PRACTICAL EXERCISES _ 453 



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

 excess of sodium hydroxide. A rose colour indicates 

 that peptones are present. More sodium hydroxide 

 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 hydroxide should therefore be used, or the 

 stick caustic soda. The addition of ammonium sul- 

 phate will cause the red colour to disappear; so will the 

 addition of an acid. Sodium hydroxide 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 curdled by 



the rcnnin. (See p. 347.) 



9. (i) To obtain Normal Chyme. Inject subcutaneously into a dog, 



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



apomorphme. Half of one of the ordinary tabloids is enough. Collect 



the vomit. 



(2) To obtain Pure Gastric Juice. If the laboratory possesses a dog 

 with Pawlow's double cesophageal and gastric fistula, the juice may 

 be obtained in large amount by sham feeding with meat (p. 395). If 

 not, proceed as follows: 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, and stitch it to 

 the abdominal wall, open it, and insert a 

 stomach cannula (Fig. 175). Make an incision Fig. 175. Stomach 



through the serosa and muscularis. Doubly Cannula. 



ligate and divide any vessels exposed in the 



submucosa. Then make an opening in the mucosa of sufficient size to 

 just admit the gastric cannula. This will go into a smaller opening if it 

 is provided with a nick in the flange which enters the stomach. Be 

 careful to prevent blood from getting into the stomach. Immediately 

 stitch the wound in the stomach over the flange of the cannula, but 

 do not pass the stitches through to the internal surface of the mucosa. 

 Suture the muscles and skin separately. Then stitch up the wound in 

 the abdomen. Wash out any stomach contents with warm water. Put 

 a cork in the cannula, and cover the animal with a cloth. The follow- 

 ing experiments may now be performed : Expose both vagi in the neck. 

 Connect two pairs of electrodes with the secondary coil of an induc- 

 torium arranged for single shocks. By means of a key in the primary 

 stimulate the nerves with slow rhythmical induction shocks at the rate 

 of about one a second. Continue the stimulation for fifteen minutes, 

 collect any juice that may have been secreted, and apply the tests in (3). 

 If secretion is slow, a little distilled water may be put into the stomach, 

 and the vagus stimulation repeated. Mechanical stimulation of the 

 mucous membrane with a feather causes no secretion of acid gastric 

 juice, but may cause a secretion of alkaline mucus. 



(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 



