140 CELLS OF THE VERTEBRATE BODY 



tribution of the chromatin of a cell to its two daughter cells. In 

 particular instances, variations in certain features may occur, but 

 the essential significance of mitosis is the same throughout the 

 animal kingdom. 



Histology 



Tissues. — In the preceding section a so-called typical cell has 

 been described. If the body of a vertebrate is examined micro- 

 scopically, it will be found that no cell conforms to the diagram of a 

 typical cell. In other words, cells differ among themselves 

 although they all contain certain features in common. Cells 

 differ in respect to size, shape, position in the body, and also 

 in the functions that they are especially fitted to perform. Cells 

 that are similar in structure and function make up groups known 

 as tissues. Tissues, then, are groups of cells specialized in the 

 same way for the performance of the same function, and are classi- 

 fied on the basis of the structure and function. There are four 

 principal classes of tissues: epithelial, sustentative, contractile, and 

 nervous. 



Epithelial Tissue. — The cells of epithelial tissues are com- 

 pactly placed with but a small amount of intercellular material, 

 and function for the covering and protection of body surfaces, both 

 internal and external, and in secretion and excretion. According 

 to the predominating form of cells, this class is subdivided into 

 squamous and columnar epithelium, each of which is again divided 

 into simple or stratified, depending upon whether it exists in single 

 or multiple layers. The cells of simple squamous epithelium, when 

 viewed from the surface, resemble tiling-blocks, and on edge they 

 are very thin (Fig. 77 A) . Such epithelium, sometimes called endo- 

 thelium, is found lining the calome, that is, forming the peritoneum ; 

 it also forms the lining of blood vessels (Fig. 77 B). In stratified 

 squamous epithelium only the outermost layers are typically flat- 

 tened cells, while the cellj of the deeper layers are progressively 

 more cuboidal (Fig. 77 C). Since blood vessels do not penetrate 

 epithelial layers, it is only the cells of the deeper layers that receive 

 abundant nourishment and consequently divide and replenish the 

 outer layers, which die and are cast off. Stratified squamous 

 epithelium is found in the outer layer of the skin and in the lining 

 of the nasal and mouth cavities of many vertebrates. In simple 

 columnar epithehum, such as that lining the digestive tract, the 



