54 THE HUMAN MECHANISM 



it does this by other chemical activities than those we have 

 described and about which we possess only fragmentary 

 knowledge at present. In picturing to ourselves the activi- 

 ties of these living mechanisms we must include all these 

 chemical processes, those of maintenance and repair as well 

 as those concerned immediately with the performance by 

 each cell of its own special functions, such as secretion by 

 a gland and contraction by a muscle. 



14. Recapitulation. We have traced the character of the 

 work done in the case of the gland and the muscle and have 

 found that it is fundamentally the work of the cells of which 

 the organs are composed. The cells of other organs are simi- 

 larly constructed to do other kinds of work, and the character 

 of their chemical changes and of the mechanisms for utilizing 

 power varies accordingly ; all, however, showing the same 

 fundamental plan of working engines. The body is a com- 

 munity of groups of cells of different kinds, each kind doing 

 some work more or less peculiar to itself. In addition to the 

 two groups (gland and muscle cells) which we have studied, 

 there are nerve cells in the brain, spinal cord, and elsewhere ; 

 cells which make blood corpuscles; cells which keep in repair 

 the connective tissues (bone, gristle, tendon, and ligament) ; 

 and many more, such as cells which manufacture or them- 

 selves form the lining of free surfaces, like the skin, the ali- 

 mentary tract, the air passages, etc. The sum total or net 

 result of the activities of these and other cells makes up the 

 work of the body as a whole. The work of the body the 

 human organism, the human mechanism is thus the outcome 

 or resultant of the work of its different component cells. 



