COMPOSITION AND PROPERTIES OF MILK 287 



is believed, derived from the degeneration of the nucleus above 

 referred to. These contents are, in time, discharged into the 

 cavity of the alveolus, and the mixture forms what we know as 

 milk. 



It will be seen that milk is not directly secreted from the 

 blood, like saliva or urine. The process is to be regarded 

 rather as one of disintegration of the cellular substance of the 

 gland itself. The latter is reformed out of the materials 

 derived from the lymph (the blood plasma which exudes 

 through the thin walls of the capillary vessels), and the process 

 is repeated indefinitely. Leucocytes (white blood corpuscles) 

 occasionally find their way from the blood into the milk ; but 

 none of the normal constituents of milk are derived directly 

 from the blood. 



The organic constituents of milk do not occur ready formed 

 in the blood. They must, therefore, be produced in the gland. 

 The same inorganic (ash) constituents occur in both, but they 

 are present in different proportions in the two fluids. Blood, 

 for instance, contains about 0*42 per cent, of soda, and 

 only 0-056 per cent, of potash, whereas milk contains about 

 o'i3 per cent, of the latter, and only 0^07 per cent, of the 

 former. 



It follows from these premises, i.e. that milk is derived 

 from the substance of the gland and not directly from the 

 blood, that the yield and quality of the milk cannot be directly 

 influenced or only to a limited extent by the food of the 

 animal (cf. p. 176). 



Chemical Composition. The more important compounds 

 which enter into the composition of milk are water, fat, pro- 

 teins (nitrogenous matter), milk-sugar, and the " mineral salts," 

 or ash. The actual and relative proportions of these con- 

 stituents fluctuate within certain ascertained limits, presently 

 to be discussed. The average composition of the milk 

 obtained in England, Germany, and America is given in the 

 following table : 



