56 SCIENCE? PROGRESS 
horse, together with the contained blood, by lgaturing the 
vessel in two places. The operation was carried out aseptically, 
with a minimum amount of damage. The corpuscles settled 
down, and from the uncoagulated plasma an insoluble fibrinogen 
separated out at 56°C. A substance varying in activity, which 
is destroyed by heat, can be obtained from coagulating or 
coagulated blood. Thisis thrombin, or fibrin-ferment (Alexander 
Schmidt, 1872), which does not exist in circulating blood, and 
therefore coagulation does not occur during life; but thrombin 
appears when blood dies, and the surplus which is not used is 
found subsequently in the serum. The ferment is destroyed by 
boiling. It cannot be obtained asa pure product. We recognise 
the substance, as is the case with all ferments, by its specific 
action, that of converting a soluble fibrinogen into an insoluble 
fibrin. Under the microscope certain changes can be witnessed 
in a film of shed blood. From isolated highly refracting bodies, 
the blood-platelets, or from groups of these, strands of less 
refractive material can be seen to extend, form a network, 
entangle both the red and white corpuscles, and form a clot 
(Mrs. E. Hart, 18791). 
These statements embody the main facts known in 1880, and, 
though they may be found in all text-books of physiology, it 
is convenient to mention them here, since the workers of the 
past twenty-five years have been compelled to take these certain 
facts into account, and, indeed, no theory of coagulation can 
ignore them. Theories are not necessarily explanations, and 
the successive output of work in connection with coagulation 
has tended to the creation of a somewhat mystifying terminology 
and to the construction of innumerable theories. Apart from 
original papers, special contributions in English,? French,’ 
and German volumes,‘ together with the admirable article by 
Morawitz,’ have rendered it possible to sift out the less definite 
from those more definite facts which have thrown light on the 
nature of the coagulation process both within and outside 
the body. 
The work of Alexander Schmidt himself, apart from that 
' Quart. Journ. of Microscopic Science, xxii, 1882. 
2 E. A. Schafer, 7ext-book of Physiology, vol. i. p. 168, 1898. 
3 Arthus, Za Coag. du Sang: Paris, 1899. 
4 E. Schwalbe, Untersuch, uber Blutgerinnung, 1890. 
5 Ergebnisse der Physiol. p. 307, 1905. 
