124 THE BLOOD 



blood vessels. The calcium is present in the blood plasma at all times. 

 The thrombokinase originates in tissue cells of the blood and of the organs of 

 the body in general. 



Rettger's view is best given in his own words: " On the basis of the work 

 here presented it is not necessary to assume the existence of a kinase in explain- 

 ing the clotting of shed blood. The prothrombin formed from the platelets 

 and leucocytes by secretion or by processes of disintegration is activated to 

 thrombin by the calcium salts present, and the thrombin so formed combines 

 in quantitative fashion with the fibrinogen to form fibrin." 



Howell's demonstration of antithrombin offers a new factor in the 

 problem of blood clotting. For example, the hastening influence of keph- 

 alin on blood clotting is probably due to its action on antithrombin, 

 rather than on either of the other clotting complexes. 



The Coagulation Time of Blood. The rapidity with which blood co- 

 agulates varies greatly in different animals. In the majority of mammals 

 the coagulation time varies from 2.5 to 5 minutes. In man this time is 

 about 3 to 3:5 minutes in normal blood. In recent experiments by Cannon 

 and Mendenhall, on the coagulation time of the blood from the dog and 

 cat, determined by a new and mechanically accurate method, the normal 

 coagulation time is given as from 3 . 5 to 4. 5 minutes. This coagulation time 

 however varies under different conditions of the animal, especially under 

 conditions which affect the activities of the glands, in particular the epine- 

 phros. Stimulation of this gland either directly by stimulation of the 

 splanchnic nerves, or indirectly through conditions that arouse fear, etc., 

 leads to a sharp diminution in the coagulation time of the blood. This 

 decrease in some instances is well within i minute, less than ^ minute in 

 their experiment 3, which was an experiment after emotional excitement. 

 The coagulation time is retarded by the elimination of the circulation of 

 the intestine and of the liver, also by nephrectomy. 



Conditions Affecting Coagulation. From the preceding discussion 

 it is evident that the rapidity of the coagulation of the blood will be 

 influenced by anything that will influence the formation of the fibrin 

 factors or their interaction. The most important influences are the 

 following: 



Condition of the Blood. The blood varies greatly through a wide range 

 in its ability to form fibrin. This depends upon the interaction of those 

 tissues that produce the fibrin factors. An efficient circulation through the 

 abdominal viscera is necessary to maintain the clotting properties of the 

 blood. The rapidity of clotting is increased following the process of 

 digestion. It is also increased (Cannon) by the injection of epinephrin or 

 by the stimulation of the splanchnic nerves which increase the output of 

 epinephrin by the suprarenal bodies. (See Influence of the Ductless 

 Glands, page 482). If those glands be removed, the time of blood clotting 



