3 o8 A MANUAL OF PHYSIOLOGY 



fatty acid series. Tyrosin, C 3 H 4 (C 6 H 4 OH)(NH 2 )O 2 , is 

 related to propionic acid (C 3 H 6 O 2 ), another of the fatty acids. 

 If one H in propionic acid is replaced by NH 2 we get amido- 

 propionic acid, C 3 H 5 (NH 2 )O 2 . If another H is replaced 

 by oxyphenyl (C 6 H 4 .OH), an aromatic radicle, we get tyrosin. 



As much as 8 to 10 per cent, of leucin, and 2 to 4 per cent, of 

 tyrosin, may be produced in artificial tryptic digestion of fibrin (Lea, 

 Kiihne), but only a portion (about the half) of the peptones formed 

 ever undergoes this change, no matter how long the digestion. 



This and other facts have led to the theory that every natural 

 proteid consists of two elements as regards the products into which 

 it may be split by digestion a hemi element and an anti element. 

 Thus, albumin is supposed to consist of hemi-albumin and anti- 

 albumin. When digested by trypsin, the hemi-albumin gives rise 

 eventually to hemi-peptone, and the anti-albumin to anti-peptone.* 

 The hemi-peptone is comparatively unstable, and is further split up 

 into leucin and tyrosin ; the anti-peptone is comparatively stable, 

 and resists further change. 



As to the method in which the ferments bring about these pro- 

 found changes, and the role played by the auxiliary acid or alkali, 

 we are almost completely in the dark. Wurtz has supposed that 

 papain, a ferment obtained from the juice of the fruit of the Carica 

 papaya, which acts powerfully on proteids in much the same way as 

 trypsin, unites temporarily with the proteid with fibrin, for instance 

 and after the hydration of the latter is complete, is again set at 

 liberty, and free to act on some more of the unchanged fibrin. He 

 compares its action with that of some inorganic bodies, such as 

 sulphuric acid, a small quantity of which may cause the hydration of 

 a large amount of certain substances by forming temporary com- 

 pounds with them, and being then set free to act again. In peptic 

 digestion, however, the hydrochloric acid seems certainly to be used 

 up. In the gastric juice it is perhaps united to the pepsin ; and it is 

 capable of forming combinations with all proteids, the lower proteids, 

 such as peptone, combining with a greater proportion of the acid 

 than the higher, such as fibrin or albumin. 



In all that we have hitherto said regarding tryptic diges- 

 tion we have supposed that putrefaction has been entirely 

 prevented. If no antiseptic is added to a tryptic digest, it 

 rapidly becomes filled with micro-organisms, and emits a 

 very disagreeable faecal odour ; and now various bodies 

 which are not found in the absence of putrefaction make 

 their appearance, such as indol, skatol, and other sub- 

 stances to which the faecal odour is due. They are not true 

 products of tryptic digestion, but are formed by the putre- 

 factive micro-organisms, which can themselves break up pro- 

 teids into leucin and tyrosin, and change tyrosin into indol. 



