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. 89.-Three plans of secreting Mem- a cluster of these follicles, 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 dllCt, WC 
a shows increase of surface by simple \ 7 j -r» . i , 1 
plaited or fringed projections; b, five bave a g land . -Dllt whether 
modes of increase by recesses, forming membrane, follicle, Or gland, 
simple glands, or follicles; c, two forms ? 5 b , 
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.* 9 
