be injected into a frog, the brain of 

 which has been destroyed, it soon loses the power of moving. 

 When the nerve to a muscle is stimulated, the muscle no 

 longer contracts. But if the muscle be directly stimulated, 

 in any of the various ways to be afterwards mentioned, it 

 at once contracts. 



It might be urged that the curare leaves unpoisoned the 

 endings of the nerve in the muscle, and that it is by the 

 stimulation of these that the muscle is made to contract. 

 But that these are poisoned is shown by the fact that, if the 

 artery to the leg be tied, just as it enters the muscle, so that 



