ond ferment which has been named thrombokinase, and
this latter is again absent from living blood. Hence the question
arises, whence are derived thrombogen and thrombokinase? In the study of
this question it has been found that if the blood of birds be collected
direct from an artery through a perfectly clean cannula into a clean and
dust-free glass vessel, it does not clot spontaneously. The plasma
collected from such blood is found to contain thrombogen but no
thrombokinase. A somewhat similar plasma may be prepared from a mammal's
blood by collecting samples of blood from an artery into vessels which
have been thoroughly coated with paraffin, though in this instance
thrombogen may be absent as well as thrombokinase. If plasma containing
thrombogen but no thrombokinase be treated with a saline extract of any
tissues it will soon clot. The saline extract contains thrombokinase.
This ferment can therefore be derived from most tissues, including also
the white blood corpuscles and the platelets. Thrombogen is produced
from the leucocytes, but it is not yet certain whether it is also formed
from the platelets. The discovery of the origin of the thrombokinase
from tissue cells explains a fact that has long been known, namely, that
if in collecting blood, it is allowed to flow over cut tissues, clotting
is most markedly accelerated. The fact that birds' blood if very
carefully collected will not clot spontaneously tends to prove that
thrombokinase is not derived from the leucocytes, and makes probable its
origin from the platelets, for it is known that birds' blood apparently
does not contain platelets, at any rate in the form in which they are
found in mammalian blood. When examining the general properties of
platelets, attention was drawn to the remarkably rapid manner in which
they undergo change on coming into contact with a foreign surface. It is
apparently the actual contact which initiates these changes, changes
which are fundamentally chemical in character, resulting in the
production of thrombokinase and possibly also of thrombogen.

Thus as our knowledge at present stands the following statement gives a
recapitulated account of the changes which constitute the many phases of
clotting. When blood escapes from a blood-vessel it comes into contact
with a foreign surface, either a tissue or the damaged walls of the cut
vessel. Very speedily this contact results in the discharge of
thrombogen and thrombokinase, the form