230 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



given to it. It was quickly found, however, that the process was complicated 

 that in the conversion to so-called " peptone " the proteid under digestion 

 passed through a number of intermediate stages. The intermediate products 

 were partially isolated and were given specific names, such as acid- albumin, 

 parapeptone, and propeptone. The two latter names, unfortunately, have not 

 always been used with the same meaning by authors, and latterly they have 

 fallen somewhat into disuse, although they are still frequently employed to 

 indicate some one or other of the intermediate stages in the formation of pep- 

 tones. The most complete investigation of the products of peptic digestion, 

 and of proteolytic digestion in general, we owe to Kiihne and to those who 

 have followed along the lines he laid down, among whom may be mentioned 

 Chittenden and Neumeister. Their work has thrown new light upon the 

 whole subject and has developed a new nomenclature. In our account of the 

 process we shall adhere to the views and terminology of this school, as they 

 seem to be generally adopted in most of the recent literature. It is well, 

 however, to add, by way of caution, that investigations of this character are 

 still going on, and the views at present accepted are liable, therefore, to 

 changes in detail as our experimental knowledge increases. Without giving 

 the historical development of Kiihne's theory, it may be said that at present 

 the following steps in peptic digestion have been described : The proteid 

 acted upon, whether soluble or insoluble, is converted first to an acid-albumiii 

 (see Chemical section) to which the name syntonin is usually given. In arti- 

 ficial digestions the solid proteid usually first swells up from the action of the 

 acid, and then slowly dissolves. Syntonin has the general properties of acid- 

 albumins, of which properties the most characteristic is that the albumin is 

 precipitated upon neutralizing the solution with dilute alkali. If, in the begin- 

 ning of a peptic digestion, the liquid is neutralized, a more or less abundant 

 precipitate of syntonin will form, the quantity depending upon the stage of 

 digestion. The formation of syntouin is due mainly to the action of the HC1, 

 although the acid seems to be much more effective in combination with pepsin 

 than in simple aqueous solutions of the same strength. Syntonin in turn, under 

 the influence of the pepsin, takes up water and undergoes hydrolytic cleavage, 

 with the formation of two soluble proteids known together as primary albumoses 

 or proteases, 1 and separately as proto-proteose and hetero-proteose. Each of these 

 proteids again takes up water and undergoes cleavage, with the formation of 

 a second set of soluble proteids known as secondary proteoses, in contradis- 

 tinction to the primary proteoses, but to which the specific name of deutero- 

 proteoses is given. Finally, the deutero-proteose, or more properly the 

 deutero-proteoses, again undergo hydrolytic cleavage, with the formation of 

 what are known as peptones. Peptic digestion can go no farther than the 

 formation of peptones, but we shall find later that other proteolytic enzymes 



1 The term protease is used by some authors in place of the older name albumose, as it has a 

 more general significance. According to this usage the name albumose is given to the proteoses 

 formed from albumin, globulose to those formed from globulin, etc., while protease is a general 

 term applying to the intermediate products from any proteid. 



