SECRETION AND EXCRETION. 121 
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 excreting organs. The lower 
tribes have nothing higher, and the apparatus for prepar- 
ing the gastric fluid attains no further development even 
in Man. When a cluster of these follicles, or sacs, dis- 
charge their contents by one common duct, we have a 
gland. But whether membrane, follicle, or gland, 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, 
secrete saliva. They exist in nearly all animals, except 
Fishes, Crocodiles, and Whales, and are most largely de- 
veloped in such as live on vegetable 
food. The saliva serves to lubricate 
or dissolve the food for swallowing, 
and, in Mammals that masticate, aids 
also in digestion.” 
2. The gastric follicles are minute 
tubes in the walls of the stomach se- 
creting gastric juice. They are found 
in all Vertebrates, and in the higher 
Mollusks and Articulates. In the low- 
er forms, a simple membrane lined 
with cells serves the same purpose. 
Under the microscope, the soft mu- 
cous membrane of the human stom- 
ach presents a honey-comb appear- go? Pontes meee 
ance, caused by numerous depressions {p0i.\e"n ‘nine of cot 
or cells. At the bottom of these de- — lumnar epithelium. 
pressions are clusters of spots, which are the orifices of 
