694 PHYSIOLOGICAL CHEMISTRY. 



cool, and neutralize. Divide solution, and heat one portion with 

 Fehling's solution, when the formation of a red precipitate indicates 

 the conversion of glycogen into dextrose. To the second portion add 

 iodine : no change. 



5. To some glycogen solution add about half its volume of saliva, 

 keep the mixture at 40 C. (104 F.) for about ten minutes, and test 

 part of solution with iodine, the other portion with Fehling's solu- 

 tion. The results show that glycogen has been changed as in 

 previous test. 



The liver has also a neutralizing function, by virtue of which it 

 retains and renders innocuous various toxins and putrefactive pro- 

 ducts which are absorbed by the intestine. 



57. MILK. 



General properties. Milk is the secretion of the mammary 

 glands, the presence of which is characteristic of mammalia. The 

 milk of different animals differs somewhat in composition, but it 

 always contains all the constituents necessary for a normal develop- 

 ment of the various tissues, liquids, organs, etc., of the young 

 mammal, which generally feeds exclusively upon milk for a shorter 

 or longer period of its early life. 



Milk is an opaque, aqueous solution of casein, albumin, lactose, and 

 inorganic salts, holding in suspension small globules of fat, invested, 

 most likely, with coatings of casein or with some other albuminous 

 envelope. The reaction of woman's milk and that of the herbivora 

 is normally alkaline, but that of carnivora is acid. Its specific 

 gravity ranges from 1.029 to 1.033, but may in extreme cases vary 

 between 1.018 and 1.045. 



Experiment 88. a. Examine milk microscopically ; notice the variously sized 

 globules of fat, and compare the appearance of milk, cream, and skimmed milk. 



b. Test with sensitive litmus-paper the reaction of fresh cows' milk and of 

 milk that has been exposed to the air for a day or two. The former will be 

 alkaline or amphoteric, due to the presence of mono- and di-calcium phos- 



QUESTIONS. What is the active principle of saliva, and how does it act on 

 starch? Explain the process of the absorption of protein. State the compo- 

 sition of gastric juice, explain its physiological action, and describe methods 

 for determining its chief constituents. What substances are formed during the 

 conversion of a simple protein into peptone? What are the functions of ^pan- 

 creatic juice? State the composition of the different kinds of calculi found in 

 feces. How are fats digested and absorbed ? State the general properties of 

 bile and mention its chief constituents ; describe Gmelin's and Pettenkofer's 

 tests. What are the principal constituents of feces ? State properties and re- 

 actions of glycogen. 



