22 A MANUAL OF VETERINARY PHYSIOLOGY 



as when the inner coat of the bloodvessel is injured, and the cells 

 then act as a foreign body. This may be seen in a ligatured 

 vessel, while pathologically the horse provides a classical example 

 in the injury occurring to the iliac arteries, which leads to 

 thrombosis. 



Circumstances influencing Coagulation. — Clotting in shed 

 blood may be retarded or hastened by certain conditions. 

 The blood of a horse received into a vessel so constructed as to 

 expose it to a freezing temperature may be kept fluid for an 

 indefinite period, though coagulation will at once occur when 

 the temperature is allowed to rise. The probable explanation of 

 the phenomenon is that the low temperature keeps the corpuscles 

 from disintegrating. 



Clotting is delayed by the addition to the blood of certain 

 neutral salts of the alkalies and alkaline earths ; the best salt to 

 employ for the purpose is magnesium sulphate. The plasma 

 so obtained is spoken of as salted plasma, and largely used in 

 physiological experiments on the blood. It is not known how 

 neutral salts prevent coagulation ; it may be by keeping 

 the corpuscles intact, or by inhibiting the conversion of pro- 

 thrombin to thrombin. The addition of dilute acetic acid or the 

 passage of a current of carbonic acid gas through blood prevents 

 coagulation by precipitating fibrinogen. The addition to blood 

 of a weak solution of potassium or sodium oxalate prevents 

 clotting by combining with lime. Without the presence of 

 calcium, thrombogen cannot, as we have seen, become converted 

 into thrombin ; if, however, a soluble calcium salt, be added, 

 the power of clotting is restored. Sodium fluoride has much the 

 same effect on blood as sodium oxalate: it combines with the 

 lime ; but the power of clotting is not restored by merely adding a 

 soluble calcium salt. Some tissue extract must also be included, 

 for the fluoride has interfered with the action of the thrombo- 

 kinase. 



The action of certain organic substances in retarding blood- 

 clotting is very remarkable. If peptone be injected into the 

 blood of a dog, such blood will not clot ; it can be shown that 

 this is not directly due to the action of the peptone, for the 

 latter may be added to blood without inhibiting coagulation. 

 Peptone introduced into the circulation causes the secretion in 

 the liver of a substance which prevents blood-clotting, an anti- 

 j thrombin. Leech extract contains a similar substance. This 

 antithrombin — designated hirudin — is secreted by the salivary 

 glands of the leech. When an extract of these is injected into 

 the circulation, the blood loses its power of clotting, and, unlike 

 peptone, leech extract, when added to blood drawn from the body, 

 prevents coagulation. The bleeding following leech-bites and 



