ion: FIG. 16.--PHAGOCYTOSIS. _a_, _b_, _c_ are the
microphages or the bacterial phagocytes. (_a_) Contains a number of
round bacteria, and (_b_) similar bacteria arranged in chains, and
(_c_) a number of rod-shaped bacteria (_d_) Is a cell phagocyte or
macrophage which contains five red blood corpuscles.]

All life in the tissues depends upon the circulation of the blood.
There is definite relation between the activity of cells and the blood
supply; a part, for instance, which is in active function receives a
greater supply of blood by means of dilatation of the arteries which
supply it. If the body be exactly balanced longitudinally on a
platform, reading or any exercise of the brain causes the head end to
sink owing to the relatively greater amount of blood which the brain
receives when in active function. The regulation of the blood supply
is effected by means of nerves which act upon the muscular walls of
the arteries causing, by the contraction or the relaxation of the
muscle, diminution or dilatation of the calibre of the vessel. After
injury the dilatation of the vessels with the greater afflux of blood
to the part is the effect of the greatly increased cell activity, and
is a necessity for this. In many forms of disease it has been found
that by increasing the blood flow to a part and producing an active
circulation in it, that recovery more readily takes place and many of
the procedures which have been found useful in inflammation, such as
hot applications, act by increasing the blood flow. So intimate is the
association between cell activity, as shown in repair and new
formation of cells, and the blood flow, that new blood vessels
frequently develop by means of which the capacity for nutrition is
still more increased. The cornea or transparent part of the eye
contains no blood vessels, the cells which it contains being nourished
by the tissue fluid which comes from the outside and circulates in
small communicating spaces. If the centre of the cornea be injured,
the cells of the blood vessels in the tissue around the cornea
multiply and form new vessels which grow into the cornea and appear as
a pink fringe around the periphery; when repair has taken place the
newly formed vessels disappear.

The exudate from the blood vessels in various ways assists in repair.
An injurious substance in the tissue may be so diluted by the fluid
that its action is minimized. A small crystal of salt is irritating to
the eye, bu