DIGESTION. 159 



filtration in a comparatively pure condition, and then redissolved in 

 water or an acidulated solution. The proportion of acid best adapted 

 for digestion is about 2 parts hydrochloric acid to 1000 parts of water. 

 The most convenient substance for showing the digestive powers of 

 such a fluid is coagulated fibrine, obtained by whipping fresh blood, 

 and cleansed from coloring matter by repeated washing with cold water. 



Physiological Action of the Gastric Juice. If gastric juice from the 

 living animal, or an acidulated solution of pepsine prepared by the 

 above method, be tested at the temperature of 38 C. with different 

 organic matters, it will be found that its action is confined to those of 

 a single class. It has no effect upon starches or fats ; but albuminous 

 matters, such as coagulated fibrine, caseine, or white of egg, or tissues 

 mainly composed of albuminous substances, are softened and liquefied, 

 and finally digested. The process by which this change takes place is 

 twofold, accomplished by the successive or simultaneous action of the 

 two essential constituents of the secretion. The first effect is produced 

 under the influence of the free acid, by which the albuminous matter 

 i- converted into syntonine. This substance is soluble in dilute acids, 

 and therefore assumes the liquid state in an acidulated solution ; but it 

 is not soluble in pure water nor in solutions of the neutral salts, and it 

 may accordingly be precipitated by neutralization with an alkali. So 

 far, the modification of albumen in the digestive act is comparatively 

 simple. Its further change is due to the presence of pepsine. By the 

 influence of this substance, acting as a ferment, the modified albu- 

 minous matter is transformed into peptone. Since peptone is soluble 

 in pure water and in neutral solutions, as well as in dilute acids and 

 alkalies, it retains the liquid form whatever may be the reaction of the 

 fluid in which it is contained. The non-precipitation of the albumenoid 

 matter, on neutralizing the solution, is therefore the indication and 

 measure of its complete transformation in the digestive proci 



As one of the distinctive features of peptone is its diffusibility through 

 animal membranes, it represents the condition of albumen when pre- 

 pared for absorption by the blood-vessels. It is not coagulable by heat, 

 the mineral acids, nor by potassium ferrocyanide, but is thrown down 

 from its solutions by alcohol in excess. 



The characters of peptone are the same, or nearly so, whether it be 

 derived from coagulated fibrine, albumen, caseine, or an organized 

 structure, like muscular or connective tissue. According to Henniger, 

 the only perceptible difference is in its rotary power on polarized light. 

 All varieties of peptone in solution deviate the plane of polarization 

 toward the left ; the amount of rotation being greatest for albumen 

 peptone, while that for fibrine peptone is the least. As to its nature, 

 it is the prevalent opinion among physiological chemists, that peptone 

 is a product of hydration ; the albuminous molecule uniting with the 

 elements of water under the influence of the gastric ferment. This 

 view is partly based on the elementary composition of peptone and its 

 power of uniting with acids and bases, as compared with albumen. It 



