30 SECRETION 



to explain all the processes of glandular secretion, especially 

 those connected with the presence of organic constituents; 

 while the undoubted presence of secretory nerves (besides the 

 vaso-motor nerves to the vessels) would seem to give a priori 

 evidence that the glandular epithelium takes some active part 

 in the formation of the secretion. 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 intestinal 

 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 associ- 

 ated 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 sebum 

 is largely made up of fatty matters. It also contains choles- 

 terin, 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 breaking off from brittle- 

 ness. Its presence over the body surface may have some influ- 

 ence in regulating the loss of heat by evaporation. 



Cerumen, smegma and the secretion from the Nabothian 

 glands are only modified forms of sebum, and the structures 

 producing these secretions belong to the class of sebaceous glands. 



