110 



THE BLOOD AND THE LYMPH 



different investigators should adopt some uniform method. For experimental pur- 

 poses the method of Cannon and Mendenhall 1 '- is no doubt the best, and it lias 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. internal diameter; 

 and a loop (of 2 mm. diameter) at the end of a copper wire (Z>), which is 8 cm. long 

 and 0.6 mm. in diameter, is allowed 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. 



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

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

 denhall.)* 



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 2 or 4 c.c. of the blood in 

 a wide tube (of 21 mm. diameter), that has been cleaned by a bichromate-acid mixture. 

 The period that elapses between the moment of the entry of fluid into the syringe and 



Fig. 20. Coagulometer. The drop of blood is placed on the upper end of the glass cone and 

 the air stream is directed against it from the side tube shown by the black dot. The apparatus 

 is placed on the stage of the microscope and the drop observed by the low power. 



that at which the clot has become firm enough so that the tube can be inverted without 

 spilling any blood, is taken as the clotting time. Since the blood does not come in 

 contact with exposed tissues, it takes from 20 to 60 minutes to clot by this method. 



For routine clinical examination of blood taken from a skin wound Brodie and 

 Bussel's method 14 is most satisfactory. This consists, in principle, in observing a drop 

 of blood, under the low power of the microscope, while a fine current of air is gently 

 blown against it at regular intervals in a tangential direction. Until clotting sets in, 



*Am. Jour. Physiol., May 



1914, xxxiv, No. 2. 



