SECRETION. 



65 



What is the function of mucus ? 



To lubricate and moisten the soft and delicate cells of mucous 

 membrane. Mucous tracts which are so protected are the digestive, 

 respiratory, and genito-urinary. 



How are these processes of secretion and excretion carried on ? 

 By means of the activity of the cells. In most cases the cells 

 are grouped in organs which are known as glands. The serous 

 fluids are the only notable exception to this rule, the endothelial 

 cells secreting without the intervention of complex anatomical 

 groups. 



What forms of secreting glands occur? 



1. Simple tubules, or tubular glands, which are pits or depres- 

 sions in epithelial surfaces lined with epithelial cells. The mucous 

 surfaces furnish the most numerous examples of this form of 

 gland, follicles of Lieberkuhn (Fig. 14), tubular (mucous) glands 



FIG. 14. 



Follicles of Lieberkuhn, from small intestine of dog. 



of the stomach ; but the skin, in the sweat-glands, shows a more 

 complicated form of tubular gland in that it is convoluted and 

 tortuous. 



2. Compound tubular glands consist of a tubular gland which 



5 Phy. 



