130 THE HUMAN BODY. 



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. A\ v3 now know that such is not the case, but that a 

 muscle-fibre is a collection of highly irritable material which 

 can have its equilibrium upset in a definite way, causing it to 

 change its shape, under the influence of certain slight disturb- 

 ing forces, one of which is a nervous impulse; and since in 

 the Body no other kind of stimulus usually reaches the mus- 

 cles, 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 con- 

 tractile and irritable, but not an automatic, tissue. 



A Simple Muscular Contraction. Most of the details con- 

 cerning 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 after re- 

 moval from the body of the animal, and so can be experi- 

 mented 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 animals to show 

 that in all essentials they agree with those of the frog or ter- 

 rapin. 



AVhen a single electric shock is sent through a muscle, the 

 nerves of which have been thrown out of action by curare, it 

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

 idly lengthens again. The whole series of phenomena from 

 the moment of stimulation until the muscle regains its rest- 

 ing form is known as a simple muscular contraction or a 

 "twitch" : it occupies in frog's muscle about one tenth of a 

 second. So brief a movement as this cannot be followed in 

 its details by direct observation, but it is possible to record it 

 and study its phases at leisure. This may be done by firmly 

 fixing the upper tendon of an isolated muscle, M, Fig. 62, 

 and attaching the other end at d to a lever, I, which can move 

 about the fulcrum/: the end cf the long arm of the lever 

 bears a point, p, which scratches on a smooth smoked surface, 

 S. Suppose the surface to be placed so that the writing point 

 of the lever is at ; if the muscle now contracts it will raise 

 the point of the lever, and a line ac will be drawn on the 



