88 THE HUMAN BODY 



has passed off the fibers suffer themselves to be extended again by 

 any force pulling upon them, and so regain their resting shape; 

 and since in the living Body almost invariably other parts are put 

 upon the stretch when any muscle contracts, these by their 

 elasticity serve to pull the latter back again to its primitive shape. 

 No muscular fiber is known to have the power of actively expand- 

 ing after it has contracted: in the active state it forcibly resists 

 extension, but once the contraction is completely over, it surfers 

 itself readily to be pulled back to its resting form. The contracted 

 state lasts always longer, however, than the mere time occupied 

 by the muscle in shortening: as will be seen later, the full state of 

 contraction is gradually attained and gradually disappears. 



Irritability. With that modification of the primitive protoplasm 

 of an amoeboid embryonic cell which gives rise to a muscular fiber 

 with its great contractility, there goes a loss of other properties. 

 Nearly all spontaneity disappears; muscles are not automatic like 

 primitive protoplasm or ciliated cells; except under certain very 

 special conditions they remain at rest unless excited from without. 

 The amount of external change required to excite the living mus- 

 cular fiber is, however, very small; muscle tissue is highly irritable, 

 a very little thing being sufficient to call forth a powerful contrac- 

 tion. In the living Human Body the exciting force, or stimulus, 

 acting upon a muscle is almost invariably a nervous impulse, trans- 

 mitted along the nerve-fiber attached to it, and upsetting the 

 molecular equilibrium of the muscle. It is through the nerves that 

 the will acts upon the muscle-fiber, and accordingly injury to the 

 nerves of a part, as the face or a limb, causes paralysis of its 

 muscles. They may still be there, intact and quite ready to work, 

 but there are no means of sending commands to them, and so they 

 remain idle. 



Although a nervous impulse is the natural physiological muscu- 

 lar stimulus it is not the only one known. If a muscle be exposed 

 in a living animal and a slight but sudden tap be given to it, or a 

 hot bar be suddenly brought near it, or an electric shock be sent 

 through it, or a drop of glycerin or of solution of ammonia be placed 

 on it, it will contract; so that in addition to the natural nervous 

 stimulus, muscles are irritable under the influence of mechanical, 

 thermal, electrical, and chemical stimuli. One condition of the 

 efficacy of each of them is that it shall act with some suddenness; 



