30 SECRETION 



simple osmosis to explain all the processes of glandular se- 

 cretion, especially those connected with the presence of or- 

 ganic constituents; while the undoubted presence of secre- 

 tory nerves (besides the vaso-motor nerves to the vessels) 

 would seem to give a priori evidence that the glandular epi- 

 thelium takes some active part in the formation of the se- 

 cretion. Such an office is granted to these cells, but whether 

 it is of chemical, or a physical, or a "vital" character is not 

 evident. 



The physiology of the salivary glands, the gastric and in- 

 testinal glands, the pancreas and liver is taken up under the 

 chapter on Digestion in which they are vitally concerned. 



Sebaceous Glands. 



The sebaceous glands (see Hair-follicles) are chiefly asso- 

 ciated with hair-follicles and, existing wherever hair is to be 

 found, cover well-nigh the whole cutaneous surface. They 

 are of the simple or compound tubular type, and discharge 

 their secretion into the hair- follicle near its outer extremity. 

 The alveoli are lined by several layers of cuboidal epithelial 

 cells. The cells of the layer nearest the lumen contain fatty 

 matter, and are thought to form the secretion by breaking 

 down and being thrown off themselves. Their place is taken 

 by cells from the deeper layers, which undergo similar 

 changes and disintegrate. 



Composition and Properties of Sebum. Chemically se- 

 bum is largely made up of fatty matters. It also contains 

 cholesterin, which is in combination with a fatty acid. It 

 forms a thin coating over the cutaneous surface, accounting 

 for the normal oiliness of the skin. It also contributes to the 

 characteristic softness of the hairs, and prevents their break- 

 ing off from brittleness. Its presence over the body surface 

 may have some influence in regulating the loss of heat by 

 evaporation. 



Cerumen, smegma and the secretion from the Nabothian 



