90 THE HUMAN BODY 



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-fiber is a 

 collection of highly irritable material which can have its equilib- 

 rium upset in a definite way, causing it to change its shape, under 

 the influence of certain slight disturbing forces, one of which is a 

 nervous 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 elec- 

 trical shocks through them. Physiologically, then, muscle is a con- 

 tractile and irritable tissue. 



A Simple Muscular Contraction. Most of the details 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 after removal from the body of the ani- 

 mal, 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 animals 

 to show that in all essentials they agree with those of the frog or 

 terrapin. 



When 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, rapidly lengthens 

 again. The whole series of phenomena from the moment of stimu- 

 lation until the muscle regains its resting 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. 50, 

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

 the fulcrum /: the end of 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 a; 

 if the muscle now contracts it will raise the point of the lever, and a 

 line ac will be drawn on the smoked surface, its vertical height, cm, 

 being dependent, first, on the extent of the shortening of the mus- 

 cle, and second, on the proportion between the long and short 

 arms of the lever: the longer fp is as compared with/d, the more 



