DISEASES OF THE ARTERIES AND VEINS 473 



The degenerative process may extend to the cells lining the vessel, when 

 they break down, and expose the underlying pasty matter. 



Should this occur, the blood in its course backwards washes up the 

 degenerated inflammatory product and carries it away, leaving the middle 

 and external coat exposed. 



In the third stage of the disease the pasty mass, instead of being thus 

 removed, becomes more or less calcified, in which case small bone-like 

 spicules are seen ramifying through the structure of the vessel wall, in 

 some instances completely surrounding it and giving it the appearance of 

 a bony tube (see Aneurism, fig. 196). 



The eflect of this disease on the wall of the vessel is seriously to spoil 

 its elastic reaction, and so interfere with the circulation. 



When the inflammatory products have undergone the softening process, 

 and become exposed to the blood current, the vessel yields to the pressure 

 from within, causing it to dilate still further, and in consequence an 

 aneurism is formed, or the blood may insinuate itself between the coats 

 of the vessel and cause them to separate, when a dissecting aneurism is 

 the result. 



When the vessel becomes thin, as is sometimes the case in this form 

 of the disease, its walls may rupture, and death take place more or less 

 suddenly, or the fibrine of the blood coagulated on the diseased surface 

 may fill up the vessel and form a thrombus. 



If in the second stage of the disease the pasty mass or some adhering 

 clot of fibrine becomes exposed to the current of the blood, and carried 

 away, some portion of it may be arrested in a distant small vessel, estab- 

 lishing the condition of embolism. 



If this should occur in the lungs, or the brain, or the kidneys, organs 

 specially predisposed to embolism, further, and perhaps fatal, mischief may 

 be the result. 



THROMBOSIS 



Both arteries and veins are liable to become more or less completely 

 obstructed during life by the coagulation of blood within them. When 

 this condition occurs it is described as " thrombosis", and the obstructing 

 clot is spoken of as a "thrombus". In the present day, thrombosis in 

 the horse is only of seldom occurrence. Formerly, when blood-letting was 

 resorted to in every conceivable ailment, it was a common afi'ection of the 

 jugular vein, and frecjuently gave rise to permanent obstruction and 

 obliteration of the vessel. 



Causes. — The causes of thrombosis are chiefly injuries in one form or 

 another, such as wounds, severe contusion, and stretching; indeed anything 



