MUSCULAR IRRITABILITY. 131 



all their nerve-fibres have been poisoned. The experiment 

 also shows that the contractility of a muscle is a property 

 belonging to itself, and that its contracting force is not 

 something derived from the nerves attached to it. The 

 nerve stimulus simply acts like the electric shock or sudden 

 blow and arouses the muscle to manifest a property which 

 it already possesses. The older physiologists seeing that 

 muscular paralysis followed when the nervous connection 

 between a muscle and the brain was interrupted, concluded 

 that the nerves gave the muscles the power of contracting. 

 They believed that in the brain there was a great store of 

 a mysterious thing called vital spirits, and that some of 

 this was ejected from the brain along the nerve to the 

 muscle, when the latter was to be set at work, and gave it 

 its working power. We now know that such is not the 

 case, but that a muscle fibre is a collection of highly irrita- 

 ble material which can have its equilibrium upset in a 

 definite way, causing it to change its shape, under the influ- 

 ence of slight disturbing forces, one of which is a ner- 

 vous impulse ; and since in the Body no other kind of 

 stimulus usually reaches the muscles, they remain at rest 

 when their nervous connections are severed. But the 

 muscles paralyzed in this way can still, in the living Body, 

 be made to contract by sending electrical shocks through 

 them. Physiologically, then, muscle is a contractile and I 

 irritable, but not automatic tissue. / 



A Simple Muscular Contraction. Most of the details r 

 concerning the physiological properties of muscles have \ 

 been studied on those of cold-blooded animals. A frog's 

 \ muscle will retain all its living properties for some time 

 j after removal from the body of the animal, and so can be 

 experimented on with ease, while the muscles of a rabbit 

 or cat soon die under those circumstances. Enough has, 

 however, been observed on the muscles of the higher ani- 

 mals to show that in all essentials they agree with those of 

 the frog or terrapin. 



AVhen a single electric shock is sent through a muscle it 

 rapidly shortens and then, if a weight be hanging on it 

 rapidly lengthens again. The whole series of phenomena 



