12 MANUAL OF EQUINE MEDICINE. 



Venous liypersemia may be caused by — 



1. Diminution of the force with which the blood should 

 be driven through the veins. 



2. Direct impediment to the return of blood by the veins. 

 We may consider these causes separately. 



(1.) Diminution of the force of the circulation may be due 

 to lessened cardiac power ; for example, in the acute febrile 

 disorders and in those degenerations of the cardiac structures 

 which lead to dilation of its cavities. 



In the arteries also the blood-current may be obstructed 

 from morbid changes in their walls. 



The current may also be obstructed in the capillaries, 

 chiefly from the pressure of inflammatory effusions, dropsy, 

 and other causes. 



Finally, the flow may be impeded in the veins, owing 

 to absence of muscular contraction of the walls, or to 

 dilation of the vessels. 



(2.) Direct impediment to the return of blood by the veins 

 may be due to various causes. 



As examples may be given the obstruction to the portal 

 circulation which occurs in cirrhosis of the liver ; the con- 

 2:estion of the lungs which occurs in mitral constriction and 

 regurgitation; and that of the lower extremities which occurs 

 from the pressure of the gravid uterus on the iliac veins. 



The Results of Venous Congestion. — Venous congestion 

 results in accumulation of blood, and diminished velocity of 

 its flow in the dilated capillaries and veins. The most im- 

 portant changes which may follow are : transudation of 

 serum, diapedesis of red blood-cells, haemorrhage, fibroid 

 induration, and frequently thrombosis. Gangrene, i.e., death 

 of the part, may result when the obstruction is very general 

 and complete. 



