ues. That is, the capillaries are closed vessels, and the
tissues lie outside of them, as asbestos packing may be used to envelop
hot-water pipes. The space between the walls of the capillaries and the
cells of the tissues is filled with lymph. As the blood flows along
the capillaries, certain parts of the plasma of the blood filter through
their walls into the lymph, and certain parts of the lymph filter through
the cell walls of the tissues and mingle with the blood current. The lymph
thus acts as a medium of exchange, in which a transfer of material takes
place between the blood in the capillaries and the lymph around them. A
similar exchange of material is constantly going on between the lymph and
the tissues themselves.

This, then, we must remember,--that in every tissue, so long as the blood
flows, and life lasts, this exchange takes place between the blood within
the capillaries and the tissues without.

The stream of blood _to_ the tissues carries to them the material,
including the all-important oxygen, with which they build themselves up
and do their work. The stream _from_ the tissues carries into the blood
the products of certain chemical changes which have taken place in these
tissues. These products may represent simple waste matter to be cast out
or material which may be of use to some other tissue.

In brief, the tissues by the help of the lymph live on the blood.
Just as our bodies, as a whole, live on the things around us, the food and
the air, so do the bodily tissues live on the blood which bathes them in
an unceasing current, and which is their immediate air and food.

178. Physical Properties of Blood. The blood has been called the
life of the body from the fact that upon it depends our bodily existence.
The blood is so essentially the nutrient element that it is called
sometimes very aptly "liquid flesh." It is a red, warm, heavy, alkaline
fluid, slightly salt in taste, and has a somewhat fetid odor. Its color
varies from bright red in the arteries and when exposed to the air, to
various tints from dark purple to red in the veins. The color of the blood
is due to the coloring constituent of the red corpuscles, _haemoglobin_,
which is brighter or darker as it contains more or less oxygen.

[Illustration: Fig. 65.--Blood Corpuscles of Various Animals. (Magnified
to the same scale.)

A, from proteus, a kind of newt;
B, salamander;
C, frog;
D, frog after addition of acetic acid, show