er from the white blood corpuscles
and also possibly from the platelets, the latter from the platelets or
from the tissue with which the blood comes in contact. The interaction
of these two bodies next results in the formation of prothrombin, which,
combining with the calcium of any soluble lime salt present, forms
thrombin or fibrin-ferment. The last step in the change is the action of
thrombin upon fibrinogen to form fibrin, and the clot is complete.

The intrinsic value to the animal of these changes is quite plain. The
power of clotting and thus stopping haemorrhage is of essential
importance, and yet this clotting must not occur within the living
blood-vessels, or it would speedily result in death. That the tissues
should be able to accelerate the process is of very obvious value. That
the inner lining of the blood-vessels does not act as a foreign tissue
is possibly due to the extreme smoothness of their surface.

Further, an animal must always be exposed to a possible danger in the
absorption of some thrombin from a mass of clotted blood still retained
within the body, and we know that if a quantity of active ferment be
injected into the blood-stream intravascular clotting does result. Under
all usual conditions this is obviated, the protective mechanism being of
a twofold character. First, it is found that thrombin becomes converted
very quickly into an inactive modification. Serum, for instance, very
quickly loses its power of inducing clotting in fibrinogen solutions.
Secondly, the body has been found to possess the power of making a
substance, antithrombin, which can combine with thrombin forming a
substance which is quite inactive as far as clotting is concerned.
Finally, there is evidence that normal blood contains a small quantity
of this substance, antithrombin, and that under certain conditions the
amount present may be enormously increased. (T. G. Br.)

_Pathology of the Blood._

The changes in the blood in disease are probably as numerous and varied
as the diseases which attack the body, for the blood is not only the
medium of respiration, but also of nutrition, of defence against
organisms and of many other functions, none of which can be affected
without corresponding alterations occurring in the circulating fluid.
The immense majority of these changes are, however, so subtle that they
escape detection by our present methods. But in certain directions,
notably in regard to the relations with