86 PHYSIOLOGY CHAP. 



protoplasmic mass, in which a number of nuclei are arranged 

 irregularly, so that for a long time these cells were thought 'to 

 be lymphoid. Eecent research, has, however, shown them to be 

 epithelial. According to some authors these epithelial bodies are 

 destitute of excretory ducts ; but they invariably exhibit a number 

 of tortuous blood-vessels which in certain cases (Klihne and Lea) 

 assume the form of glomeruli. They are generally known as the 

 islets of Langerhans (Fig. 27). 



The special significance of these islets has recently been much 





* ** 



tvV> 



FIG. 27. Section of rabbit's pancreas. (Marassini.) The periphery shows a number of glandulnr 

 acini, which are darker in colour ; at the centre is a large islet of Langerhans of a lighter 

 colour, composed of cells with indistinct outlines. 



discussed. ~ Many observers conclude from their morphological 

 characters (absence of ducts, abundance of blood-vessels) that they 

 are responsible for the internal secretion of the pancreas, while 

 others claim that the alveoli, too, participate in this function. 



As regards the relation between the islets and the alveoli, 

 Lewaschew (1886) suggested that the islets represent phases in 

 advancing exhaustion of the secreting alveoli. According as the 

 latter are more or less fatigued, they exhibit cells which ap- 

 proximate to the characters of the islets or the acini. He 

 supported his hypothesis by observations which showed that the 

 islets are more numerous in the pancreas of over -fed animals, 

 and during the action of pilocarpine. Lewaschew's theory was 

 adopted, as regards the possible derivation of islets from alveoli, 

 by Dogiel, Perdisgeat and Tribondeau, Dale, Laguesse, and many 

 others. 



Laguesse, however, modified its physiological interpretation, 



