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 

 tribes have nothing higher, 

 and the apparatus for pre- 

 paring the gastric fluid at- 

 tains no further develop- 

 ment even in Man. When 



FIG. S9.-Three plans of secreting Mem- a c l us t er O f tllCSC f ollicleS, Ol' 

 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, WC 



a shows increase of surface by simple \ 7 ? p , i , r 



plaited or fringed projections ; 6, five haVe a fa*- Bttt whether 



modes of increase by recesses, forming membrane, f ollicle. Or gland, 

 simple glands, or follicles; c, two forms 



of compound glands. the organ is covered with a 



net-work of blood-vessels, and lined witli 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. 8 " 



