

THE CLOTTING OF BLOOD. 31 



to be extracted by special methods, are called extractives ; of these some are 

 nitrogenous, some non-nitrogenous. Serum contains besides important inor- 

 ganic saline substances ; but to these we shall return. 



18. With the knowledge which we have gained of the proteids of clotted 

 blood we may go back to the question : Clotting being due to the appearance 

 in blood plasma of a proteid substance, fibrin, which previously did not exist 

 in it as such, what are the causes which led to the appearance of fibrin ? 



We learn something by studying circumstances which affect the rapidity 

 with which the blood of the same individual clots when shed. These are as 

 follows : 



A temperature of 40 C., which is about or slightly above the tempera- 

 ture of the blood of warm-blooded animals, is perhaps the most favorable to 

 clotting. A further rise of a few degrees is apparently also beneficial, or at 

 least not injurious ; but upon a still further rise the effect changes, and when 

 blood is rapidly heated to 56 C. no clotting at all may take place. At this 

 temperature certain proteids of the blood are coagulated and precipitated 

 before clotting can take place, and with this change the power of the blood 

 to clot is wholly lost. If, however, the heating be not 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 fluid, and yet capable of clotting when withdrawn from the adverse circum- 

 stances, 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 influence 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 influence of surface, hasten clotting. Con- 

 versely, 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 flow for a 

 longer time without clotting through a tube smeared inside with oil than 

 through a tube not so smeared. The influence 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 appear- 

 ance, not the amount of clot, not the quantity 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 bloodvessel over mercury so as 

 wholly to exclude the air, clots 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 



