174 ESSENTIALS OF PHYSIOLOGY. 



oxalate or of sodium fluoride to blood. (b) Sodium fluoride preci- 

 pitates not only calcium but also thrombogen, so that fluoride blood 

 will not clot on the subsequent addition of lime salts, (c) If one 

 volume of saturated solution of magnesium sulphate be added to four 

 volumes of blood, and the mixture be allowed to stand for twenty-four 

 hours, the thrombokinase is precipitated, and clotting ^will not take 

 place on dilution. For the first few hours after the addition of the 

 sulphate the clotting is merely retarded by the excess of salt, and the 

 blood will clot if it is diluted, (d) Sodium citrate may also be used 

 to prevent coagulation. It forms with calcium a double salt, calcium 

 sodium citrate, which is soluble, and in which the calcium is combined 

 with the acidic radical and does not become a free ion. Calcium will 

 not combine with thrombogen unless it is in the ionised condition. 



(3) (a) Coagulation may be prevented by cooling freshly shed blood 

 to C. (b) The addition to the blood of an equal volume of saturated 

 solution of sodium sulphate will prevent or delay clotting, but co- 

 agulation will take place when the mixture is diluted, showing that 

 the action of the salt is purely mechanical. 



(4) Hirudin, a substance obtained by extracting the glands in the 

 head of the leech, is an anticoagulin, and will prevent clotting either 

 if added to shed blood or if previously injected into a blood-vessel. 

 The injection of peptone will render blood incoagulable, and such blood, 

 when added to blood shed in the ordinary way, will prevent coagulation 

 of the latter ; but peptone itself has no retarding effect on coagulation 

 when added to shed blood. From experiments such as these it is 

 clear that peptone has no direct action in preventing coagulation, but 

 that it leads to the production in the liver of an anticoagulin, which 

 is discharged into the circulating blood. 



Coagulation Time. The time taken for coagulation of a sample 

 of blood may be conveniently estimated by means of Dale and Laidlaw's 

 coagulometer. This consists of a short capillary tube containing a 

 leaden shot. The finger is pricked and blood is run into the tube, 

 which is then immersed in water at a selected temperature, the ends 

 of the tube being closed. The tube is moved so as to keep the shot 

 rolling until the latter stops dead with the tube vertical. The time 

 between the appearance of blood on the finger and the stopping of the 

 shot is the coagulation time. It varies from one and a half minutes 

 at 40 C. to about eight minutes at 19 C. for normal blood. 



