DIGESTION 325 



which is not too fresh also possesses peptic powers ; but it 

 requires the addition of a sufficient quantity of acid to render 

 them available. 



Well-washed fibrin obtained from blood is a convenient protein 

 for use in experiments on digestion. Since the blood contains 

 traces of pepsin, the fibrin should be boiled to destroy any which 

 may be present (see also p. 422). 



If we place a little fibrin in a beaker, cover it with gastric juice 

 obtained from a dog or with 0*4 per cent, hydrochloric acid, to 

 which a small quantity of pepsin or of a gastric extract has been 

 added, and put the beaker in a water-bath at 40 C., the fibrin soon 

 swells up and becomes translucent, then begins to be dissolved, and 

 in a short time has disappeared (see Practical Exercises, p. 426). If 

 we examine the liquid before digestion has proceeded very far, we 

 shall find chiefly acid-albumin in solution ; later on, chiefly albu- 

 moses ; and of these the primary albumoses (proto-albumose and 

 hetero-albumose) are the first to appear in quantity, followed by 

 secondary or deutero-albumose (p. 10) . Still later, peptones in large 

 and always relatively increasing amounts will be present along with 

 the albumoses. From this we conclude that acid-albumin is a stage 

 in the conversion of fibrin into albumose, and albumose a half-way 

 house between acid-albumin and peptone. It must not be supposed, 

 however, that all the protein is first changed into acid-albumin before 

 any of the acid-albumin is changed into albumose, or that all the 

 protein has already reached the albumose stage before peptone 

 begins to appear. On the contrary, a certain amount of albumoses 

 and of peptones are present very early in peptic digestion, while the 

 greater part of the original protein is still unaltered. Similar, but 

 not identical, intermediate substances occur in the digestion of the 

 other proteins, including that of bodies like gelatin, which are not 

 ordinary proteins, but which pepsin can digest. The generic name 

 of proteose properly includes all bodies of the albumose type, the 

 term ' albumose ' itself being sometimes reserved for such inter- 

 mediate products of the digestion of albumin ; while those of fibrin 

 are called fibrinoses ; of globulin, globuloses ; of casein, caseoses ; 

 and so on. The peptones produced from different proteins are also 

 not absolutely identical. If the digestion is prolonged, the peptones 

 are in turn further hydrolysed, so that eventually a considerable 

 proportion of the original protein is converted into amino-acids and 

 other substances (p. 332). 



In the stomach, however, during the four or five hours for 

 which gastric digestion ordinarily lasts, little, if any, of the 

 protein passes beyond the stage of proteose and peptone, and it 

 is in these forms that the bulk, at any rate, of the protein food 

 enters the duodenum. The pancreatic juice, as we shall see 

 later on, not only effects a more complete conversion into 

 peptone, but can split up the whole or a very large proportion 

 of the peptone itself into substances which are no longer 

 protein. Since the subject of protein digestion must come up 

 again, it will be well to postpone any closer discussion of the 

 process till we can view it as a whole. In the meantime it is 



