206 COMPARATIVE PHYSIOLOGY. 



plasmic activity is accompanied by chemical change, and that 

 some of these processes result in the formation of products 

 which are hurtful and are usually rapidly expelled. 



Muscular contraction is accompanied by chemical changes, 

 in which the formation of carbon dioxide, and some substance 

 that causes an acid reaction to take the place of an alkaline or 

 neutral one. Since free oxygen is not required for the act of 

 contraction, but is still used iip by a contracting muscle, it may 

 be assumed that the oxygen that plays a part in actual contrac- 

 tion is intra-molecular. 



Chemical changes are inseparable from the vital processes 

 of all protoplasm, and the phenomena of muscle show that 

 they are constantly in operation, but exalted during ordinary 

 contraction and that tetanic condition which precedes and 

 may end in coagulation of muscle plasma and the formation of 

 myosin. The latter is a result of the disorganization of muscle, 

 and has points of resemblance to the coagulation of the blood. 



The contraction of a muscle and the passage of a nervous 

 impulse are accompanied by electrical changes. Whether cur- 

 rents exist in uninjured muscle and nerve is a matter of contro- 

 versy. All physiologists agree that they exist in muscle (and 

 nerve) duiing functional activity. 



During the passage of a constant (polarizing) current from 

 a battery through a nerve, it undergoes a change in its irrita- 

 bility and shows a variation in the electro-motive force of the 

 ordinary nerve-current (electrotonus). This fact is of thera- 

 peutic importance. The electrical phenomena of nerve are 

 altogether more prominent than the chemical, the reverse of 

 which is true of muscle. The activity of a muscle (and nerve 

 probably) is accompanied by the generation of heat, an exalta- 

 tion of which takes place during muscular contraction. 



Rigor mortis causes an increase in temperature and the 

 chemical interchanges which accompany the other phenomena. 

 A muscle may also become rigid by passing into rigor caloris. 

 Living muscle is translucent, alkaline or neutral in reaction, 

 and elastic ; dead muscle, opaque, acid in reaction, and devoid 

 of elasticity, but firmer than living muscle, owing to coagula- 

 tion of the muscle-plasma. Dead nerve undergoes similar 

 changes. 



The elasticity of muscle is restricted but perfect within its 

 own limits. It difFers from that of inorganic bodies in that the 

 increments of extension are not directly proportional to the 



