44 A MANUAL OF PHYSIOLOGY 



great a part in the clotting of shed blood ; but we know that 

 leucocytes are constantly breaking down in the lymph and the 

 blood, and we have to inquire how it is that coagulation does not 

 occur, except in disease, within the vessels. Calcium is not wanting 

 to the circulating plasma, fibrinogen is not wanting, and it has already 

 been mentioned (p. 41) that a small amount of fibrin-ferment can 

 be obtained from the perfectly fresh and, as we might almost say, 

 still living blood. Why, then, does it not coagulate ? Some have 

 said that the quantity of fibrin-ferment is too small ; but if any is 

 present, some coagulation ought to occur if the conditions were 

 exactly the same as in a test-tube. Others have said that coagulation 

 is ' restrained ' by the contact of the living walls of the bloodvessels ; 

 but although it is certain that the contact of foreign matter, and all 

 dead matter is foreign to living cells, does hasten the destruction of 

 leucocytes, and therefore the liberation of fibrin-ferment, it is 

 evident that it is just this 'restraining' influence of the vessels 

 which has to be explained. Schmidt has attempted a chemical 

 explanation. He starts with the assumption that some ready-made 

 fibrin-ferment, or its precursor, exists not only in the circulating 

 blood, but in the circulating plasma, for he finds that the blood- 

 plasma of the horse, entirely freed from formed elements by 

 filtration through several folds of filter-paper at a temperature 

 of o to 0*5 C., remains fluid at the ordinary temperature of the 

 air for hours, but eventually coagulates. On this and other evidence 

 he bases the view that substances formed by the breaking down 

 of white blood-corpuscles in shed blood are not the only cause 

 of coagulation, although they undoubtedly greatly accelerate it. 

 According to Schmidt, a precursor, or mother-substance of fibrin- 

 ferment, is produced in the body from all, or most, proto- 

 plasmic cells, from white blood-corpuscles among the rest, but not 

 exclusively, nor even pre-eminently, from them. This substance 

 passes continually into the blood, and fibrin-ferment is continually 

 formed from it, but is always being neutralized by other chemical 

 processes. So that living blood within the living vessels may be said 

 to be acted upon by two sets of influences, one tending to coagula- 

 tion, the other opposing it. Under normal conditions, the processes 

 that make for coagulation never obtain the upper hand ; but any- 

 thing which interrupts the circulation, and consequently the free 

 interchange between blood and tissues, interferes with the entrance 

 of the substances that render the fibrin-ferment inactive. In the 

 clotting of extravascular plasma, free from corpuscles, Schmidt sees 

 the continuation, under modified conditions, of a normal process 

 always going on within the bloodvessels. In the lungs it would 

 seem that the forces which favour coagulation are feeble, or the 

 forces that resist it strong, for blood, after passing many times 

 through the pulmonary circulation without being allowed to enter 

 the systemic vessels, loses its power of clotting (Ludwig and Pawlow). 

 The liver is another organ whose relations to the coagulation of 

 the blood are peculiar. We have already mentioned that the injection 

 of commercial peptone, which consists chiefly of proteoses, into the 

 blood of dogs causes it to lose its coagulability. The effect gradually 



