VARIATIONS IN THE COMPOSITION OF MILK. 337 



fluidity of the caseine. It is not determined precisely in what form iron 

 exists in the milk, but its presence here is undoubted. A comparison of the 

 composition of the milk with that of the blood shows that most of the impor- 

 tant inorganic matters found in the latter fluid exist also in the milk. 



Hoppe has indicated the presence of carbon dioxide, nitrogen and oxygen, 

 in solution in milk. Of these gases, carbon dioxide is the most abundant. 

 It is well known that the presence of gases in solution in liquids renders them 

 more agreeable to the taste, and carbon dioxide increases very materially 

 their solvent properties. Aside from these considerations, the uses of the 

 gaseous constituents of the milk are not apparent. 



In addition to the constituents given in the table of composition, the 

 milk contains small quantities of peptone, nucleine, dextrine, urea, lecithine, 

 hypoxanthiue, fluorine and silica. 



A study of the composition of the milk fully confirms the fact that this 

 is a typical alimentary fluid and presents in itself the proper proportion and 

 variety of material for the nourishment of the body during the period when 

 the development of the system is going on with its maximum of activity. 

 The form in which its different nutritive constituents exist is such that they 

 are easily digested and are assimilated with great rapidity. 



Variations in the Composition of Milk. If the composition of the milk 

 be compared at different periods of lactation, it will be found to undergo 

 great changes during the first few days. In fact, the first fluid secreted after 

 parturition is so different from ordinary milk, that it has been called by an- 

 other name. It is then known as colostrum, the peculiar properties of which 

 will be considered more fully under a distinct head. As the secretion of milk 

 becomes established, the fluid, from the first to the fifteenth day, becomes 

 gradually diminished in density and in its proportion of water and of sugar, 

 while there is a progressive increase in the proportion of most of the other 

 constituents ; viz., butter, caseine and the inorganic salts. The milk, there- 

 fore, as far as one. can judge from its composition, as it increases in quantity 

 during the first few days of lactation, is constantly increasing in its nutritive 

 properties. 



The differences in the composition of the milk, taken from month to 

 month during the entire period of lactation, are not so distinctly marked. 

 It is difficult, indeed, to indicate any constant variations of sufficient impor- 

 tance to lead to the view that the milk varies much in its nutritive properties 

 at different times, during the ordinary period of lactation. The differences 

 between the milk of primiparae and multipart are slight and unimportant. 

 As a rule, however, the milk of primiparae approaches more nearly the normal 

 standard. 



In normal lactation, there is no marked and constant difference in com- 

 position between milk that has been secreted in great abundance and milk 

 which is produced in comparatively small quantity ; and the difference be- 

 tween the fluid first drawn from the breast and that taken when the ducts 

 are nearly empty, which is observed in the milk of the cow, has not been 

 noted in human inilk. 



