CONSTITUENTS OF MILK. 27 



It will be observed that from the foregoing analyses, the composition of the organs and 

 parts vary essentially from each other ; and though it. may not be precisely the same in 

 individuals at different times, still the departure from a certain standard is never very 

 wide. The law of the vital economy secures a composition constant within narrow limits. 



MILK: 



A secretion of the mammary glands, of a white color, and, like the blood, is of a com- 

 plex constitution. 



Milk, when pure and healthy, has an alkaline reaction. This alkaline reaction con- 

 tinues for periods varying according to the state of the weather, and the meteorological 

 condition of the atmosphere. The kind of milk, also, varies in this respect ; human milk 

 remaining longer in its alkaline condition than cow's milk. 



Milk consists of casein, a coagulable fluid in which fat vesicles may be observed under 

 the microscope. In addition to the casein and butter, certain salts and sugar are invariably 

 dissolved in a large quantity of water. When milk is allowed to stand, the fat vesicles or 

 butter rise to the surface. When boiled, casein rises to the surface and forms a pellicle. 

 The same substance is coagulated by rennet, sulphuric and other acids. The solid matter 

 of milk is composed of the following substances : 



I. II. 



Phosphate of lime 47 • 1 50 • 7 



Phosphate of magnesia 8*6 9-5 



Phosphate of peroxide of iron 1"4 l'O 



Chloride of potassium 29-4 27-1 



Chloride of sodium 4"9 5 - 



Soda 8-6 6-7 



The fluid matter of cow's milk has the following constitution in 1000 parts : 



I. II. 



Water 857-0 853-0 



Solid constituents 143-0 177-0 



Butter 40-0 38-9 



Casein 72-0 69-8 



Sugar and extractive matter 28-0 31-3 



Fixed salts 6-2 Simon. 7 - Herberger. 



The milk may be diseased, by the general disease or" the animal. When the cow has 

 vaccinia, or vaccine disease, the butter is diminished, and pus is found mixed with casein. 

 The milk from a healthy teat contains no pus, and has the appearance of healthy milk. 

 Cows affected by the grease, do not yield healthy milk. 



Milk is also liable to certain changes which can not be explained very satisfactorily. It 

 sometimes becomes blue and yellow, from the presence of two species of animalcula : the 



