292 ANIMAL PHYSIOLOGY. 
Casein, with some fat, makes up the greater part of cheese. 
2. Fats——Milk is an emulsion—i. e., contains fat suspended 
in a fine state of division. The globules, which vary greatly in 
size, are surrounded by an envelope of proteid matter. This 
covering is broken up by churning, allowing the fatty globules 
to run together and form butter. 
Butter consists chiefly of olein, palmitin, and stearin, but 
contains in smaller quantity a variety of other fats. The ran- 
cidity of butter is due to the presence of free fatty acids, espe- 
cially butyric. 
The fat of milk usually rises to the surface as cream when 
milk is allowed to stand. 
3. Milk-sugar, which is converted into lactic acid, probably 
by the agency of some form of micro-organism, thus furnish- 
ing acid sufficient to cause the precipitation or coagulation of 
the casein. 
Milk-sugar. Lactic acid. 
C,H,,0, = 2C,H,0, 
Milk, when fresh, should be neutral or faintly alkaline. 
4, Salts (and other extractives), consisting of phosphates of 
calcium, potassium, and magnesium, potassium chloride, with 
traces of iron and other substances. 
It can be readily understood why children fed on milk rarely 
suffer from that deficiency of calcium salts in the bones leading 
to rickets, so common in ill-fed children. It thus appears that 
milk contains all the constituents requisite for the building up 
of the healthy mammalian body; and experiments prove that 
these exist in proper proportions and in a readily digestible 
form. The author has found that a large number of animals, 
into the usual food of which, in the adult form, milk does not 
enter, like most of our wild mammals, as well as most birds, 
will not only take milk but soon learn to like it, and thrive well 
upon it. Since the embryo chick lives upon the egg, it might 
have been supposed that eggs would form excellent food for 
adult animals, and common experience proves this to be the 
case; while chemical analysis shows that they, like milk, con- 
tain all the necessary food constituents. Meat (muscle, with 
fat chiefly) is also, of course, a valuable food, abounding in 
proteids. Cereals contain starch in large proportion, but also a 
mixture of proteids. Green vegetables contain little actual nu- 
tritive material, but are useful in furnishing salts and special 
substances, as certain compounds of sulphur which, in some ill- 
understood way, act beneficially on the metabolism of the body. 
