128 THE HUMAN BODY. 



cles move parts attached to their tendons. AVhen the state of 

 activity has passed off the fibres suffer themselves to be ex- 

 tended again by any force pulling upon them, and so regain 

 their resting shape; and since in the living Body almost in- 

 variably other parts are put upon the stretch when any mus- 

 cle contracts, these by their elasticity serve to pull the latter 

 back again to its primitive shape.' No muscular fibre is 

 known to have the power of actively expanding after it has 

 contracted: in the active state it forcibly resists extension, but 

 once the contraction is completely over, it suffers 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 fibre with its great contractility, there goes a loss 

 of other properties. Nearly all spontaneity disappears; mus- 

 cles 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 muscular fibre is, how- 

 ever, 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 stim- 

 ulus, acting upon a muscle is almost invariably a nervous 

 impulse, a molecular movement transmitted along the nerve- 

 fibre attached to it, and upsetting the molecular equilibrium 

 of the muscle. It is through the nerves that the will acts 

 upon the muscle-fibre, 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 

 muscular 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 elec- 

 tric shock be sent through it, or a drop of glycerin or of solu- 

 tion 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 



