Introduction. 3 



are called tissues. A tissue is a collection of similar cells 

 devoted to the same work ; or, in other words, a tissue 

 is a set of cells having the same structure and the same 

 function. Thus we have, muscular tissue, nervous tissue, 

 bony tissue, etc. 



Cells. The whole body is made up of small parts, called 

 cells, which are to be compared to the bricks in a house. 

 These cells are of various shapes in the different tissues. 

 The living material of the body is called protoplasm. It is 

 a jelly-like substance resembling the 

 white of an egg, though often pre- 

 senting a dotted appearance. A cell, 

 in its simplest form, is merely a dis- 

 tinct particle of protoplasm. Each Nucleus 

 cell usually has, however, a more Fig. i. Epithelial ceils from 



the Inside of the Cheek. 



dense central part, called the nucleus. 



The great majority of cells have a distinct covering or 

 cell-wall. A grape or cherry serves very well to illustrate 

 a cell. The skin represents the cell-wall, the pulp corre- 

 sponds to the protoplasm, and the seed to the nucleus. 

 (See Fig. 39, Cells of the Epidermis.) 



Division of Labor in a Community. We are all aware 

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

 each person learns to do one thing well, all can work 

 together economically for the common good, time is saved, 

 and cheaper and better goods are produced. 



Division of Labor in the Body. In the body there is a 

 division of labor similar to that in a community. In the 

 first place each organ has its special work, and the various 

 organs act helpfully together, each working for all the rest 

 and worked for by them. 



