4 I'HYSIOLOGY. 



These "primary building materials" of the body we call 

 tissues. A tissue may be defined as an aggregation of 

 similar cells devoted to a common work. 



Cells. The whole body is made up of small parts 

 called cells, comparable to the bricks in a house. These 

 cells are of various shapes in the different tissues. 



In the more active tissues the cells are alive, and each 

 cell may be compared to the ameba, a little mass of living 

 jelly-like substance called protoplasm. The ameba is a 

 protozoan often found in the slime at bottom of stagnant 

 water. Within this is a small, rounded part called the 

 nucleus. Most of the cells of the body differ from 

 the ameba in having a distinct 

 outer covering or cell wall. A grape 

 serves very well to show what a cell 

 is like. The whole body is built up 

 Nucleus. f cells, few of them large enough 



Fig. i. Epithelial Ceils from to be seen by the naked eye. 

 Although the cells are closely 



packed together, each cell leads, in one sense, an inde- 

 pendent life. But all work together to maintain the life 

 of the body. The cell is like the individual in a com- 

 munity. Each lives primarily for itself, yet all work 

 together for the good of the whole. 



Epithelial Cells from the Inside of the Cheek. With the blade of 

 a very dull knife, or the handle of a scalpel, gently scrape the inside of 

 the cheek. Place a little of the white scraping on a slide in a drop 

 of water, cover with a cover slip, and examine under a quarter-inch 

 objective. Many cells will be seen, some of them showing nuclei. 

 Compare these cells with the accompanying figure. 



The Physiological Division of Labor. We are aware 

 of the advantages of division of labor in a community. If 



