GLAND SECRETION 29 



concerned in actual secretion, but simply in carrying the pro- 

 duct away from the secreting terminal ramifications of the 

 subdivisions of the involution which terminations are called 

 acini or alveoli. It follows, of course, that a collection of 

 acini may discharge their secretion into the main duct by a 

 smaller duct that is, that the gland may have various subdi- 

 visions 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 an- 

 other. 



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 se- 

 cretion 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 anatomical arrangement favors osmotic transudation 

 from the vessels, 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 



