DIGESTION AND THE DIGESTIVE AGENTS. 333 



difference between a proteid and a peptone is in the power 

 of dialysis. A proteid dialyzes little if at all, while a pep- 

 tone does so readily. Egg albumens even when in a liquid 

 form, as we find it in an egg which has been subjected to 

 the process of incubation for several days, is by no means a 

 peptone. In spite of its liquid form it is but little dia- 

 lyzable, and if taken into the alimentary canal would have 

 to be transformed into dialyzable peptones in no less a way 

 than a bit of solid meat. Peptones further possess the 

 property of not being coagulated when subjected to heat. 

 This is, of course, also true of certain proteids; as, for in- 

 stance, the proteid in milk which may be boiled without 

 coagulating the milk. But the peptones, in addition to not 

 being coagulated by boiling, are not precipitated when 

 treated with alkalies, mineral acids or various salts. In 

 short, a peptone is an albumen in such a liquid form that it 

 is easily dialyzable and is not taken out of its liquid form 

 by any of the common reactions which coagulate and pre- 

 cipitate ordinary albumens. Every one is familiar with the 

 fact that most common albumens will coagulate when 

 heated, and that the albumen in milk will coagulate when 

 treated with an acid. This latter is the common process of 

 curdling. While the introduction of alkalies or strong 

 mineral salts will at once precipitate these albumens in the 

 form of white insoluble flakes it does not precipitate pep- 

 tones. 



It is not possible to get pepsin in an absolutely pure 

 form. For commercial purposes, however, it may be 

 secured with satisfactory purity. For these purposes it is 

 extracted from the gastric mucous membrane of various 

 animals, and then mixed with starch or sugar of milk and 

 placed on the market. For laboratory experiments to show 

 artificial gastric digestion the pepsin may be secured by 

 taking the gastric mucous membrane of an animal, cutting 

 it up very finely, and then extracting the pepsin from it 

 with glycerine. These glycerine extracts if properly diluted 

 and acidulated, readily digest proteids subjected to their 

 action. 



