THE WORK OF ORGANS AND CELLS 53 



12. Heat production by the working muscle. One other 

 point of similarity between the working muscle fiber and the 

 working steam engine should be pointed out; namely, that 

 both produce heat. It is a familiar fact that muscular activity 

 makes us feel warm. This is the direct result of the libera- 

 tion of heat by the oxidations within the working muscle 

 fiber. The same thing is true of the steam engine, the liber- 

 ated heat going in that case to warm the engine or passing 

 away in the gases which escape from the smokestack, steam 

 vents, etc. It is important that the student of physiology 

 bear clearly in mind this feature of muscular action, since 

 the active muscles not only supply power for work but also 

 the heat necessary to maintain the temperature of the body, 

 and no muscle can be thrown into contraction without liber- 

 ating a certain amount of heat. For a full discussion of this 

 matter see Chapter XII. 



13. The repair and maintenance of the cellular nu 



Thus far we have considered only those chemical activities 

 of gland and muscle cells which are directly concerned with 

 secretion and contraction or which prepare the cell for the 

 performance of these functions. This is only a part, how- 

 ever, of the work of living cells, for, like all machines, cells 

 may be injured by overwork or by accident, and their parts 

 (nucleus, cytoplasm, fibrils, etc.) must be kept in working 

 order. Just here the living mechanism differs from the life- 

 less engine, for the living mechanism is itself capable of 

 repairing damage to itself. The locomotive must be sent to 

 the shops and be repaired by work done upon it by other 

 machines; if the boiler rusts, it must be taken out and a 

 new one put in ; if the wheels wear unevenly, they must be 

 made true again by turning in a lathe or new ones must 

 be substituted ; when the grate burns out, a new one must be 

 put in its place. The living cell, on the other hand, itself 

 makes these repairs from certain constituents of the same 

 food out of which fuel and zymogen granules are made, and 



