CH. I.] 



INTRODUCTORY. 



5 



Suppose 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 connec- 

 tive tissues which are not so eminently 

 living as the rest, the amount of cement 

 or intercellular 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 consists of a 

 single little mass of living material, 

 a single cell. As development pro- 

 gresses it becomes an adherent mass of 

 cells. In the later stages of develop- 

 ment various tissues become differen- 

 tiated 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 connective 

 tissues, there is a preponderating amount of intercellular mate- 

 rial which may become permeated with fibres, or be the seat 

 of the deposition of calcareous salts, as in bone. Instances 

 of chemical changes in the cells themselves are seen on the 

 surface of the body where the superficial layers of the epidermis 

 become horny ; in the mucous glands, where they become filled 



Space con- 

 taining 

 liquid. 



Protoplasm. 



Nucleus. 



Cull-wall. 



Fig. i. Vegetable cells. 



