58 PHYSIOLOGY OF FARM ANIMALS [CH. 



The blood plasma contains about ninety-tAvo per cent, water, 

 and the remaming eight per cent, consists of proteins, fats, 

 carbohydrates, salts and gases, and all the various products 

 of metabolism which need to be transported to or from the 

 different parts of the body. The proteins consist largely of 

 albumen and globulin together with fibrinogen which as men- 

 tioned below plays an important part in the process of clotting. 

 The reaction of blood plasina is alkaline. 



If blood is withdrawn from the body and allowed to stand in 

 a vessel for a short time (three or four minutes for a man or a 

 little longer for a horse) it becomes thick and eventually coagulates 

 or clots. After a space of a few hours the clot shrinks like curd 

 of milk, and finally we get a tangled mass containing the corpuscles 

 enmeshed in it. floating in a colourless fluid which is called serum. 

 The network of the tangled mass or clot consists of fibrin, a 

 protein substance white and stringy in appearance, and produced 

 by the alteration of fibrinogen under the influence of another 

 substance of unknown composition called thrombin. Thus, pure 

 fibrinogen obtained by repeated precipitation is without the 

 propertj^ of spontaneous coagulation, but if a little serum be 

 added coagulation proceeds to take place, since the serum con- 

 tains thrombin. Moreover, serum from the pericardial or other 

 cavities Avill sometimes not clot spontaneously, but on the addition 

 of other serum coagulation sets in. This again is explained on 

 the assumption ' that the pericardial serum did not contain 

 thrombin, but that this substance was introduced in the addition 

 of other serum. It seems certain that thrombin is not a ferment, 

 since its activity is not permanently destroyed by boiling. 



Thrombin, like fibrin, is not present in the blood as such, but 

 in the form of a precursor called prothrombin or thrombogen, 

 from which it is converted through the agency of thrombokinase 

 in the presence of lime salts. Normally however, if lime salts 

 are not present (e.g. if they have been precipitated out from 

 freshly shed blood, as by adding potassium oxalate) the blood 

 will not clot. The thrombokinase which is formed in injured 

 animal tissues and escapes into the blood, brings about the union 

 of the lime salts with prothrombin, the resulting product being 

 thrombin. This explains why it is that blood clots readily when 

 in contact with broken or torn vessels, but does not easily do so 

 in uninjured tissues, since thrombokinase does not occur (at any 



