174 PHYSIOLOGY CHAP. 



a carbohydrate substance akin to the peptones. On the other 

 hand, the gastric juice has no effect on keratin, elastin, and certain 

 other substances of this group. 



The analysis of the percentage composition of proteins, pro- 

 teoses, and peptones, as performed by Kiihne and Chittenden, 

 suggests that the proteolytic process effected by the pepsin and 

 gastric juice consists not so much in a profound alteration of the 

 structure of the large original protein molecule, as in its sub- 

 division, accompanied by hydration, i.e. taking up of water. This 

 theoretical concept explains why many properties are common to 

 all the chemical aggregates represented by the products of diges- 

 tion in particular, the great facility with which the peptones 

 and their amino-acid derivatives are reconverted into natural 

 protein in order that the body may utilise them. 



Gastric juice acts on the caseinogeu of milk and clots it 

 in virtue of its chymosin or rennin, which (as we saw in the 

 last chapter) is an enzyme distinct from pepsin. According to 

 Hammarsten's admirable researches, this curdling is a process 

 quite distinct from the flocky precipitation of caseinogen, which 

 takes place in the presence of hydrochloric acid, and redissolves 

 on neutralisation. Chymosin splits the caseinogen of milk into 

 two substances a proteose, which remains dissolved in the serum 

 of milk, and is not precipitated by boiling or the addition of acids, 

 and the so-called casein or paracasein, which in combination with 

 the calcium salts of milk forms the true clot or cheese, which is 

 then digested by the action of the pepsin and hydrochloric acid. 



A phenomenon apparently analogous with curdling is that 

 first described by Danilewsky (1886) of the precipitation of a 

 clot from a highly concentrated solution of proteoses and peptones 

 (Witte's peptone) by chymosin, pepsin, and papain, as well as by 

 extracts of many organs (pancreas, liver, small intestine, etc.). If 

 the mixture is placed in the thermostat at 35 C., a more or less 

 abundant precipitate is formed after a certain time, to which 

 Sawjawlow gave the name of plastein and Kurajew of coagulose, 

 and as to the nature of which nothing definite is known. 



Since the normal stomach (or intestine) never contains a 

 concentrated solution of proteoses and peptones, Danilewsky's 

 phenomenon cannot be utilised in the complex study of digestion. 



It was believed for a long time that the gastric juice had no 

 important action on fats, starches, and sugars. According, however, 

 to Cash (1880) and Ogata (1881), neutral fats can be split up in 

 a minor degree, with liberation of fatty acids. Volhard (1900-2) 

 demonstrated a lipolytic ferment in the gastric juice. His pupil, 

 Stade, not only confirmed the existence of this ferment, but also 

 showed that it conforms to the Schiitz law. 



On the strength of this, Connstein (1904) concluded that the 

 lipolytic ferment is of great importance in the assimilation of 



