THE COAGULATION OF THE BLOOD 219 



also be mentioned that the effects of Witte's peptone, or of hirudin are 

 only temporary. It seems, therefore, that certain tissue cells possess 

 the power of rendering these substances inert, the probabiUty being 

 that this neutrahzation is brought about by the discharge of an anti- 

 coagulating agent. 



The fact that the blood does not clot while traversing the normal 

 circulatory channels, may therefore be explained in two ways, namely: 

 (a) by saying that thrombokinase is not liberated as long as the blood 

 is prevented from coming in contact with a destructive agent and (6), 

 that a certain amount of an anticoagulating substance is always pres- 

 ent in the blood which serves the purpose of retaining the thrombin 

 in its inactive condition. 



THE TIME OF COAGULATION 



The period intervening between the moment of the withdrawal of 

 the blood and the moment when it has assumed a jelly-Hke consistency, 

 is known as the coagulation time. Various methods have been devised 

 to determine its length, but none of them gives absolutely reliable 

 results. Vierordt^ employed a glass tube possessing a diameter of 1 

 mm. and a length of 5 cm. A white horse hair 

 having been placed lengthwise in this tube, the* [ j 



latter was then filled with the blood to be tested. 

 After a few moments the hair was withdrawn ' — i r\ B Tr" 



at intervals and a short distance each time, until 

 small coagula began to adhere to it. Possibly 

 the simplest procedure is to collect a small quan- 

 tity of blood in a test tube of ordinary size, ^'""'TiMETF^Bro'L^™'' 

 noting the time of its withdrawal, and to deter- ^^ chamber in which 

 mine again the moment when it is possible to drop is suspended from 

 invert this tube without causing the blood to """^,^''^ f "?^^ (^^ "^^^^ 



no 1 ocular of microscope. 



flow out. Brodie and RusselP advocate the 



following method: A drop of freshly drawn blood is placed upon 

 the poHshed tip of a conical piece of glass (Fig. 117). The latter is 

 then inverted and placed in a small compartment underneath a lens 

 magnifying thirty diameters. Very weak currents of air are brought to 

 bear upon the lateral surface of this suspended drop at intervals of 

 thirty seconds until the corpuscles cease to spin around and the ex- 

 ternal layers have assumed a gelatinous consistency. Btirker^ employs 

 a glass sHde the central area of which is depressed and surrounded by 

 a low wall of glass. A drop of boiled water is then placed in this com- 

 partment, to which is added a drop of fresh blood. The time of mixing 

 these fluids is accurately recorded by means of a kymograph and 



1 Archiv fur Heilkunde, 1878, 193. 



2 Jour, of Physiol, xxi, 1897, 403; also see^ Pratt, Archiv fur Exp. Path, 

 und Pharm., xlix, 1903, 299. 



3 Pfluger's Archiv, cii, 1904, 57. 



