12 



ELEMENTS OF PHYSIOLOGY 



FIG. ii. 



A, typical structure of a mucous 

 membrane with two layers of epi- 

 thelial cells, a, 6; c, the connec- 

 tive tissue beneath, with e, blood- 

 vessels, and f t connective tissue 

 cells. 



B, the same with one layer of cells 

 resting on 3, the so-called base- 

 ment membrane. 



FIG. 12. 



A simple tubular gland with blood- 

 vessel. 



FIG. 13. 



A tubular gland dividing. 



FIG. 14. 

 A sac-like gland. 



Diagrams to illustrate the Struc- 

 ture of Glands. 



15. The Mucous Membrane. 



Epithelial cells form not only the 

 epidermis to cover the body exter- 

 nally, but they form also a lining 

 for the interior cavities of the body. 

 Any interior cavities to which air 

 has access, as the mouth, stomach, 

 and lungs, are lined with one or 

 more layers of closely packed epi- 

 thelial cells called mucous mem- 

 brane. Any cavities from which 

 the air is cut off, as the heart, are 

 called serous cavities, and have a 

 lining of epithelial cells forming 

 what is called a serous membrane. 

 These membranes secrete a lubri- 

 cating fluid. The fluid formed by 

 the mucous membrane is called 

 mucus. The line where the epi- 

 dermis and the mucous membrane 

 of the lips join can be plainly seen, 

 as the latter membrane is so thin 

 that the color of the blood shows 

 through it. Mucous membranes 

 are so smooth and their cells are so 

 tightly packed that they are harder 

 to cut than the skin, notwithstand- 

 ing they are very thin. 



16. Glands (Figs. 11 to 17) are 

 always lined with epithelial cells. 

 Glands are cavities in the body, 

 usually bag-shaped or tubular, 

 whose walls secrete fluids which 

 are of various uses in the body. 

 The gland cells make use of the 



