ANATOMY AND PHYSIOLOGY 



The principal varieties are: 



Hyaline cartilage which has few cells and much intercellular sub- 

 stance. (Fig. 4.) 

 While fibro-cartilage which contains many white fibers, giving to it 



additional strength. 



Yellow or elastic fibro-cartilage which contains elastic fibers giving ad- 

 ditional elasticity. 

 Note. Most bones are formed in cartilage. (See Ossification, page 14.) 



i Epithelial tissue forms the surface layers 



of the body both within and without. It is 

 ^ G composed of layers of cells resting upon a base 

 -. of the simplest possible substance, which holds 

 the cells together and which bears vessels and 

 jr> nerves for their use. The form of epithelial 

 - cells varies with their location and use or func- 

 tion. (Fig. 5.) 



CARTILAGE. tSfr.) The epithelium oj the exterior of the body is 

 formed by flattened cells, arranged in few or 

 many layers according to the degree of friction or pressure to 

 which the skin of the part may be exposed. The covering thus 

 formed varies therefore in thickness, from that of the delicate 

 covering of the lips to the tough sole of the foot. 



FIG. 5. EPITHELIAL CELLS OF RABBIT, ISOLATED. Xs6o. i. Squamous cells 

 (mucous membrane of mouth). 2. Columnar cells (corneal epithelium) . 3. Columnar 

 cells with cuticular border s (intestinal epithelium). 4. Ciliated cells; h, cilia 

 (bronchial epithelium). (Lewis and Stohr.) 



The epithelium of interior surfaces is quite different. Its cells 

 may be flattened, spherical, cuboid or columnar in shape and it is 

 always moist. (All body surfaces are epithelial surfaces.) 



In the lining of the air passages the epithelial cells are ciliated, that is, 

 they bear tiny hair-like projections of their substance called cilia, which are 

 in constant waving motion, always in the same direction, sometimes slow, 

 sometimes rapid. (See p. 235.) 



