4 Physiology. 



The general structure of all the cells is about the same, 

 yet they differ enough for us to tell them apart. They 

 differ more in their work than in their appearance. Each 

 has some one kind of work that it can do well, and to 

 which it devotes itself. The nerve cells receive impressions 

 from the outer world, carry nerve currents, and control the 

 various actions of the body. The muscle cells have as 

 their work the production of motion. 



The Life of Cells. Each cell must take food for itself 

 and grow. Each has a birth, life, and death, as each indi- 

 vidual in a community of men; and as the community 

 continues, while the individual members are constantly 

 changing, so, in the body, while the form remains about 

 the same from year to year (in the adult), the cells are 

 continually changing, some dying, and others taking their 

 places. Thus it is seen that though the cells are packed 

 closely together and though they work in groups, each cell 

 leads, in one sense, an independent life. Like the indi- 

 vidual in the community, each lives for itself, yet all work 

 together for the common good. 



