330 



COMPARATIVE ZOOLOGY 



type of all secreting and excreting organs. The lower 

 animals have nothing more complex, and the apparatus 

 for preparing the gastric fluid attains no further devel- 

 opment even in man. When a cluster of these follicles, 

 or sacs, discharge their contents by one common duct, 



we have a gland. But 

 whether membrane, folli- 

 cle, or gland, the organ 

 is covered with a network 

 of blood vessels, and lined 

 with epithelial cells, which 

 are the real agents in the 

 process. 



The Chief Secreting Or- 

 gans are the salivary 

 glands, gastric follicles, 

 pancreas, and liver, all 

 situated along the diges- 

 tive tract. 



i. The salivary glands, 



FIG. 286. Three plans of secreting Mem- which Open into the 

 . branes. The heavy line represents the 



areolar-vascular layer; the next line is the mOUth, SCCrCte Saliva. 

 basement, or limiting membrane; and the Thp v pvici- in n<=arl T all 

 dotted line the epithelial layer: a, shows ney eXlbt ] nearly ail 

 increase of surface by simple plaited or vertebrates, hig her mol- 

 fringed projections; b, five modes of in- 

 crease by recesses, forming simple glands, lusks, and inSCCtS, and are 

 or follicles: c, two forms of compound . , , . , . 



glands. most largely developed in 



such as live on vegetable 



food. The saliva serves to lubricate or dissolve the 

 food for swallowing, and, in some mammals, aids also 

 in digestion of starch. 123 



2. The gastric follicles are minute tubes in the walls 

 of the stomach secreting gastric juice. They are found 

 in all vertebrates, and in the higher mollusks and arthro- 

 pods. In the lower forms, a simple membrane lined 

 with cells serves the same purpose. Under the micro- 



