CHAP, i.] BLOOD. 21 



very rapid, the blood may clot before this change has time to come 

 on. When the temperature instead of being raised is lowered 

 below 40 C. the clotting becomes delayed and prolonged ; and at 

 the temperature of or 1 C. the blood will remain fluid, and yet 

 capable of clotting when withdrawn from the adverse circumstances, 

 for a very long, it might almost be said for an indefinite, time. 



A small quantity of blood shed into a small vessel clots sooner 

 than a large quantity shed into a larger one ; and in general the 

 greater the amount of foreign surface with which the blood comes 

 in contact the more rapid the clotting. When shed blood is 

 stirred or " whipped " the fibrin makes its appearance sooner than 

 when the blood is left to clot in the ordinary way ; so that here 

 too the accelerating inttuence of contact with foreign bodies makes 

 itself felt. Similarly, movement of shed blood hastens clotting, 

 since it increases the amount of contact with foreign bodies. So 

 also the addition of spongy platinum or of powdered charcoal, or 

 of other inert powders, to tardily clotting blood, will by inttuence 

 of surface, hasten clotting. Conversely, blood brought into contact 

 with pure oil does not clot so rapidly as when in contact with glass 

 or metal ; and blood will continue to ttow for a longer time without 

 clotting through a tube smeared inside with oil than through a 

 tube not so smeared. The inttuence of the oil in such cases is a 

 physical not a chemical one ; any pure, neutral, inert oil will do. 

 As far as we know, these influences affect only the rapidity with 

 which the clotting takes place ; that is, the rapidity with which the 

 fibrin makes its appearance, not the amount of clot, not the quan- 

 tity of fibrin formed, though when clotting is very much retarded 

 by cold changes may ensue whereby the amount of clotting which 

 eventually takes place is indirectly affected. 



Mere exposure to air exerts apparently little influence on the 

 process of clotting. Blood collected direct from a blood-vessel 

 over mercury so as wholly to exclude the air, clots, in a general 

 way, as readily as blood freely exposed to the air. It is only when 

 blood is much laden with carbonic acid, the presence of which is 

 antagonistic to clotting, that exclusion of air, by hindering the 

 escape of the excess of carbonic acid, delays clotting. 



These facts teach us that fibrin does not as was once thought 

 make its appearance in shed blood because the blood when shed 

 ceases to share in the movement of the circulation, or because the 

 blood is cooled on leaving the warm body, or because the blood is 

 then more freely exposed to the air ; they further suggest the view 

 that the fibrin is the result of some chemical change, the conversion 

 into fibrin of something which is not fibrin, the change like other 

 chemical changes being most active at an optimum temperature, 

 and like so many other chemical changes being assisted by the 

 influences exerted by the presence of inert bodies. 



And we have direct experimental evidence that plasma does 

 contain an antecedent of fibrin which by chemical change is 

 converted into fibrin. 



