

OF THE SALTS, BUTTEE, AND CUED OF MILK. 31 



particularly for showing that there is nothing whatever mysterious in 

 the casein or curd of milk arising from -the albuminous serum of the 

 mother's blood, and being transmuted into the fibrin structure of the 

 muscular tissues of the infant. 



Returning now to our examination of the composition of milk, as set 

 forth in the preceding table, we find that two respiratory el- The sugar and 

 ements are next upon the list : 1st. Sugar of milk, which is butter of milk - 

 to be converted into lactic acid, partly by the agency of the saliva, and 

 chiefly in intestinal digestion ; 2d. Butter, which is the oleaginous or 

 fatty portion, and of which a part is to be deposited in the adipose tis- 

 sues for a time of need, and a part, along with the lactic acid and excess 

 of sugar, is to be burned at once for the production of heat. 



The inorganic body, phosphate of lime, is necessary for the earthy por- 

 tion of the skeleton, and probably the reason of the introduction The ^^ of 

 of casein, to the exclusion of other protein compounds, depends milk, particu- 

 on the power it possesses of holding phosphate of lime in solu- eaSh^nd" 

 tion, not less than 6 per cent, of its weight of this earthy body chloride of so- 

 being often obtainable from it. Among the other salts of 

 the milk, chloride of sodium may be pointed out as of special importance. 

 It undergoes decomposition in the system of the infant, its hydrochloric 

 acid giving acidity to the gastric juice, its soda entering into the compo- 

 sition of the bile and various salivary secretions. It also imparts solu- 

 bility to albumen, and, in some degree, regulates the facility with which 

 that substance coagulates. It impedes the coagulation of fibrin. 



Milk is not a chemical compound, but a variable mixture of different 

 ingredients, which, under proper circumstances, may be sepa- Making of 

 rated. When the fluid is allowed to rest for some hours at the butter - 

 ordinary temperature, the fat-globules rise to the surface as cream, Vhich, 

 submitted to a strong agitation with air in the process of churning, forms 

 butter. 



The casein of milk can be readily coagulated by rennet (which is the 

 mucous membrane of the stomach of the calf) at a temperature Making of 

 of 120. If parted from the residual whey, mixed with a little cheese - 

 salt and yellow coloring matter, and subjected to the action of a suitable 

 press, it is formed into cheese. No better examples of the tissue-mak- 

 ing and heat-making elements of food can be offered than cheese and 

 butter respectively. 



When milk is exposed to the air, its sugar, under the influence of the 

 casein or curd, gradually disappears, turning into lactic acid, Lactic acid in 

 and the milk becomes sour. The composition of sugar and sour milk - 

 lactic acid is such, that we might, without much error, say that an atom 

 of sugar symmetrically bisected will yield two atoms of lactic acid. This 

 effect is produced by the casein commencing to pass into a state of de- 



