BLOOD CLOTTING 



109 



cause the clotting sets in gradually and it is difficult to tell precisely 

 when it occurs. 



3. The blood must always be collected in the same sort of vessel and 

 come in contact with the same kind and amount of foreign material. 

 To this it may be added that the receiving vessel must be scrupulously 

 clean ; any trace of old blood clot or of serum is especially to be guarded 

 against. 



4. The end point must be sharp. It is here that the greatest technical 

 difficulties are met with in making precise measurements, and it is 

 greatly to be desired that different investigators should adopt some uni- 

 form method. For experimental purposes the method of Cannon and 

 Mendenhall 12 is no doubt the best, and it has the added advantage of 

 giving a graphic record of the observations. The accompanying figure 

 (Fig. 19) shows the principle of the method. The blood is received 

 through a standard cannula (C) into a tube (T) 5 cm. long and of 5 mm. 



Fig. 19. Diagram of the graphic coagulometer. The cannula at ihe right rested in a water 

 bath not shown in this diagram. For further description see text. (From Cannon and Men- 

 denhall.)* 



internal diameter; and a loop (of 2 mm. diameter) at the end of a 

 copper wire (D), which is 8 cm. long and 0.6 mm. in diameter, is al- 

 lowed to fall gently into the blood at regular intervals. The upper end 

 of the wire is articulated with the short arm of a light lever so counter- 

 poised that when the stop (R), which ordinarily holds it in a horizontal 

 position, is released, the wire, now having a net weight of 30 mg., falls 

 on the blood in the tube. The long arm of the lever is provided with 

 a writing point, which is made to inscribe its movements on a drum. 

 So long as the blood is unclotted the loop sinks into it when the lever 

 is released and a vertical line is traced, but whenever clotting occurs 

 the loop sticks on the blood and the writing point does not rise. 



For clinical purposes where blood collected in a syringe by venipuncture 

 is used, the method of Howell 13 is most accurate. It consists in placing 



*Am. Jour. Physiol., May 1, 1914, xxxiv, No. 2. 



