MUSCULAR IRRITABILITY. 



as they have fewer other things to do, they do that one better. 

 This contractility of the muscular fibres may be best con- 

 ceived by considering each to possess two natural shapes; 

 one, the state of rest, in which the fibres are long and nar- 

 row; and the other the state of activity, in which they are- 

 shorter and thicker: under certain conditions the fibres 

 tend to pass, with considerable force, from their resting to 

 their active form, and in so doing they move parts attached 

 to their tendons. When the state of activity passes off the 

 fibres suffer themselves to be passively extended again by 

 any force pulling upon them, and they so regain their rest- | 

 ing shape; and since in the living Body other parts are 

 * nearly invariably put upon the stretch when any given 

 muscle 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 ex- 

 tension, but once the contraction is over it suffers itself 

 readily to be pulled out to its resting form. 



Irritability. With that modification of the primitive 

 protoplasm of an amoeboid cell which gives rise to a mus- 

 cular fibre with its great contractility, there goes a loss of 

 other properties. Nearly all spontaneity disappears; muscles 

 are not automatic like native protoplasm or ciliated cells; 



1 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, 

 however, very small; muscle tissue is highly irritable, a>| 

 very little thing being sufficient to call forth a powerful 

 contraction. In the living Human Body the exciting force, 

 or stimulus, acting upon a muscle is almost invariably a 

 nervous impulse, a molecular movement transmitted along 

 the nerve-fibres attached to it, and upsetting the equili- 

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

 acts upon the muscles, and accordingly injury to the nerve 

 of a part, as the face or a limb, will cause 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 permanently idle. Although a 



