184 THE DIGESTIVE SYSTEM 



cytoplasmic i]:;raniiles and a large oval nucleus with little chromatin 

 apparent is found most frequently in the center of the island. The 

 second type, the B cell, is smaller and more numerous, with a 

 smaller nucleus containing large chromatin granules. The secre- 

 tion of these island cells, insulin, passes into the blood and governs 

 the metabolism of carbohydrates. The appearance of these islands 

 may \'ary considerably, even occurring as a separate gland outside 

 the pancreas in some fishes. 



In lower vertebrates the pancreas is usually a more compact 

 gland, but in general closely resembles the mammalian structure 

 just described. Intercalated ducts are usually not present and 

 centro-acinar cells are absent, 



THE LIVER. 



The liver arises as a diverticulum of the mid-gut and de\'elops 

 into a compound tubular gland. Among fishes it may retain this 

 simple glandular condition with blindly ending secretory tulniles. 

 ^Yith the amphibia and reptiles the tul)ules fuse to form a network 

 and the li\'er cells surround a central lumen into which secre- 

 tions are ])oure(l from bile capillaries wdiich form as a network of 

 grooves between the adjacent faces of the cells. The connective 

 tissue is not abundant, and no lobulation occurs. In mammals 

 there is an increase in amount of connective tissue and lobulations 

 are indicated where connective tissue accompanies the larger })lood- 

 vessels. Among the mammals the lumens of the embryonic end- 

 pieces of the gland are lost, so that only cords of cells are apparent 

 and an increasing amount of connective tissue results in a di\ision 

 of the gland into lobules, and groups of these form lobes. 



Lower Vertebrates.— The liver of fishes is a typical com])ound 

 tubular gland with the clear cells of the secreting end-pieces arranged 

 about the lumen into which the secretion collects. Small bile ducts 

 are formed of cuboidal cells. Between adjacent end-pieces and 

 ducts and lobular masses is a small amount of connective tissue 

 which supports capillaries and small vessels. Another type of cell 

 with granular pigmented cytoplasm often occurs around the larger 

 excretory ducts and l)lo()dvessels. 



The liver of amphibians is a modified compound tubular gland. 

 (Fig. 119.) The peripheral end-pieces composed of the secreting 

 cells form a series of differently directed short tubules that connect 

 and form a network. The cells are large and pyramidal shaped. 



