BY DR. LAUDER BRUNTON. 479 



the residue dissolved in a small quantity of water. A little of 

 the solution is now put in a test-tube, and a few drops of a 

 neutral solution of ferric chloride added. If acetic acid is 

 present, the fluid will become of a dark red color, and when 

 boiled will deposit a yellow precipitate. A solution of silver 

 nitrate may be added to second portion. If hydrochloric acid 

 is present, a white precipitate will fall, and will not be dis- 

 solved on adding nitric acid, but will be dissolved by ammonia. 

 To the remainder, dilute sulphuric acid is added, and the 

 mixture allowed to stand for some time. If butyric acid is 

 present, a smell like rancid butter will be perceived. The 

 residue of the gastric juice, which remained in the retort after 

 the hydrochloric and other acids were distilled off, is poured 

 into a large test-tube or flask, and agitated with ether. The 

 ethereal la}*er is then decanted off and evaporated over a water- 

 bath. If acetic acid be present in the gastric juice, it will 

 remain as an acid residue. Ci'3'stals of zinc lactate (square 

 prisms with one or two oblique surfaces at the ends) may be 

 obtained on allowing the residue to stand after the addition of 

 zinc, oxide, and water. 



108. Action of Gastric Juice. The power of gastric 



juice to dissolve coagulated albuminous substances is best 



shown by using fibrin from blood. To prepare fibrin the blood 



is to be stirred, as it flows from the vessel, with a rough stick 



or piece of ragged whalebone, and the fibrin collected and 



washed till it is perfectly white. It may be preserved for a 



considerable time under glycerin, from which it must be 



washed before it is used. Put a small piece of fibrin into a 



test-tube along with gastric juice, and place the tube for an 



/hour or two in the water-bath at 35 C. The fibrin will swellA 



Ibecome somewhat transparent, and then dissolve, forming an\ 



lopalescent fluid, which is not precipitated by boiling, and I 



(slightly, or not at all, by neutralization. As no other fluid 



except gasti'i' 1 . juice has this action on fibrin, the production 



of all these effects is used as a test for it, and is called the 



pepsin test. Pepsin alone will not produce them, however, 



unless free acid be present as it is in gastric juice. In this 



process, boiled fibrin may also be used as recommended by 



Kiihne. 



** 109. Artificial Gastric Juice. All the actions of gas- 

 tric juice can be more conveniently studied with an artificial 

 juice than with the natural secretion, as the former can be ob- 

 tained in much larger quantities. The method of preparing it 

 is as follows : Open the stomach of a newly-killed pig or rabbit, 

 or the fourth stomach of a calf, remove its contents and wash 

 it thoroughly with a gentle stream of water without much rub- 

 bing. Lay it on a piece of board with its mucous surface up- 

 wards, fasten it down with a few pins, and then with the back 



