1912.] Islets of Langerhans to the Pancreatic Acini. 75 



kinds of granules in the islet cell, differing chemically from each other and 

 from the zymogen of the acini. 



Finally, Bensley (1911), in a most exhaustive study of the pancreas of the 

 guinea-pig, has cleared up much of the doubt surrounding this subject. By 

 the use of vital stains he shows that the islets can be readily identified and 

 counted, and that their number is not influenced by starvation, or any known 

 method of stimulation. He demonstrates, by the use of refined methods of 

 fixation and by granule stains, that the islet cells have a structure distinct 

 from that of the acinous cells, as well as from the duct and centroacinous 

 cells with which they are likely to be confused, and among which they are 

 frequently found. He points out with justice the error in the common 

 practice of identifying the islets by purely negative means, that is, by 

 fixations and stains which do not bring out their distinguishing characters. 

 I have repeated some of his experiments with vital stains to satisfy myself of 

 the fairness of his basis of identification, and this research would have little 

 justification if it did not take a direction in which Bensley's work has not 

 carried him. I have borrowed freely from his methods. 



Technique. 



The animals used in this research were dogs and guinea-pigs. No especial 

 selection of animals was made. The experiments* include two exhaustions of 

 the dog's pancreas by secretin, five nearly complete extirpations and one duct 

 ligation in the dog, and three pancreas exhaustions in the guinea-pig by 

 purified secretin. In addition, a number of injections of neutral red, methylene 

 blue, and pyronin have been made in guinea-pigs, according to. the methods of 

 Bensley. 



Fixing and Staining Methods. — All specimens were removed immediately after the 

 animals had been killed by bleeding under anesthetics. Pieces of pancreas not more 

 than 2 mm. in thickness were placed at once in a number of fixative solutions and 

 stained by various methods. 



I. Acetic-osmic-bichromate (Bensley). 



Osmic acid 4 c.c. 



Potassium bichromate 15 c.c. 



Acetic acid 2 drops. 



Tissues fixed from 16 to 24 hours were washed in water and passed through alcohol to 

 paraffin. Ordinary tissues were cleared in benzol before paraffin, but pieces containing 

 considerable scar tissue were passed instead through carbon bi-sulphide for greater ease 



* All experiments in which the animals have been allowed to recover from the 

 anesthetic — five extirpations of the pancreas and one duct ligation in the dog — were 

 performed by Prof. E. H. Starling. The exhaustion experiments were performed by the 

 writer. 



