PEPSIN DIGESTION. 473 



bones, from which last the acid dissolves only the inorganic substances, 

 is converted into gelatin by digesting with gastric juice. The gelatin is 

 further changed so that it loses its property of gelatinizing and is con- 

 verted into gelatoses and peptone (see page 120). True mucin (from the 

 submaxillary) is dissolved by the gastric juice, yielding substances similar 

 to peptone, and a reducing substance similar to that obtained by boil- 

 ing with a mineral acid. Mucoids from tendons, cartilage, and bones 

 dissolve, according to POSNER and GiES, 1 in pepsin-hydrochloric acid, 

 but leave a residue which amounts to about 10 per cent of the original 

 material and which, as it seems, consists in great part, if not entirely, 

 of a combination of proteid with glucothionic acid (Chapters VI and 

 VII). The solution contains primary, and secondary mucoproteoses 

 and mucopeptones. The former contain glucothionic acid, but the latter 

 do not. Elastin is dissolved more slowly and yields the previously men- 

 tioned substances (page 117). Keratin and the epidermal formations 

 are insoluble. The nudeins are dissolved with difficulty, and the cell 

 nuclei, therefore, remain in great part undissolved in the gastric juice. 

 According to LONDON 2 and his collaborates the nucleic acids are not 

 attacked in the stomach. The animal cell-membrane is, as a rule, more 

 easily dissolved the nearer it stands to elastin, and it dissolves with 

 greater difficulty the more closely it is related to keratin. The mem- 

 brane of the plant-cell is not dissolved. Oxyhcemoglobin is changed into 

 haematin and protein, the latter undergoing further digestion. It is 

 for this reason that blood is changed into a dark-brown mass in the 

 stomach. The gastric juice does not act upon }at, but, on the contrary, 

 dissolves the c/ell-membrane of fatty tissue, setting the fat free. < Gastric 

 juice has no action on starch or the simple varieties of sugar. The 

 statements in regard to the ability of gastric juice to invert cane-sugar 

 are very contradictory. At least this action of the gastric juice is not 

 constant, and if it is present at all, it is probably due to the action of the 

 acid. 



Pepsin alone, as above stated, has no action on proteins, and an acid of the 

 intensity of the gastric juice can only very slowly, if at all, dissolve coagulated 

 albumin at the temperature of the body. Pepsin and acid together not only 

 act more quickly, but qualitatively they act otherwise than the acid alone, at 

 least upon dissolved protein. This has led to the assumption of the presence of 

 a pepsin-hydrochloric acid whose existence and action are only hypothetical. 

 As pepsin digestion, it seems, yields finally the same products as the hydrolytic 

 cleavage with acids, we can say for the present only that this enzyme acts like 

 other catalysts in very powerfully accelerating a process which would also pro- 

 ceed without the catalvte. 



1 Amer. Journ. of Physiol., 11. 



8 Zeitschr. f. physiol. Chem. 70, 72. 



