1 90 HUMAN PHYSIOLOGY 



the development in the liver of some body which retards 

 coagulation, and if the liver be excluded from the circulation 

 they are incapable of acting. 



Why is it that blood does not coagulate in the vessels and 

 does coagulate when shed ? Such a general statement is 

 not absolutely correct, for blood may be made to coagulate 

 in the vessels of a living animal in various ways. If inflam- 

 mation is induced in the course of a vessel, coagulation at 

 once occurs. If the inner coat of a vessel be torn, as by a 

 ligature, or if any roughness occurs on the inner wall of a 

 vessel, coagulation is apt to be set up. Again, various sub- 

 stances injected into the blood stream may cause the blood 

 to coagulate, and thus rapidly kill the animal. Among such 

 substances are extracts of various organs thymus, testis, 

 and lymph glands which yield prothrombin, although 

 curiously enough the injection of pure thrombin does not 

 usually act in this way. Nor does blood necessarily coagulate 

 when shed. If it is received into castor oil, or into a vessel 

 anointed with vaseline and filled with paraffin oil, it will 

 remain fluid for a considerable time. 



Apparently some roughness in the wall of the blood vessel 

 or of the vessel in which the blood is received is required to 

 start the process, acting as a focus from which it can spread 

 outwards. 



The advantages of coagulation of blood are manifest. 

 By means of it wounds in blood vessels are sealed and 

 haemorrhage stopped. 



Although an important and very prominent change in 

 the blood, clotting is really produced by change in one 

 constituent of the plasma, which is present in very small 

 quantities. 



II. Plasma and Serum. 



These may be considered together, since serum is merely 

 plasma minus fibrinogen. As serum is so much easier to 

 procure, it is generally employed for analysis. 



Both are straw-coloured fluids, the colour being due to a 

 yellow lipochrome. Sometimes they are clear and trans- 

 parent, but after a fatty diet they become milky. They are 

 alkaline in reaction, and have a specific gravity of about 



