9 2 HUMAN PHYSIOLOGY 



laries. The glands also contain lymph vessels, muscles and 

 nerves. 



The secreting cells are generally only found at the closed 

 end of the gland duct, while the other part serves as an 

 excretory duct for the secretion. 



The process of secretion is not merely a filtration of the 

 fluids of the blood through the walls of the gland, but is 

 brought about by a special activity of the secreting gland- 

 cell, as the following shows: 



1 . Most of the secretions contain substances not found as 

 such in the blood, which must therefore have been made by 

 chemical processes in the gland-cells (e.g. the ferments of 

 digestive fluids, the caseinogen and the lactose of milk, etc.). 



2. Secretion, in many cases, does not occur continually 

 but only at stated times, while blood pressure should cause 

 a continual filtration. 



3. The pressure of the secretion in the duct of the gland 

 may be higher than that of the blood. Furthermore, secre- 

 tion can take place in glands free from blood and even in 

 excised glands. 



4. In many cases the secretion is accompanied by morpho- 

 logical changes in the cells of the gland. 



5. Many secretions are under the influence of specific 

 secretory nerves. The nerve-fibres in the salivary glands 

 end in the gland-cells. 



2. SALIVARY SECRETION 



I. Composition of saliva. The saliva of the mouth is a 

 secretion of all the glands of the mouth-cavity. It is a 

 colorless, cloudy, stringy fluid, having a weak alkaline 

 reaction. The amount secreted in twenty-four hours is 

 estimated at from one to two litres. 



The cloudiness of the saliva is due to the mucin, salivary 

 corpuscles and discarded epithelial cells of the mouth-cavity. 

 The salivary corpuscles are recently loosened gland-cells 

 or migrated leucocytes. 



The saliva contains 99-99.5$ water, 0.1-0.2$ salts (in- 



