INTRODUCTION 



13 



underlying tissues. Epithelial cells are 

 usually transparent ; for instance, the 

 blood is visible beneath the mucous 

 membrane of the lips. The finger nails 

 are made of epithelial cells, and they 

 are nearly transparent. 



There are two classes of epithelial 

 cells ; one class forms protective cover- 

 ings (Fig. 15) ; the other class forms the 

 lining of glands (Fig. 16). Glands are 

 cavities whose lining of epithelial cells 

 (Fig. 17) form either useful fluids called 

 secretions to aid the body in its work, or 

 harmful fluids called excretions to be cast 

 out, or excreted. Most glands empty 

 their fluids through tubes called ducts. 



Cartilag'inous tissue is tough, yet 

 elastic. Cartilage or gristle may be 

 readily felt in the ears, the windpipe, 

 and the lower half of the nose. This 

 tissue consists of cartilage cells embedded 

 in an intercellular substance through 

 which run connective tissue fibers (see 

 Fig. 18). If yellow fibers predominate, 

 the cartilage is yellow and very elastic, 

 as in the ear ; if white fibers predomi- 

 nate, it is white and less elastic, as in 

 the pads of gristle between the bones 

 of the spinal column. Cartilage is to 

 prevent jars, and, in movable joints, to 

 lessen friction. 



Bony (Osseous) Tissue. — Solid bone 

 is seen under the microscope to contain 



Fig. 15. — Epithelial 

 Tissue (epidermis of 

 skin, magnified). 



•*Sw«Sf 



Fig. 16. — Epithelial 

 Tissue; cells form- 

 ing two glands in 

 wall of stomach. 



Fig. 17. — Six Gland 

 Cells : at left, 

 shrunken after activ- 

 ity ; at right, rested, 

 full of granules. 



