THE TISSUES 45 



Tonus of Muscle. The tense condition of resting 

 between its points of origin and insertion is not merely due 

 to passive elasticity, but is in part caused by a continuous 

 contraction, kepj^up by the action of the nervous system. Jf^ 

 the nerve to a group of muscles be cut, the muscles become 

 soft aridHabby, and lose their tense feeling. 



3. Heat Production. Muscles, like all other living proto- 

 plasm, is in a state of continued chemical change, constantly 

 undergoing decomposition and reconstruction. As a result 

 of this chemical change, heat is evolved. This is a matter 

 of considerable importance, as it explains how, even when the 

 muscles are at rest, the body is still kept at a comparatively 

 high temperature. 



4. Electrical Conditions. Muscle when at rest is iso- 

 electric, but if one part is injured, it acts to the rest like the 

 zinc plate in a galvanic battery becomes electro-positive ; and 

 hence if a wire passes from the injured to the uninjured part 

 round a galvanometer, a current is found to flow along the 

 wire from the uninjured to the injured part just as, when the 

 zinc and copper plates in a galvanic cell are connected, a cur- 

 rent flows through the wire from copper to zinc. This is the 

 Current of Injury (p. 65). (Practical Physiology, Chap. 

 VIII.) 



2. METHODS OF MAKING MUSCLE CONTRACT. 



Skeletal muscle remains at rest indefinitely until stimu- 

 lated to contract, usually by changes in the nerves. We 

 desire to contract our biceps : certain changes occur in our 

 brain, these set up changes in the nerves passing to the 

 biceps and the muscle contracts. 



Can skeletal muscle be made to contract without the 

 intervention of nerves can it be directly stimulated ? 



To answer this, some means of throwing the nerves out of 

 action must be had recourse to. If curare, a South American 

 arrow poison, be injected into an animal e.g. into a frog, the 

 brain of which has been destroyed it soon loses the power 

 of moving. When the nerve to a muscle is stimulated, the 

 muscle no longer' contracts. But if th^ TmiP fi1ft ^ Q ^ir^tly 

 stimulated by any of the various agents to be__afterwards 

 m entidneH, it at once, contracts. ~~ 



