CHAPTER VIII. 



GLANDS. 



Glands are organs of secretion or of excretion. In structure they in- 

 volve all five fundamental bodily tissues — epithelial, connective and vas- 

 cular in all cases, nervous tissue in nearly all, and muscular tissue in the 

 larger glands — -but the secreting or excreting agents in the glands are cells 

 of epithelial origin. 



Secretion is the production from the bodily fluids — or, in some cases, 

 the mere separation from the bodily fluid — of substances which are directly 

 useful to cells other than those secreting them, or which are useful to the 

 organism as a whole : e. g., saliva, produced by the salivary glands, is nec- 

 essary for the digestion of starch; and sweat, produced by glands in the 

 skin, helps to regulate the skin temperature. Excretion is the separation 

 or the elimination from the bodily fluids of waste products of cells other 

 than those excreting them : e. g., urine, excreted by the kidneys. Some- 

 times a gland combines both functions, as in the case of the liver the se- 

 cretion of which (bile) both contains waste products and is also an im- 

 portant agent in intestinal digestion. The processes of secretion and 

 excretion are in a sense, common to all cells. All cells take up from the 

 blood and lymph substances required for their own nutritive processes and 

 give off waste products. But technically the terms ' excretion ' and ' secre- 

 tion ' apply only to the processes as described above, viz., in which certain 

 specialized cells are carrying on the functions for the direct benefit of other 

 cells. 



There are many secreting cells — gland cells — which are not located in 

 glands, but are scattered in epithelial membranes, especially in the mucous 

 membrane. Such are the ' goblet cells ', earlier described. Goblet cells, 

 which, during their period of activity, form a mass of secretion within 

 themselves and then liberate it at the end of the period of activity, repre- 

 sent the middle type of gland-cell. At one extreme are cells which prob- 

 ably produce and liberate their secretions continuously during the active 

 period : such are the cells of the parotid gland. At the other extreme are 

 cells which, having become filled with products during the active period, 

 are themselves broken up, and mingle with their secretions in the process 

 of discharging them; such are the lacteal cells in the mammary glands, 

 and the cells of the sebaceous glands in the skin. 



