PRACTICAL EXERCISES 379 



nothing can be inferred as to the presence or the 

 nature of proteids in the liquid. 



(y) Heat another portion of the nitrate from (a) to 30 C., 

 and add crystals of ammonium sulphate to satura- 

 tion. A precipitate of proteoses (albumoses) may 

 be obtained. Filter off. 



(fl) Add to the filtrate from (y) a trace of cupric sulphate 

 and excess of sodium hydrate. A rose colour in- 

 dicates that peptones are present. More sodium 

 hydrate must be added than is sufficient to break 

 up all the ammonium sulphate, for the biuret 

 reaction requires the presence of free fixed alkali. 

 A strong solution of the sodium hydrate should 

 therefore be used, or the stick caustic soda. The 

 addition of ammonium sulphate will cause the red 

 colour to disappear; so will the addition of an 

 acid. Sodium hydrate will bring it back. Ammonia 

 does not affect the colour. 

 (e) To some milk in a test-tube add a drop or two of rennet 



extract, and place in a bath at 40 C. In a short time the milk is 



curuled by the rennin. 

 5. ( i ) To obtain Normal Chyme. Inject subcutaneously into a 



dog, one and a half hours after a meal of meat, 2 mg. of 



apomorphine. One-half of one of the ordinary tabloids is enough. 



Collect the vomit. 



(2) To obtain Pure Gastric Juice. Put a fasting dog under ether, 

 and fasten on the holder. Clip the hair and shave the skin in the 

 middle line below the sternum. Make a longitudinal incision 

 through the skin and subcutaneous tissue 



from the xiphoid cartilage downwards for 



3 or 4 inches. The linea alba will now 



be seen as a white mesial streak. Open 



the abdomen by an incision through it. 



Pull over the stomach towards the right, 



stitch it to the abdominal wall, open it, and 



insert a stomach-cannula (Fig. 114). By 



mechanically stimulating the mucous " cl NN u^ 



membrane of the stomach with a feather, 



or a smooth glass rod, or by the intioduction of pieces of indiarubber, 



a flow (not very copious) of gastric juice can be caused. 



(3) (a) Test the reaction to litmus of the chyme obtained in (i), 

 and of the pure juice obtained in (2). 



(b) Test their proteolytic powers by putting in a bath at 40 C. for two 

 hours two test-tubes containing respectively filtered chyme and fibrin, 

 and gastric juice and fibrin. The fibrin will be digested in both. 



(c) Add a few drops of the chyme and gastric juice to milk in two 

 test-tubes, and place them in a bath at 40 C. Repeat (c) after 

 neutralizing the liquids. Probably neither will curdle milk when 

 neutralized. For the gastric juice of many animals contains no 

 rennin, although the fully-formed ferment or its zymogen may be 



