What a great variety of ingredients must enter into the composition of the blood?
MRS. B.
You must observe that there is also a great variety of substances to be secreted from it. We may compare the blood to a general receptacle or storehouse for all kinds of commodities, which are afterwards fashioned, arranged, and disposed of as circumstances require.
There is another set of absorbent vessels in females which is destined to secrete milk for the nourishment of the young.
EMILY.
Pray is not milk very analogous in its composition to blood; for, since the nursling derives its nourishment from that source only, it must contain every principle which the animal system requires?
MRS. B.
Very true. Milk is found, by its analysis, to contain the principal materials of animal matter, albumen, oil, and phosphat of lime; so that the suckling has but little trouble to digest and assimilate this nourishment. But we shall examine the composition of milk more fully afterwards.
In many parts of the body numbers of small vessels are collected together in little bundles called glands, from a Latin word meaning acorn, on account of the resemblance which some of them bear in shape to that fruit. The function of the glands is to secrete, or separate certain matters from the blood.
The secretions are not only mechanical, but chemical separations from the blood; for the substances thus formed, though contained in the blood, are not ready combined in that fluid. The secretions are of two kinds, those which form peculiar animal fluids, as bile, tears, saliva, &c.; and those which produce the general materials of the animal system, for the purpose of recruiting and nourishing the several organs of the body; such as albumen, gelatine, and fibrine; the latter may be distinguished by the name of nutritive secretions.