All milk is composed of a watery serum called 'whey', and a
consistent substance called curd (or cheese); and the thicker the
milk, the more abundant the curd. The milk, then, of non-ambidentals
coagulates, and that is why cheese is made of the milk of such animals
under domestication; but the milk of ambidentals does not coagulate,
nor their fat either, and the milk is thin and sweet. Now the
camel's milk is the thinnest, and that of the human species next after
it, and that of the ass next again, but cow's milk is the thickest.
Milk does not coagulate under the influence of cold, but rather runs
to whey; but under the influence of heat it coagulates and thickens.
As a general rule milk only comes to animals in pregnancy. When the
animal is pregnant milk is found, but for a while it is unfit for use,
and then after an interval of usefulness it becomes unfit for use
again. In the case of female animals not pregnant a small quantity
of milk has been procured by the employment of special food, and cases
have been actually known where women advanced in years on being
submitted to the process of milking have produced milk, and in some
cases have produced it in sufficient quantities to enable them to
suckle an infant.