226 THE BLOOD 



the cobra, for example, inhibits the coagulation even in very minute 

 doses in vivo as well as in vitro, because it prevents the conversion 

 of the prothT-ombin into thrombin. The venoms of other snakes, 

 for example, that of pseudechis porphytaceus,^ behave in the same 

 manner as tissue-extracts, but the question whether their action is 

 identical with that of thrombin or of thrombokinase, has not been 

 definitely decided. 



Defibrination. — It is possible to hasten the formation of the fibrin 

 by vigorously whipping the blood during its withdrawal with a rough 

 stick of wood or with a bundle of fine wires. The shreds of fibrin 

 then adhere to the wood, while the blood from which they have been 

 removed remains fiuid for an indefinite period of time. Obviously, 

 this procedure causes a rapid destruction of those cellular elements 

 from which the thrombokinase is derived. The fact that the fibrin 

 may be separated in this way, is made use of at times in rendering cer- 

 tain inoperative aneurysms less dangerous to life. The blood con- 

 tained in these saccular enlargements of the blood-vessels, is coagulated 

 by the insertion of several long needles of steel. Acting as foreign 

 bodies, these needles incite a deposition of fibrin in constantly in- 

 creasing mass until the entire lumen of the tumor has been occluded. 



Menstrual blood is commonly regarded as being non-coagulable. 

 This belief is erroneous, because coagula are always present in the 

 upper portion of the vagina. Only the fluid cruor mixed with mucus 

 escapes. It is true, however, that the mucus retards the clotting, 

 because it tends to smoothen the surface of this passage and to separate 

 the individual masses of fibrin more widely from one another. 



CHAPTER XX 



THE TOTAL QUANTITY AND DISTRIBUTION OF THE BLOOD. 



LOSS OF BLOOD 



Quantity of Blood. — It was formerly thought possible to determine 

 the total amount of blood present in an animal by simply opening an 

 artery and permitting the blood to escape until it ceased flowing. 

 It must be evident, however, that this procedure is open to certain 

 objections, because a considerable portion of the blood is always 

 entrapped in the finer ramifications of the vascular system as well as in 

 the central veins. Welker^ has advocated the following method. 

 A small amount of blood is withdrawn and diluted with saline solu- 

 tion in the proportion of 1 : 500. This mixture, designated as solution 



^ Martin, Jour, of Physiol., xxxii, 1905, 207. 



2 Zeitschr. fiir rat. Med., iv, 1858 (modified by Heidenhain). 



