PHYSIOLOGY OF MILK SECRETION. 199 



By many, yellowishness is regarded as an indication of richness. While 

 yellowish milk is usually rich, it does not necessarily follow that white milk 

 may not be rich. The yellowish character is to a certain extent also a 

 breed characteristic. The bluish color has the effect of neutralizing the 

 yellow and making the whole have a whiter appearance than it otherwise 

 would. 



The specific gravity of milk varies from 1.029 to 1.035, the average 

 being 1.032. The variation is with the per cent, of total solids present 

 and not with the per cent, of fat. A very high per cent, of fat will have 

 the effect of lowering the specific gravity, but the relation between the spe- 

 cific gravity and fat is not sufficiently close to warrant using the former 

 in estimating the latter. 



The viscosity of milk is greater than that of water, on account of the 

 character of the solids it contains, and especially upon the state of aggrega- 

 tion of the fat globules. 



The reaction of milk is variable. The milk of the herbivora is generally 

 described as normally alkaline, but at times it is also said to be possessed 

 of an amphigenic reaction, that is both alkaline and acid. In 1892, Pro- 

 fessor Huston of this station, made a series of experiments to determine 

 the reaction of milk, and obtained an acid reaction in all cases. His 

 method differed from those usually described and is as follows: To 25 c. c. 

 of milk 5 c. c. of normal H. Cl. solution was added to get a positive acid 

 reaction, then titrating with one-half normal potassic hydrate solution until 

 a neutral reaction occurred. In freshly drawn milk (28 samples) it re- 

 quired .01648 c. c. of normal potassic hydrate to neutralize 1 c. c. of warm 

 milk, or .01694 c. c. to neutralize the same quantity of cooled milk. 

 The acidity of milk after standing, is from changes which it undergoes 

 through bacterial fermentation. 



The milk of all animals shows essentially the same constituents, and 

 analysis shows that these apparently differ only in the relative proportion 

 of the several constituents present, a condition analogous to the body fat. 



The composition of milk is of a very complex character. It consists 

 of a mixture containing water, proteids, carbohydrates, organic and in- 

 organic acids, and animal salts. Minute quantities of ammonia, urea, 

 hypoxanthin, chyme, chyle, biliverdin, cholestrin, mucin, lecithin, kreatin, 

 leucin and ty rosin are sometimes present. 



A much used classification is as follows: Water, fats, albuminoids 

 and ash. All the constituents taken together, except the water are known 

 as milk solids. These comprise from 12 to 15 per cent, of the total. Vieth 

 compiled the results of 120,540 analyses, covering a period of eleven 

 years and found the average per cent, of solids in milk to be as follows: 



Total solids 12.9 



Solids not fat 8.8 



Fat. . 4.1 



