COMPOSITION OF MILK. 29 



Food which is typically perfect, is presented by nature to the young 

 of various animals. In milk, or in the egg, we should ex- Miikasanarti- 

 pect to find whatever is necessary for the growth of the tis- cic of food: its 



i f .1 f *MI' * - M* A composition. 



sues, and for the performance 01 the junctions. An exam- 

 ination of milk will therefore illustrate the essential characters of the 

 different elements of food. 



Composition of JUiffc. 



Water 873. 



Casein 48. 



Sugar of milk 44. 



Butter 30. 



Phosphate of lime 2.30 



Other salts 2.70 



1000.00 



In this we notice, first, the large proportion of water present, almost 

 nine tenths of the whole amount. The double duty of this The water of 

 water has already been mentioned, to remove from the sys- milk - 

 tern effete substances which are not of a vaporous or gaseous form, and 

 which can not escape through the lungs, and to regulate the temperature 

 by evaporation. We might have added to these that it imparts a due 

 fluidity to the blood. These are conditions as necessary to the infant 

 as to the adult, and it should be remembered that two thirds of the 

 weight of the body are water. 



Next follows the nitrogenized principle casein, which is closely re- 

 lated in composition to muscular flesh. It is the tissue-mak- The casein of 

 ing, histogenetic, or nutritive element of the milk, and has been milk - 

 elaborated from the albumenoid substances of the mother's system. It 

 is to be converted into the muscular, gelatinous, and other soft tissues of 

 the infant. 



Casein is one of a group designated as the neutral nitrogenized bodies, 

 of which some of the more prominent are albumen, fibrin, Nature of pro- 

 and globulin. From an opinion that these all contain the tein bodies - 

 same organic radical, they are often termed the protein bodies. They 

 appear to exist in two different physical conditions, soluble and insolu- 

 ble in water; they all contain sulphur, and exhibit. a proneness to pass 

 into the putrefactive fermentation. As this takes place when they have 

 reached a certain stage of decay, they act upon other bodies as ferments. 

 Their constitution is represented in common by the formula 



C 48 H 36 M N, 



Of the whole group, albumen may be taken as the type and most import- 

 ant member. Indeed, as will be found hereafter, in the process 

 of digestion the others are invariably converted into it. The 

 white of the egg and the serum of the blood are usually referred to as 

 examples of albumen, though they differ in several particulars from one 



