232 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



substance that takes stains readily, while the inner portion tnrned toward 

 the lumen is rilled with conspicuous granules. In addition to this type of 

 cell, which is the characteristic secreting element of the organ, the pancreas 

 contains a number of irregular masses of cells of a different character (bodies 

 of Langerhans). These latter cells are clear and small, frequently have ill- 

 defined cell-bodies, but contain nuclei which stain readily with ordinary 

 reagents. By some these eel Is are supposed to be immature secreting cells of 

 the ordinary pancreatic type. By others it is thought that they are a separate 

 type of cell and take some special part in the secretory functions of the pan- 

 creas. Nothing definite, however, is known as to their physiological import- 

 ance. 



Composition of the Pancreatic Secretion. — The pancreatic secretion is 

 a clear alkaline liquid which in some animals (dog) is thick and mucilaginous. 

 Its phvsical characters seem to vary greatly, even in the same animal, accord- 

 ing to the duration of the secretion or the time siuce the establishment of the 

 fistula by which it is obtained (see p. 300). In a newly made fistula in the 

 dog the secretion is thick, but in a permanent fistula it becomes much thinner 

 and more watery. The main constituents of the secretion are three enzymes, 

 a large percentage of proteid material the exact nature of which is not known, 

 some fats, soaps, a slight amount of lecithin, and inorganic salts. The strongly 

 alkaline nature seems to be due chiefly to sodium carbonate, which may 

 be present in amounts equal to 0.2 to 0.4 per ceut. The three enyzmes are 

 known respectively as trypsin, a proteolytic ferment ; amylopsin, a diastatic 

 ferment, and steapsin, a fat-splitiug ferment. The action of these enzymes 

 in digestion is described in the section on Digestion. 



Action of the Nerves on the Secretion of the Pancreas. — In animals 

 like the dog, in which the process of digestion is not continuous, the secretion 

 of the pancreas is also supposed to be intermittent. A study of the flow of 

 secretion as observed in cases of pancreatic fistula indicates that it is connected 

 with the beginning of digestion in the stomach, and is therefore probably a 

 reflex act. Until recently, however, little direct evidence had been obtained 

 of the existence of secretory nerves. Stimulation of the medulla was known 

 to increase the flow of pancreatic juice and to alter its composition as regards 

 the organic constituents, but direct stimulation of the vagus and the sympa- 

 thetic nerves gave only negative results. Lately, however, Pawlow 1 and some 

 of his students have been able to overcome the technical difficulties in the way, 

 and have given what seems to be perfectly satisfactory proof of the existence of 

 distinct secretory fibres comparable in their nature to those described for the 

 salivary glands. The results that they have obtained may be stated briefly as 

 follow- : Stimulation of either the vagus nerve or the sympathetic causes, after 

 a considerable latent period, a marked flow of pancreatic secretion. The failure 

 of other experimenters to ^{ this result was due apparently to the sensitive- 

 ness of the gland to variations in its blood-supply. Either direct or reflex 



'Paw-Lav : /'» Bois-Reymond 's Archiv fur Physiologie, 1893, Suppl. Bd.; Mett: Ibid., 1894; 

 Kudrewetsky: Ibid., 1894 ; Pawlow: Die Arbeit der Verdauungsdriisen, Wiesbaden, 1898. 



