the blood pressure and weakening down the heart, are so immensely
effective that a slash across the great artery of the thigh in the
groin of a dog will be closed completely before he can bleed to death.
So delicate and so purposeful is this adjustment that the blood will
continue as fluid as milk for ten, twenty, forty, eighty years--as long
as it remains in contact with healthy blood-vessels. But the instant it
is brought in contact with a broken or wounded piece of a vessel-wall,
that instant it will begin to clot. So inevitable is this result that it
gives rise to some of the sudden forms of death by bloodclot in the
brain or lung (apoplexy, "stroke"), the clot having formed upon the
roughened inner surface of the heart or of one of the blood-vessels and
then floated into the brain or lung.

Then take that matchless and ingenious process of the healing of wounds,
whose wondrousness increases with every step that we take into the
deeper details of its study. First, the quick outpouring and clotting of
the blood after enough has escaped to wash most poisonous or offending
substances out of the wound. This living, surgical cement, elastic,
self-moulding, soothing, not only plugs the cut or torn mouths of the
blood-vessels, but fills the gap of the wound level with the surface.
Here, by contact with the air and in combination with the hairs of the
animal it forms a tough, firm, protective coating or scab, completely
shutting out cold, heat, irritants, or infectious germs.

Into the wedge-shaped, elastic clot which now fills the wound from
bottom to top like jelly in a mould, the leucocytes or white blood-cells
promptly migrate and convert it into a mesh of living cells. They are
merely the cavalry and skirmishers of the repair brigade and are
quickly followed by the heavy infantry of the line in the shape of cells
born of the injured tissues on either side of the wound. These join
hands across the gap, the engineer corps and the commissariat department
move up promptly to their support in the form of little
vein-construction switches, which bud out from the wounded
blood-vessels. The clot is transformed into what we term granulation
tissue and begins to organize. A few days later this granulation tissue
begins to contract and pull the lips of the wound together. If the gap
has not been too wide the wound will be completely closed, its lips and
deeper parts drawn together in nearly perfect line, separated only by a
thin