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Basic Structure of Vertebrates 



"ALVEOLUS 



■feta* ISLAND 



.■--' 8 ; '^ -..■■*■ •■■'^* ■■■■ ^'ass* 



BLOOD VESSEL 



Fig. 29. A section of the pancreas, showing an island among the alveoli of the 

 gland. (X 200.) (Courtesy, Neal and Rand: "Comparative Anatomy," Phila- 

 delphia, The Blakiston Company.) 



tissue makes it unsafe to assert that any fish lacks a pancreas. The 

 absence of a duct in cyclostomes indicates that their pancreas is a 

 gland of "internal secretion" — i.e., secreting into the blood-stream — 

 not a digestive gland. 



Viewed in its entirety, the liver is impressive only by virtue of its 

 great bulk. Devoid of definite form, not to say "lumpish," it seems 

 anatomically uninteresting. In its histologic architecture, however, the 

 relations of its secretory and circulatory structures are characterized 

 by a degree of precision, refinement, and intricacy which perhaps is not 

 exceeded in any other visceral organ. In number and diversity of its 

 functions, the liver even goes beyond what might be inferred from its 

 structure. The combined output of its secretory activity is the bile, a 

 fluid usually of strong color — green, yellow, or brown. Constituents of 

 the bile aid in the emulsifying of fats in the intestine and, interacting 

 with secretions of the pancreas and intestine, are concerned with other 

 digestive processes. From the hepatic-portal venous blood flowing 

 through the liver, the hepatic cells extract sugar and some protein sub- 

 stances and convert them into chemically starchlike glycogen, which 

 is stored in the cells and, as need arises, is reconverted to sugar and 

 returned to the blood. Some waste nitrogenous substances are removed 

 from the blood and converted into urea, which is returned to the blood 

 to be eliminated by the kidneys. The colored substances and some 



