IX.] 



GASTRIC DIGESTION. 



characters of the final product peptone, and the intermediate 

 albumoses in contrast with those of a native proteid like albumin. 



(The b indicates precipitated.) 



7. Action of Gastric Juice on Milk. 



(a.) Mix 5 cc. of fresh milk in a test-tube with a few drops of 

 neutral artificial gastric juice ; keep at 40 C. In a short time the 

 milk curdles, so that the tube can be inverted without the curd 

 falling out. By-and-by whey is squeezed out of the clot. The 

 curdling of milk by the rennet ferment present in the gastric juice 

 is quite different from that produced by the " souring of milk," 

 or by the precipitation of caseinogen by acids. Here the casein 

 (carrying with it most of the fats) is precipitated in a neutral fluid. 



(6.) To the test-tube add 5 cc. of 0.4 per cent, hydrochloric acid, 

 and keep at 40 C. for two hours. The pepsin in the presence of 

 the acid digests the casein, gradually dissolving it, forming a straw- 

 yellow-coloured fluid containing peptones. The " peptonised milk " 

 has a peculiar odour and bitter taste. 



(c.) Peptonised Milk. To 5 cc. of milk in a test-tube add a 

 few drops of Benger's liquor pepticus, and place in a water-bath. 

 Observe how the caseinogen first clots, and is then partially dissolved 

 to form a yellowish-coloured fluid, with a bitter taste and peculiar 

 odour. There generally remains a very considerable clot of casein ; 

 and, in fact, the gastric digestion of milk is slow, especially if com- 

 pared with its tryptic digestion (Lesson X. 11). Test the fluid for 

 peptones with the biuret reaction, and observe the light-pink colour 

 obtained. The bitter taste renders milk " peptonised " by gastric 

 juice unsuitable for feeding purposes. 



8. Action of Rennet on Milk. (Rennin the enzyme.) 



(a.) Place milk in a test-tube, add a drop or two of rennet, and 



