122 



COMPARATIVE ZOOLOGY. 



be poured. Usually, however, the cells of the membrane 

 manufacture the secretion from materials furnished by the 

 blood. Even in the higher animals, there are such secret- 

 ing membranes. The membranes lining the nose and ali- 



mentary canal and enclosing 

 the lungs, heart, and joints, 

 secrete lubricating fluids. 



The infolding of such a 

 membrane into little sacs or 

 short tubes (follicles), each 

 having its own outlet, is the 

 type of all secreting and ex- 

 creting organs. The lower 

 animals have nothing more 

 complex, and the apparatus 

 for preparing the gastric fluid 

 attains no further develop- 

 ment even in Man. When 



FIG. S9.-Three plans of secreting Mem- a c l us ter of tllCSG f ollicleS, Or 

 branes. The heavy line represents the 



areolar-vascular layer; the next line is saCS, discharge their Contents 

 the basement, or limiting membrane; 

 and the dotted line the epithelial layer: by One Common QUCt, WG 



a gland. But whether 

 mbrane, M\^, or gland, 



of compound glands. the organ is covered with a 



net- work of blood-vessels, and lined with epithelial cells, 

 which are the real agents in the process. 



The chief Secreting Organs are the salivary glands, 

 gastric follicles, pancreas, and liver, all situated along the 

 digestive tract. 



1. The salivary glands, which open into the mouth, se- 

 crete saliva. They exist in nearly all Vertebrates, higher 

 Mollusks, and Insects, and are most largely developed in 

 such as live on vegetable food. The saliva serves to lu- 

 bricate or dissolve the food for swallowing, and, in some 

 Mammals, aids also in digestion of starch. 89 



