32 ELEMENTS OF PHYSIOLOGY 



officiate, teachers, and preachers of any ordinary community 

 of human beings. One man learns to do one kind of work 

 and can do this better than anything else. Often the 

 nature of his work can be judged by his appearance: so 

 it is with the cells. Still, the general structure of all 

 cells is essentially the same, just as all men are alike in 

 their plan of structure. Thus the tissues of the higher 

 animals beautifully exemplify the principle called 



50. Division of Labor. By this means a community of 

 men working together can accomplish vastly more than 

 the same number working separately. Each, learning to 

 do a few things well, can turn out perfect products of 

 skilled labor, and by a system of interchange can come to 

 possess the products of the skilled labor of others. With- 

 out this the human race could have made but very little 

 progress and would have remained in the state of savages, 

 who do not practice the division of labor to any great 

 extent. 



51. In the human body, the several groups of cells 

 called tissues have divided among themselves the various 

 physiological processes : the muscle cells are for move- 

 ment, nerve cells for feeling, gland cells for digestion, etc. 

 They are thus unlike the ameba, whose one cell carries on 

 all the functions, unaided by any other cell. Still, in the 

 body of a higher animal, each cell has many forms of 

 activity common to the ameba and all other living cells; 

 it requires food as much as if it lived alone, and its food 

 is digested and brought to it in a state ready for use ; it 

 requires oxygen, and the oxygen is separated by special 

 cells and brought to it purer than that found in air or 

 water. Cells all have at least sometime in their lives the 

 power to increase in size, and to reproduce themselves by 

 dividing just as the ameba does (Fig. 31). In this way, the 

 tissues grow, and thus they may be repaired if injured 

 or broken. 



