1 68 THE BLOOD. 
Theories of Coagulation. 
That the coagulation of the blood is due to the formation of an 
insoluble substance (fibrin) in the plasma, was proved by Hewson, 1 who 
showed that a coagulable plasma can be obtained by skimming, after 
allowing the corpuscles to subside, in blood the coagulation of which 
is delayed in any way, as by cold, by neutral salts, or by its retention 
within a living vein. The old theories which ascribed the coagulation to 
the cooling of the blood, to its coming to rest, to the running together 
of the corpuscles into rouleaux, were all effectually disproved by the 
same careful observer. Hewson also showed that fibrinogen (" coagul- 
able lymph ") is precipitable and removable from plasma by a tempera- 
ture of a little over 50° C. 2 Many of Hewson's observations upon 
coagulation were forgotten, and the facts rediscovered by subsequent 
observers, but their accuracy was such that until comparatively modern 
times no addition of any permanent value to the knowledge of the 
subject was made. The most important of such additions (which was 
also overlooked for many years) 3 was the observation of Andrew 
Buchanan, that a substance could be extracted by water and solutions of 
salt from lymphatic glands, from blood clot (especially the buffy coat), 
and from various tissues, which had the property of producing the 
coagulation of serous fluids, not themselves spontaneously coagulable, 
such as hydrocele and pericardial fluid ; such action being comparable 
to that of a ferment. But it is only quite recently that the active sub- 
stance extracted by Buchanan has been examined, and found to belong 
to the class of bodies known as nucleo-proteids. 
Schmidt's theory. — A theory of coagulation, which was long accepted, 
was that of Alexander Schmidt. Schmidt noticed that fluids which 
contained fibrinogen but were not spontaneously coagulable, such 
as pericardial or hydrocele fluid, coagulated on the addition of serum. 
He ascribed the fibrin formation which resulted to the action (fibrino- 
plastic action) of the globulin in the serum upon the fibrinogen of the 
pericardial fluid. Since, however, the same globulin is already present 
in abundance in pericardial and hydrocele fluid, it became clear that 
this explanation of the action of serum was insufficient. It was, how- 
ever, shown by Schmidt that a substance is extracted by water from the 
alcohol precipitate of blood or serum, which possesses the property of 
causing coagulation in these fibrinogenous liquids, or of causing coagula- 
tion in plasma, the coagulation of which has been prevented by the 
addition of neutral salts. To this substance the name of fibrin ferment 
was applied, on account of its action resembling in general that of the 
unorganised ferments or enzymes. Thus it was found to have its 
activity accelerated by warmth, and destroyed by a high tempera- 
ture (65° C), and also to be capable of producing the coagulation of a 
relatively large amount of fibrinogen. It was still held by Schmidt that 
the globulin of serum takes an important share in the formation of fibrin. 
Hammarstcn 's earlier researches. — Haminarsten showed that serum 
1 Op. cit. 
2 For the history of this see Schafer, Journ. Physiol., Cambridge and London, 1880, 
vol. iii. p. 185. 
3 Cf. Gamgee, "Physiol. Chemistry," 1880, vol. i., where will also be found an excellent 
account of the earlier history of the subject of blood coagulation. See also Arthus, " Coag. 
des liquides organiques," Paris, 1894, for a good epitome of the history of the subject up 
to that date. 
