INTRODUCTORY 



[CH. I. 



organ, we see that it consists of various textures, or, as they are 

 called, elementary tissues. Just as one's garments are made up of 

 textures (cloth, lining, buttons, etc.), so each organ is composed of 

 corresponding tissues. The elementary tissues come under the 

 following four headings: 



1. Epithelial tissues. 3. Muscular tissues. 



2. Connective tissues. 4. Nervous tissues. 



Each of these is again divisible into subgroups. 



If we continue our anatomical analysis still further, we find 

 that the individual tissues are built up of 

 structures which require the microscope for 

 their accurate study. Just as the textures 

 of a garment are made up of threads of 

 various kinds, so also in many of the animal 

 tissues we find threads or fibres, as they are 

 called. But more important than the threads 

 are little masses of living material. Just as 

 the wall of a house is made up of bricks 

 united by cement, so the body walls are built 

 of extremely minute living bricks, united 

 together by different amounts of cementing 

 material. Each one of these living units is 

 called a cell. 



Some of the tissues already enumerated 

 consist of cells with only very little cement 

 material binding them together; this, for 

 instance, is seen in the epithelial tissues; 



but in other tissues, particularly the connective tissues, which are 

 not so eminently living as the rest, the amount of cement or inter- 

 cellular material is much greater, and in this it is that the fibres 

 are developed that confer the necessary strength upon these binding 

 tissues. 



If, instead of going to the adult animal, we look at the animal 

 in its earliest stage of development, the ovum, we find that it con- 

 sists of a single little mass of living material, a single cell. As 

 development progresses it becomes an adherent mass of cells. In the 

 later stages of development various tissues become differentiated 

 from each other by the cells becoming grouped in different ways, by 

 alterations in the shape of the cells, by deposition of intercellular 

 matter between the cells, and by chemical changes in the living 

 matter of the cells themselves. Thus in some situations the cells are 

 grouped into the various epithelial linings ; in others the cells become 

 elongated and form muscular fibres; and in others, as in the con- 

 nective tissues, there is a preponderating amount of intercellular 



Space con- 

 taining 

 liquid. 



Protoplasm. 



' ' Nucleus. 



Cell-wall. 



Fm. 1. Vegetable cells. 



