36 SCIENCE PROGRESS. 



heating with dilute mineral acids or superheated steam. 

 The final product of this action is called peptone, and this 

 substance compared to the original albumin is very diffusible. 

 But between the albumin and the peptone are several 

 intermediate stages of intermediately diffusible substances. 

 One of these produced in small quantities is acid albumin, 

 but the oreater number come under the "eneral headino- of 

 the proteoses. 



The pancreatic digestion of proteids is closely similar ; 

 it, however, occurs in an alkaline medium, and is more 

 energetic, and, moreover, if long continued leads to a 

 breaking up of some of the peptone into simpler nitrogenous 

 substances like leucine, tyrosine, aspartic acid, and ammonia. 



It should be mentioned that though highly diffusible 

 compounds like proteoses and peptone are formed in the 

 alimentary canal, none find their way as such into the 

 circulating" blood, even during the periods of most energetic 

 digestion. These substances are in fact pretty powerful 

 poisons when injected into the blood stream. The epithel- 

 ial lining of the alimentary canal normally protects us from 

 these toxic products by once more dehydrating the peptone 

 in virtue of the vital activity of its cells. Though lymphoid 

 tissue, which is abundant in the intestinal walls, has been 

 considered by some to have a share in this action, most 

 observers are pretty well agreed that it is the columnar 

 epithelium which is the main agent in the " regeneration of 

 albumin ". 



But to return to the products of digestion, it was very 

 soon recognised that they are numerous ; Meissner described 

 the varieties of peptone as parapeptone, dyspeptone, meta- 

 peptone, a, b and c peptone. Schmidt-Mulheim distin- 

 guished between parapeptone, propeptone and peptone. 

 Parapeptone is the acid albumin, and propeptone is a very 

 good name for what we now call the proteoses. Nearly 

 all of our present knowledge of the chemistry of digestion 

 is due to the work of Kuhne and those associated with him 

 in his researches, particularly Chittenden and Neumeister. 

 A most valuable method of isolating peptone was discovered 

 by Wenz, one of Kuhne's pupils. It consists in the use of 



