LIVER AND PANCREAS 



531 



passages which form a reticulum between the cells and so goes 

 by the bile duct to the intestine, but the main function of the 

 liver is to take raw materials from the blood stream, change 

 them chemically, and return both useful products and waste 

 material to the blood. It is richly provided with blood vessels, 

 and the blood itself penetrates into the cells. 



The pancreas (Fig. 413) has a more obviously diverticular 

 nature. It is a compound tubuloalveolar gland, that is, it branches 

 many times and both the finer tubules and their terminal 

 swellings or alveoli are secretory. The cells are somewhat poly- 



Red cells m 

 blood vessel. 



Alveolus. 



Duct of 

 alveolus 



Partofanisiet 

 of Langerhans. 



Connective 

 tissue. 



Smalt duct leading 

 out of alveolus. 



Fig. 413. — A small portion of the pancreas of a guinea- 

 pig in section x c. 400. 



hedral. The islets of Langerhans, or islet tissue of the pancreas, 

 are masses of cells which have become separated from the ducts 

 and are endocrine in function. 



LUNGS 



The lungs are also outgrowths of the alimentary canal. In the 

 frog each is a simple sac, but in the mammal the original diverticu- 

 lum has become very finely divided. Each terminal bronchiole 

 expands into a group of air sacs or infundibula, and each of these 

 has its wall pushed out into several pimples, called air cells or 

 alveoU, so that the final appearance is not unlike that of a bunch 

 of raisins. The alveoU are hned by a thin pavement epithelium, 

 and outside this are the blood vessels and some connective 

 tissue with yellow fibres. Covering the whole lung is a serous 



