THE BLOOD 263 



and converts it into thrombin, after which it combines with the fibrinogen. 

 For this reason the plastic agent has been termed thrombo-plastin. Experi- 

 ments indicate that this agent belongs to the group of bodies known as 

 phosphatids and similar in its properties to one member of this group, viz., 

 kephalin. 



Intra-vascular Coagulation. So long as the relations of the blood 

 and the vascular apparatus remain physiologic, no coagulation occurs in the 

 vessels. The reasons assigned for this are: (i) the absence of thrombo- 

 kinase in sufficient amounts; or (2) the presence of anti- thrombin. On either 

 assumption the reaction between pro thrombin and calcium with the forma- 

 tion of thrombin does not take place. If the vessels are injured as they are 

 when ligated or torn or in any way impaired, coagulation promptly takes 

 place with the subsequent occlusion of the vessel. As to whether the injured 

 tissues or the blood-cells now generate an agent, thrombo-kinase, which 

 activates the prothrombin and calcium, or whether they generate an 

 agent thrombo-plastin, which neutralizes an anti-thrombin, is a subject of 

 discussion. 



Under pathologic conditions of the circulatory apparatus, especially of 

 the internal lining, intra-vascular coagulation frequently arises, though the 

 process cannot be considered as identical with extra-vascular coagulation. 

 Many pathologists assert that in its origin, mode of formation, and structure 

 the intra-vascular coagulum or thrombus is not a true coagulum as ordinarily 

 understood, but rather a conglutination of blood-plaques and leukocytes. 

 Whenever the integrity of the internal wall of the vessel is impaired by 

 disease or by the introduction of foreign bodies, there is primarily a deposition 

 and accumulation of blood-plaques at the injured area or on the foreign body 

 which constitutes to a large extent the mass of the thrombus which at once 

 forms. The thrombi which form on the surface of atheromatous ulcers, on 

 the valves of the heart, and in the veins in consequence of diseased states, on 

 threads or needles passed through the vessels, at the orifices of torn blood- 

 vessels, consist largely of blood-plaques. A thrombus so formed may con- 

 tain a number of delicate fibrin threads, which, however, present a different 

 appearance from the fibrin of the extra-vascular clot. In the thrombi which 

 form around foreign bodies there is a larger quantity of fibrin than in those 

 originating from causes wholly within the vessel. 



