352 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



As has been already stated, albumen is precipitated from its solutions 

 by boiling or by a concentrated mineral acid. If the filtrate from this 

 atitodigestion of the stomach is boiled no coagulum will be formed, nor 

 will an addition of nitric acid cause any precipitate. If albumen is 

 present in this solution, it must, therefore, exist in a modified form. 



Millon's test and the xanthoproteic reaction will indicate in this 

 filtrate the presence of an albuminoid body. If the filtrate be neutralized 

 by the careful addition, drop by drop, of a little liquor potassse, when the 

 fluid is perfectly neutral a precipitate will be formed. These tests show 

 that while albuminoid bodies do exist in the filtrate, they have been trans- 

 formed by the gastric secretions into other members of the proteid group. 



Recollecting the statement made in a preceding chapter as to the 

 effect of dilute acid on albumen, it was found that if a dilute acid was 

 added to a solution of albumen, the albumen totalhlost its power of 

 coagulating by heat and was rendered insoluble in water. It was there- 

 fore thrown out of solution by neutralization. When such a solution of 

 acid albumen was exactly neutralized, the filtrate was found to be entirely 

 free from proteid in solution. If, on the other hand, in this experiment 

 of autodigestion by the stomach the precipitate produced by neutraliza- 

 tion is filtered off, the filtrate will still show the presence of proteid in 

 large amounts. The results of gastric digestion are not, therefore, entirely 

 identical to the action of a dilute acid, for we find a portion of proteid 

 which is still soluble in neutral solutions and is nevertheless not coagu- 

 lated by boiling ; consequently, the modifications of albuminoids produced 

 by the action of the gastric juice are not due solely to the acid alone. 



This fact can be still further demonstrated by a simple experiment. 



In four test-tubes may be placed some fragments of boiled blood-fibrin. In 

 one tube are placed ten cubic centimeters of hydrochloric acid of .02 per cent.; 

 in No. 2 are placed ten cubic centimeters of artificial gastric juice made by 

 adding a few drops of glycerin-pepsin extract to dilute hydrochloric acid, as 

 already described ; in No. 3 are placed ten cubic centimeters of the same arti- 

 ficial gastric juice carefully neutralized ; and in No. 4 ten cubic centimeters 

 of gastric juice thoroughly boiled. All of these tubes are then to be placed in 

 an oven heated by 40 degrees centigrade. At the same time, duplicates of tube 

 No. 2 are to be prepared, one being surrounded with ice and the other kept at the 

 temperature of the room. On examination of these tubes after four or five hours, 

 it will be found that in tube No. 1, which contained acid alone, the fibrin is 

 swollen up into a stiff jelly, but has not been dissolved. In No. 2, which con- 

 tained artificial gastric juice, the fibrin will have entirely disappeared. In No. 3. 

 which contained gastric juice neutralized, or, in otherwords, pepsin in solution, the 

 fibrin will be unaltered, while in No. 4, which contained the boiled gastric juice, 

 the appearances will be identical with those of No. 1. It may be learned from 

 this that fibrin is not dissolved by acid alone nor by pepsin alone, but that their 

 combination is necessary for its solution, while it is also seen that pepsin is 

 destroyed by heat. By referring to the other two tubes, it will be seen that the 

 fibrin will present the same appearance almost as seen in tube No. 1, showing, 

 therefore, that cold prevents the action of gastric juice. If, however, the tube 

 which was in the ice is placed in a warm oven, the fibrin will be rapidly digested, 

 showing that its solution was simply suspended by cold. The solvent action of 

 the gastric juice is, however, totally destroyed by boiling. If tube No. 1 be exactly 



