SECRETION AND EXCRETION. 
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 Mol- 
lusks and Arthropods. In the lower 
forms, a simple membrane lined with 
cells serves the same purpose. Under 
the microscope, the soft mucous mem¬ 
brane of the human stomach presents a 
honey-comb appearance, caused by nu¬ 
merous depressions or cells. At the bot¬ 
tom of these depressions are clusters of 
spots, which are the orifices of the tubu¬ 
lar follicles. The follicles are about 
of an inch in diameter, and number mill¬ 
ions. 
3. The pancreas, or “ sweetbread,” so 
. , . - /» i. . Fig. 00.—Follicles from the 
important in the process ot digestion, stomach of a Dog, x 
when present, exists only in the Yerte- 
brates, and perhaps in the higher Mol- lumnar epithelium. 
Insks. In its structure and its secretion it closely resem¬ 
bles the salivary glands. In the Cuttle-fish, it is repre¬ 
sented by a sac; in Fish¬ 
es, by a group of follicles. 
It is proportionally larg¬ 
est in Birds whose sali¬ 
vary glands are deficient. 
The pancreatic juice en¬ 
ters the duodenum, 
4. A liver in some form 
is found in all animals 
having a distinct diges¬ 
tive cavity. In Mollusks 
and Vertebrates, it is the 
largest gland in the body. 
The higher the animal, the more compact the organ. 
Fxg. 01.—Pancreas of Man, o; g , gall-bladder 
*, cystic duct; c, duct from the liver; p, py 
loric valve; e , duodenum. 
