ng, seem in this
respect to occupy an intermediate place between birds and cold-blooded
animals, such as reptiles and fishes.

EMILY.

The different degrees of firmness and solidity in the muscles of these
several species of animals proceed, I imagine, from the different nature
of the food on which they subsist?

MRS. B.

No; that is not supposed to be the case: for the human species, who are
of the mammiferous tribe, live on more substantial food than birds, and
yet the latter exceed them in muscular strength. We shall hereafter
attempt to account for this difference; but let us now proceed in the
examination of the animal functions.

The next class of organs is that of the _vessels_ of the body, the
office of which is to convey the various fluids throughout the frame.
These vessels are innumerable. The most considerable of them are those
through which the blood circulates, which are of two kinds: the
_arteries_, which convey it from the heart to the extremities of the
body, and the _veins_, which bring it back into the heart.

Besides these, there are a numerous set of small transparent vessels,
destined to absorb and convey different fluids into the blood; they are
generally called the _absorbent_ or _lymphatic_ vessels: but it is to a
portion of them only that the function of conveying into the blood the
fluid called _lymph_ is assigned.

EMILY.

Pray what is the nature of that fluid?

MRS. B.

The nature and use of the lymph have, I believe, never been perfectly
ascertained; but it is supposed to consist of matter that has been
previously animalised, and which, after answering the purpose for which
it was intended, must, in regular rotation, make way for the fresh
supplies produced by nourishment. The lymphatic vessels pump up this
fluid from every part of the system, and convey it into the veins to be
mixed with the blood which runs through them, and which is commonly
called venous blood.

CAROLINE.

But does it not again enter into the animal system through that channel?

MRS. B.

Not entirely; for the venous blood does not return into the circulation
until it has undergone a peculiar change, in which it throws off
whatever is become useless.

Another set of absorbent vessels pump up the _chyle_ from the stomach
and intestines, and convey it, after many circumvolutions, into the
great vein near the heart.

EMILY.

Pray what is chyle?

MRS. B.

It is the substance into which food is co