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 T ^ ir 

 of an inch in diameter, and number mill- 

 ions. 



3. The pancreas, or " sweetbread," so 



, . ,i n t , . Fig. 90.— Follicles from the 



important in the process of digestion, stomach of a Dog, x 

 when present, exists only in the Verte- J^T.^iT^i 

 brates, and perhaps in the higher Mol- Iumnar epithelium, 

 lusks. 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. 



Fjg. 91. — Pancreas of Man, o; gsi 11 -bin elder 

 «, cystic duct; c, duct from the liver; p, py 

 loric valve ; e, «, duodenum. 



