84 PHYSIOLOGICAL CHEMISTRY. 



the action of the acid gastric juice upon the albuminous elements of the 

 food. It is the first stage in the digestive process, by which all these 

 substances are reduced to the form of syntonine. It may be precipi- 

 tated from its solution in the gastric juice by careful neutralization 

 with an alkali. This is the only situation in the body in which 

 syntonine is known to be normally present. 



8. Peptone. 



This substance is the final product of the stomach digestion of albu- 

 minous matters. These matters, first converted into syntonine by the 

 acid of the gastric juice, are further transformed by the action of its 

 ferment or "pepsine." The result of this transformation is peptone; 

 and it appears to be essentially the same substance, whether derived 

 from the digestion of coagulated albumen, fibrine, myosine, or other 

 nutritious albumen oids. It is soluble in water, in dilate acid and 

 alkaline liquids, and in neutral saline solutions in all proportions. Even 

 strong mineral acids have no effect upon it. It is not coagulated by 

 heat, nor by potassium ferrocyanide in acidulated solution, a reagent 

 so prompt and effective for albuminous matters in general ; though it 

 is thrown down by alcohol in excess and by the metallic salts. It has 

 therefore acquired, in comparison with other substances of this group, 

 an increased range of solubility. 



But its most distinctive feature is its diffusibility. Unlike other 

 albuminous matters, it passes readily through animal membranes or 

 parchment paper. Comparative experiments on the two substances 

 show that the diffusibility of peptone is about twelve times as great as 

 that of albumen. By this means it is enabled to leave the cavity of 

 the alimentary canal, and to pass through the walls of the blood-vessels 

 into the circulation. 



The transformation of albuminous matters into peptone is a phenom- 

 enon of catalysis. It does not represent any fundamental change in 

 the chemical composition of these bodies, since the elementary analyses 

 of peptone, thus far made, show that it contains nearly the same pro- 

 portions of carbon, hydrogen, nitrogen, oxygen, and sulphur as the 

 substances from which it is produced. The principal modification 

 which takes place seems to consist in the assumption, by the original 

 albuminous matter, of the elements of water, that is, in a hydration. 

 This is claimed as proved by direct experiment ; and it is held by 

 observers of high repute,* that peptone is simply an albuminous 

 substance in its state of maximum hydration, retaining its chemical 

 qualities and nutritive value, but altered in its physical properties of 

 solubility and diffusibility. If this view be correct, peptone will 

 stand in its relation to albumen very much as glucose in its relation to 

 glycogen and starch. 



The fresh juices of growing plants, and especially of the succulent 

 vegetables, contain a nitrogenous substance coagulable by heat, and 



* Hoppe-Seyler, Physiologische Chemie. Berlin, 1878, p. 227. 



