126 



The Endocrine Organs 



FIG. 88. Section of pancreas of dog, showing an islet 

 of Langerhans between the alveoli. Magnified 200 

 diameters. 



and disappearance which in many cases are probably not justifiable. Thus 

 Bensley found a variation in the guinea-pig in animals of different ages 



of from 10 to 189 islets per 

 milligramme of pancreas: in 

 the mature animal the varia- 

 tion was from 10 to 25. The 

 enumerations of Clerk led 

 him to the conclusion that 

 in the normal human pan- 

 creas there may average 10 

 to 20 islets in each milli- 

 gramme of the gland, which 

 roughly would give about 

 three-quarters of a million 

 to a million and a half for 

 the whole pancreas. 



In Teleostean fishes 

 Rennie has shown that there 

 is one very large mass of islet 

 tissue which is encapsuled by 

 connective tissue and practi- 

 cally forms a separate organ. 

 But in other animals the 

 islets are closely encircled by and in contact with the alveolar tissue, and 

 sometimes appear to be continued into it. According to Pensa, they have an 

 abundant supply of nerve fibres. 

 Each islet has a specialised blood- 

 supply (fig. 89) in the form of 

 a network of irregular-shaped 

 sinus-like capillaries which are 

 both larger and relatively more 

 numerous than the capillaries of 

 the alveoli (Klihne and Lea). 



The cells of the islets are 

 generally much less stained by 

 the ordinary dyes used in histo- 

 logy than are those of the alveoli. 

 They are therefore usually de- 

 scribed as chromophobe. But 

 they have an especial affinity, as 

 Bensley has shown, for neutral 



red and janus green, employed as intra vital stains. Although they contain 

 granules, these are much finer than the zymogen granules of the alveolar 

 cells. According to Lane, there are two kinds of cell in the islets, dis- 

 tinguishable from one another by the nature of their granules. The cells 



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FIG. 89. An islet of Langerhans of the pancreas, 

 with its blood-vessels injected. (Kiihne and Lea.) 



