THE BLOOD 107 



The next point to consider is why blood obtained after the previous 

 injection of proteoses (or commercial peptone) into the circulation 

 should not clot. It certainly contains calcium salts, and probably 

 both thrombogen and thrombo-kinase, for it can be made to clot with- 

 out the addition of either ; for instance, by dilution or the passage of 

 a stream of carbon dioxide through it. There must be something in 

 peptone blood which antagonises the action of thrombin. This some- 

 thing is an excess of antithrombin. Peptone will not hinder blood 

 coagulation, or only very slightly if it is added to the blood after it is 

 shed. The antithrombin must therefore have been added to the 

 blood while it was circulating in the body. We can even go further 

 than this and say what part of the body it is which is concerned in the 

 production of antithrombin : it is the liver, for if the liver is shut 

 off from the circulation, peptone is ineffective in its action. The 

 converse experiment confirms this conclusion, for if a solution of 

 peptone is artificially perfused through an excised surviving liver, a 

 substance is formed which has the power of hindering or preventing 

 the coagulation of shed blood. 



We are thus justified in two conclusions : 



(1) That the antithrombin which is normally present in healthy 

 blood in sufficient quantities to prevent intravascular clotting, is 

 formed in the liver. 



(2) That commercial peptone in virtue of the proteoses it contains 

 stimulates this action of the liver to such an extraordinary degree, 

 that the accumulation of antithrombin in the blood becomes suffi- 

 ciently great to prevent the blood from clotting even after it is shed. 



It should be noted, however, that this effect upon the liver varies 

 in different animals, and is most marked in the dog. 



We shall conclude by considering only one more of the hindrances 

 to coagulation, and that by no means the least interesting. The 

 leech lives by sucking the blood of other animals ; from the leech's 

 point of view it is therefore necessary that the blood should flow 

 freely and not clot. The glands at the head end of the leech, often 

 spoken of roughly as its salivary glands, secrete something which 

 hinders the blood from coagulating, and everyone knows by experience 

 who has been treated by leeches how difficult it is to prevent a leech- 

 bite from bleeding after the leech has been removed ; complete 

 cleansing is necessary to wash away the leech's secretion from the 

 wound. Now if an extract of leeches' heads is made with salt solution 

 and filtered, that fluid will prevent coagulation, whether it is injected 

 into the blood-stream or added to shed blood. The substance in the 

 extract is called hirudiii, and this is believed to be antithrombin itself. 



