ii EXTERNAL DIGESTIVE SECRETIONS 87 



and regards this process as an alternation between the external 

 and internal secreting conditions of pancreatic tissue, which is 

 necessary to enable the organ to accomplish its double function. 



The theory of the development of alveoli into islets, and 

 vice versa, was soon contested. 



Vassale (1891) first showed that after ligation of Wirsung's 

 duct in rabbit, the islets remain, while the alveoli disappear. 

 Massari (1898) found that in eels the islets were constant and 

 invariable, without any true transitional forms. Immediately 

 after, Giannelli, Renaut, Diainare, Jacotsky, W. Schultze, Opie, 

 and others, on repeating the experiments of Lewaschew's school 

 (hyperalimentation, pilocarpinisation, inanition, etc.), failed to find 

 any constant difference in the number and appearance of the 

 islands, in the various animals experimented on. Since that 

 time the theory of Lewaschew and Laguesse has been unanimously 

 abandoned. 



It remained to be seen whether the islands were in com- 

 munication with the excretory ducts. The results obtained by 

 certain authors who attempted to solve the problem by injecting 

 the ducts with coloured solutions which are readily recognised 

 under the microscope, are directly contradictory. Thus Lewaschew 

 (1886), Mankowski (1901), asserted that the injected substance 

 penetrates to the islets. V. Ebner (1872), G. Rossi (1902), and 

 others affirm that it never reaches the islets. But it must be 

 noted that injection by the ducts does not always reach the whole 

 of the alveoli, as observed by Rossi. These negative results have 

 therefore no decisive value. 



Simple histological examination, on the contrary, does tend to 

 support the hypothesis of communication between the islets and 

 the excretory ductules. Laguesse, in a fragment of human pan- 

 creas, noted that out of 56 islets followed in serial section, only 

 4 were entirely independent. Of the other 52, many were either 

 in direct relation with the excretory system, or were joined to it 

 by the alveoli. 



A highly important detail, as to which opinions differ, is the 

 existence of a connective capsule, surrounding the islets com- 

 pletely, and separating them sharply from the alveolar tissue. 

 Renaut (1879) pointed out a reticulum enclosing the islets, as 

 did also Opie and Pugnat. On the other hand, Gibbes, Diamare, 

 and Hansemann denied these observations. To solve the problem 

 Marshall Flint (1903) employed tryptic digestion, which spares 

 this capsule ; and decided that it existed. 



Laguesse also admitted it, but stated that the capsule 

 (membrana propria) does not completely surround the islets, which 

 contract relations with the excretory system at the points at 

 which they are not invested. 



Golgi's method demonstrates the origin of the excretory 



G 3 



