THE INTERNAL SECRETION OF THE PANCREAS 433 



But in addition to the acini and their excretory ducts, it was known 

 from Langerhans's description (1869), that the pancreas contained 

 irregularly distributed cell groups, which were later named, after 

 their discoverer, the islands of Langerhans. The structure of 

 these elements suggested that they might possess an internal 

 secretory function. 



The islands are composed of polygonal cells with ill-defined 

 boundaries, containing large round nuclei and comparatively 

 small and sparsely distributed granules. They are remarkably 

 well furnished with capillary blood-vessels which are intimately 

 associated with the cells, and they are surrounded with a delicate 

 connective tissue sheath (Flint, Laguesse). 



A question of paramount importance in the determination 

 of function, namely, whether these islands contain lumina com- 

 municating with the excretory ducts of the pancreas, is not as yet 

 definitely decided. Lumina within the insular area have been 

 described by Gianelli and Giacomini in reptiles, by Dale in toads, 

 and by Vincent and Thompson in Kinosternon and in Amia 

 call) a. According to both Lewaschew and Mankowski, the islands 

 in mammals may be injected from the excretory ducts, though 

 this is denied by other authors (v. Ebner, Dogiel, Rossi). 

 Laguesse found that, in the snake, the islands contained lumina 

 which had an unbroken continuity with those of the acini. Well- 

 developed excretory ducts have also been occasionally observed 

 in the interior of the islands of the human pancreas. In his 

 investigation of the regenerative processes which take place in 

 portions of pancreatic tissue implanted in the spleen, Kyrle ob- 

 served proliferation of the epithelial cells of the excretory ducts, 

 in consequence of which the acini and the islands were formed 

 separately and excretory ducts were present in the islands when 

 fully developed. The embryological investigations of Weichsel- 

 baum and Kyrle led them to conclude that the islands of Langer- 

 hans are derived from the epithelium of the excretory ducts, and 

 that their formation takes place, not during foetal, but during the 

 early part of post-fcetal life. 



The islands of Langerhans were regarded by the older scien- 

 tists, as well as many later authors, as lymph follicles to which 

 no particular functional significance was attaching. A more 

 exact knowledge of their genesis showed, however, that the cells 

 of the islands are of epithelial origin. By some authors (Gian- 

 nelli, Oppel) they are even regarded as the rudimentary remains 

 of an atavistic gland. Many scientists held the view that, though 

 the acini and the islands differ structurally from one another, 

 they are not functionally differentiated. Harris and Gow, Gibbes, 

 Gianelli and Giacomini assume that the islands play a part in 

 the production of the external pancreatic secretion, especially of 

 the diastatic ferment. Jarotsky observed signs of increased secre- 

 tory activity on the part of the cells of the islands after the ad- 



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