42 VETERINARY PHYSIOLOGY 



muscles tends to bring the parts back to their normal position 

 when the muscles have ceased to contract. It must not, 

 however, be imagined that, in all movements of one set of 

 muscles, the antagonistic muscles are relaxed, although they 

 may be elongated. Often they are in a state of activity so as 

 to guide the movements which are being produced (see p. 58). 



Tonus of Muscle. The tense condition of resting muscle 

 between its points of origin and insertion is not merely due to 

 passive elasticity, but is in part caused by a continuous con- 

 traction kept up by the action of the nervous system. If the 

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

 flabby and lose their tense feeling. 



3. Heat Production. Muscle, 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. But the heat evolved 

 during rest of muscle is trivial. 



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 current flows 

 through the wire from copper to zinc. This is the Current of 

 Injury (p. 63). (Practical Physiology.) 



2. MUSCLE IN ACTION 



A. Skeletal Muscle 

 1. Methods of making Muscle Contract 



Skeletal muscle remains at rest indefinitely until stimulated 

 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 inter- 

 vention of nerves can it be directly stimulated ? 



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



