Thrombosis. 137 



laryngeal entrance (oedema of the glottis) is dangerous, occasion- 

 ing asphyxiation by narrowing the air passages; oedema of the 

 lungs and of the brain may be fatal by causing functional dis- 

 turbances of these organs. The mechanical pressure effects of 

 collections of fluid in tlie serous cavities upon the blood circula- 

 tion, the compression and displacement of viscera (pressure on 

 the diaphragm), as secondary results of the dropsy, increase the 

 severity of the primary disease which was the cause of the dropsy 

 (affections of the heart, liver or kidneys). 



Obstruction of Blood Vessels. Thrombosis and Embolism. 



Obstruction of blood vessels may be due to the formation within 

 them of relatively solid plugs made up of the blood constituents, 

 this process being known as thrombosis ( 6 ep6ix(3os, mass; from 

 rp^cpw, to make firm or compact), or to forcible lodgment in their 

 limien of materials conveyed by the blood current, this latter pre cess 

 being known as embolism ( e/j.-pdWeiv . to throw in). 



It is well known that blood outside the body clots in a few 

 minutes after its escape from the vessels, as after venesection. 

 There is formed from the blood a red jelly-like mass, the bloody 

 cnior saiigiiinis: over the surface of which there collects a pale 

 yellow fluid, the blood serum. The clot is primarily constituted 

 of fibres of coagulated fibrin, which include the blood plaques. In 

 case of slow coagulation the red corpuscles sink to the bottom of 

 the containing vessel and thus the superior part of the coagulum is 

 largely occupied by the lighter and therefore less readily sinking 

 white blood corpuscles ; for which reason the surface of the clot 

 shows a yellowish-white layer composed of fibrin and leucocytes, 

 the so-called huffy coat. The formation of fibrin is the main fea- 

 ture of extravascular coagulation, and can be directly followed in a 

 drop of blood under the microscope. In a blood drop placed upon 

 the cold slide the film can be observed to separate as fine threads, 

 and these, with the platelets adhering, unite with each other to form 

 a network (Birch-Hirschfeld). Fibrin is an albiuninous substance, 

 existing in solution in the circulating blood in the plasma (the 

 so-called fibrinogenous material or fibrinogen), which separates as 

 a coagulating calcium compound with proteid when acted upon in 

 th.e presence of calcium salts by a ferment known as the fibrin 

 ferment. 



Following the apt comparison of Birch-Hirschfeld, blood coagulation 

 rnay be thought of as somewhat analogous to rennet coagulation of milk. 



