302 A MANUAL OF PHYSIOLOGY 



and not by organic acids, even when present in much greater 

 strength.* Finally, when the bases and acid radicles of the 

 juice are quantitatively compared, it is found that there 

 is more chlorine than is required to combine with the 

 bases ; the excess must be present as free hydrochloric acid. 

 The quantity of gastric juice secreted is very great ; it has 

 been estimated at as much as 5 to 10 litres in twenty-four 

 hours, or five times as much as the quantity of saliva 

 secreted in the same time. But such estimates are loose 

 and uncertain. 



The great action of gastric juice is upon proteids. In this 

 two of its constituents have a share, the pepsin and the free 

 acid. One member of this chemical copartnery cannot act 

 without the other; peptic digestion requires the presence 

 both of pepsin and of acid ; and, indeed, an active artificial 

 juice can be obtained by digesting the gastric mucous 

 membrane with '2 per cent, hydrochloric acid. A glycerine 

 extract of a stomach 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 

 proteid 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. 



If we place a little fibrin in a beaker, cover it with -2 per cent, 

 hydrochloric acid, add a small quantity of pepsin or of a gastric 

 extract, 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. 378). If 

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

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

 still later, peptones 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. Similar, but not identical, intermediate sub- 

 stances occur in the digestion of the other proteids, as well as in that 

 of bodies like gelatin, which are not true proteids, 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 intermediate products of the digestion of albumin ; 



* A dilute solution of congo-red is turned violet by organic and blue by 

 inorganic acids ; the gastric juice turns it blue. Methyl violet is rendered 

 blue by an inorganic acid like hydrochloric acid, and green if more of the 

 acid be added. It is not altered by organic acids. Gastric juice turns it 

 blue. 



