396 A TEXTBOOK OF PHYSIOLOGY 



bigger. In the rabbit, the orifice of the pancreatic duct is consider- 

 ably below that of the bile (35 centimetres). The structure of the 

 living gland, can be particularly well studied in this animal, since the 

 gland is spread in the mesentery in a thin, transparent layer which 

 can be examined under the microscope. The cells are seen to 

 resemble closely those of the parotid gland. Before secretion, they 

 have been observed to be swollen and distended with granules; after 

 secretion, the cells become shrunken, most of the granules have dis- 

 appeared, and are only seen near the lumen. 



In this gland, also, are certain little aggregations of tissue known 

 as the " islets of Langerhans." The exact significance of these has 

 been much debated. Some hold that they are the secreting alveoli 

 in an exhausted condition. It is now, however, generally conceded 

 that they represent the inclusion of another gland within the pancreas 

 a gland which is formed separate in certain fishes. The function of 

 these islets is discussed in the chapter on internal secretions (p. 511). 



The Pancreatic Juice can be obtained by the insertion of a cannula 

 into the duct, and making a temporary or permanent fistula. It is a 

 clear, slightly viscid, strongly alkaline fluid. The composition of 

 the juice varies, that secreted upon the establishment of a fistula 

 being as a rule considerably richer in solids than that secreted some 

 days later. 



Directly after Pancreatic 



Operation. Fistida. 



Water . . . . 90-08 97-68 



Total solids 9-92 2-32 



Organic 9-04 1-64 



Inorganic V. . .- 0-88 0-68 



In cases where a pancreatic fistula has resulted from an operation 

 in man, the amount of the secretion has been found to vary from 

 600 to 800 c.c. per diem, and to have a specific gravity of 1007. 



The organic constituents are due to the enzymes contained in it, 

 together with some heat-coagulable protein. There are also traces of 

 leucin, tyrosin, xanthin, and soaps. The inorganic salts are mostly 

 the chloride, carbonate, and phosphate of sodium. The alkalinity of 

 the juice is due to the two latter. In addition, there is present potas- 

 sium chloride and the phosphates of calcium and magnesium. 



This juice is the most powerful digestive fluid of the alimentary 

 canal. It contains several enzymes, the best known being the protein- 

 splitting (trypsin), fat-splitting (steapsin or lipase), starch-splitting 

 (amylopsin), and the rennet enzyme. It is probable that rennin 

 and trypsin are to be regarded as side-chains of the same body. 

 The other enzymes can apparently be separated from each other, and 

 are stated to vary in amount in the juice according to the nature of 

 the food. Thus, when the secretion on a milk diet is compared with 

 that on a bread diet, it is found to contain half the tryptic, one- 

 quarter the amylolytic, and six times the fat-splitting potency. 

 Further evidence of the independence of the enzymes is 



