IX.] 



GASTRIC DIGESTION. 



75 



characters of the final product peptone, and the intermediate 

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



(The 4, 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. 



(ft.) 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 



