ii EXTERNAL DIGESTIVE SECRETIONS 117 



cells of the fundus glands. The pyloric glands, which are destitute 

 of these cells, yield a persistently alkaline secretion. Miss Green- 

 wood provided direct evidence in favour of this theory, by showing 

 that when the gastric mucosa is treated with silver nitrate the 

 border cells alone stain black, while all the other cells, which do 

 not secrete acid, are unstained. 



(6) The dissolving (proteolytic] action of the gastric juice upon 

 proteins, which we shall examine in discussing digestion, is due to 

 a special enzyme, guessed at or noticed by Spallanzani, Eberle, 

 Beaumont, and Joh. Miiller, and to which Schwann in 1836 gave 

 the name of pepsin. 



Wassman first isolated it in the impure state. Briicke, 

 Wittich, Pettit, and others, perfected the methods of extraction 

 and purification in various ways, but little is even yet known as 

 to its chemical constitution, or if it contains nitrogen, and it is 

 doubtful whether it belongs to the protein group. 



Pepsin is an amorphous substance, greyish-yellow, odourless, 

 soluble in water and in glycerol, particularly if acidulated, insoluble 

 in alcohol, which precipitates it from its solutions. It is not 

 dialysable, and can thus be easily separated from the acids, salts, 

 and peptones, which dialyse readily (Hammarsteri). 



An acid medium is an indispensable condition to the exhibition 

 of the digestive activity of pepsin. In neutral solution it is inert ; 

 it is destroyed in an alkaline medium. It neither increases nor 

 diminishes in quantity during the process of digestion, but 

 according to Griitzner it loses some of its activity. 



In consequence of Wassinann's experiments, which showed 

 that the artificial juice prepared from the mucous membrane of 

 the fundus digests a given quantity of fibrin in an hour and a 

 half, while with that made from the mucous membrane of the 

 pyloric portion the same digestion requires 6 to 8 hours, it was 

 supposed that only the fundus glands secrete pepsin, and that the 

 pyloric glands secreted mucin only, the small amount of pepsin 

 they contain being due to infiltration of that secretion from 

 the fundus. 



The ingenious experiments of Heidenhain and his disciples 

 Ebstein and Griitzner, however, showed that the pyloric glands 

 also secrete pepsin, although to a minor extent, because the 

 glandular substance is less abundant there. Langendorff, again, 

 found pepsin in the pyloric part of the calf's embryo. But the 

 most decisive proof of the digestive activity of the juice secreted 

 by the pyloric end was given by Klemensiewicz and Heidenhain, 

 who showed that this part of the stomach when isolated and 

 converted into a cul de sac, secretes an alkaline juice containing 

 pepsin, so that it is capable of digesting protein on the simple 

 addition of acid. According to Klemensiewicz, this pyloric juice in 

 acid solution usually digests better than the secretion of the 



