616 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



in all of its characteristics closely resembles serum-albumen. It is 

 present in about 0.5 per cent., though in colostrum it is in much larger 

 percentage. It is there present in about 15 per cent., but rapidly de- 

 creases for about four weeks, when it reaches its average- percentage of 

 0.5 per cent. 



When the plasma of milk is slightly acidulated and boiled it coagu- 

 lates between 70° and 80° C. Peptone is also present in small amounts 

 as a transudation from the blood. 



3. Milk- Sugar. — Milk-sugar is an animal carbohydrate found only 

 in milk ; its average percentage is 4.5 per cent., varying from 3 to 1 per 

 cent. Of all the constituents of the milk the milk-sugar is least influ- 

 enced by external conditions. It is found dissolved in milk-serum and in 

 whey, whether after the spontaneous coagulation of the milk (acid whey) 

 or after the action of rennet (sweet whey). It may be obtained by coag- 

 ulating milk and evaporating the whey until crj^stals form. It occurs 

 with one molecule of water in rhombic prisms soluble in from five to six 

 parts of cold water and in three parts of boiling water. It is, therefore, 

 very insoluble in comparison with the other forms of sugar. It is only 

 slightly soluble in alcohol and water, and is not readily crystallizable. 



Its formula may be placed at C 6 H ls e . Heated up to 100° C. it 

 loses some of its water, and may thus be represented b} r the formula 



The specific rotation of milk-sugar containing water of crystalliza- 

 tion is -f 52.53° at 20° C. ; anhydrous milk-sugar = + 81.3°. When 

 warmed with alkalies it becomes brown, like dextrose (Moore's test), and 

 likewise reduces Fehling's solution. 



The milk-sugar of the human female, the cow, and the goat agree in 

 their chemical characteristics, their form of crystallization, and their 

 action on polarized light. 



The spontaneous coagulation of milk is due' to the formation of 

 lactic acid from milk-sugar, one molecule of milk-sugar forming four 

 molecules of lactic acid. The formula may be represented as follows :— 



Ci.H^O,! +H 2 0=4(C 3 H 6 3 ). 



When lactic acid forms, it unites with the alkaline phosphates to 

 form lactates of the alkalies and acid salts, and coagulation only occurs 

 when all the alkaline phosphates have been converted into acid salts. A 

 slight further development of lactic acid is then sufficient to cause coag- 

 ulation. 



If two glasses, one containing milk and the other a pure solution of 

 milk-sugar, are subjected to the same conditions, after a few-days the 

 former will contain so much lactic acid that the milk will be coagulated, 

 while not the slightest trace of acidity will be found in the milk-su^ar 



