38 



CHAPTER III. 

 ELEMENTARY HISTOLOGY. 



When examined under the microscope, all the different tissues 

 and organs of the body are found to consist of elementary bodies 

 called cells and of an intercellular substance, connecting the 

 several cells together ; in much the same way as a wall is built 

 of bricks cemented together with mortar. These cells, of which a 

 white blood corpuscle is a typical example, vary much in shape, 

 size, and structure in different tissues, but are to be considered as 

 fundamentally equivalent to one another. The intercellular 

 substance varies very much in quantity; it may be almost 

 absent, so that the several cells are practically in contact with 

 one another ; or it may be so abundant as to separate them 

 widely : it is to be viewed as formed by the cells, and, therefore, 

 as secondar}^ in importance to these. 



When drawing histological preparations, it is well to look out 

 for, and draw, a few red hlood corpuscles, to the same scale as the 

 rest of the drawing. The hlood corpuscles form most useful 

 standards of measurement, as their dimensions are already known, 

 (p. 35). 



A. Epithelium. 



Epithelium consists of cells placed side by side so as to form 

 layers, which form the surface covering, or epidermis, of the 

 body, and line the alimentary canal, the blood vessels, and the 

 various internal cavities of the body. At the external apertures 

 of the body, the epidermis is directly continuous with the 

 epithelial lining of the internal cavities. 



The layers may be one or more cells in thickness ; in the 

 former case the epithelium is said to be simple, in the latter 

 stratified. 



Epithelium is of different kinds, according to the shape and 

 structure of its component cells. 



I. Squamous Epithelium. In this the component cells are 

 flattened parallel to the surface they cover : if the epithelium is 

 stratified, the flattening is most marked in the superficial cells. 



