GLAND SECRETION 29 



cerned in actual secretion, but simply in carrying the product 

 away from the secreting terminal ramifications of the subdivi- 

 sions of the involution which terminations are called acini or 

 alveoli. It follows, of course, that a collection of acini may dis- 

 charge their secretion into the main duct by a smaller duct 

 that is, that the gland may have various subdivisions of the duct 

 proper. 



Furthermore secretions are classified as external when they 

 are discharged upon a surface communicating with the external 

 air, such as the alimentary canal, or skin, and internal when 

 they are discharged upon surfaces not in communication with 

 the exterior, such as blood-vessels. Both external and internal 

 secretions are liquid or semi-liquid in character, for they must 

 contain water as a vehicle for the salts and organic substances 

 which are present in all of them and which, in fact, distinguish 

 them from one another. 



Glands in general have been divided into serous and mucous 

 by Heidenhain, according as the secreted fluid is watery and 

 thin, or viscid and stringy from the presence of mucin. This 

 division is further warranted by histologic differences in the cells 

 concerned in each kind of secretion. The cells in a serous 

 gland are small and finely granular, and are in close apposition 

 to each other. Those of mucous glands are larger, almost 

 square and are definitely separated. Many glands contain both 

 kinds of cells, but since their secretion contains mucin, such 

 glands are usually spoken of as belonging to the mucous variety. 

 It will be seen that the salivary glands illustrate these varieties. 



Gland Secretion. Underneath the basement membrane of a 

 gland (that is, on the side opposite the epithelial cells) ramifies 

 an abundant network of blood and lymph capillaries. This an- 

 atomical arrangement favors osmotic transudation from the ves- 

 sels, especially since the pressure in the vessels is normally 

 greater than in the acini and ducts of the gland. Numerous 

 experiments, however, prove the inadequacy of simple osmosis 



