CHAPTER IV. 

 DISEASES OF THE LUNGS. 



CIRCULATORY DISTURBANCES. 



Congestion of the Lungs. — ^Definition. — ^An engorgement of 

 the pulmonary capillaries with blood, due either to a heart 

 which is overexerting itself (active hyperemia), or to some 

 obstacle which prevents the prompt return of the pulmonary 

 blood to the heart (passive hj^eremia). 



Occurrence and Etiology. — ^Active congestion is quite com- 

 mon in horses especially during hot weather in unconditioned, 

 untrained animals which are put to violent exertion such as 

 racing, hunting, hard pulling. In all probability the primary 

 lesion is cardiac rather than pulmonary and represents an 

 acute dilatation of the heart. (See this.) Fat cattle and 

 swine are often attacked when being driven to market on a 

 hot day. 



An active congestion of the lungs may attend heat stroke. 

 It can arise from the inhalation of irritant fumes (stable 

 fires). The first stage of pneumonia is congestion. 



Passive Congestion. — Two forms of passive congestion 

 are recognized : (o) Mechanical, and (b) hypostatic. 



(a) The former occurs whenever there is some condition of 

 the heart present which prevents the free return of blood to 

 it. It, therefore, may attend mitral insufficiency, valvular 

 stenosis, cloudy swelling, or dilatation of the heart. It 

 can occur during the course of infectious diseases which 

 affect the heart. Passive congestion may follow filling of 

 the pericardium (pericarditis), and develops secondary to 

 gastric or intestinal bloat. 



(6) Hypostatic congestion occurs chiefly in large animals 

 which are forced to lie in one posture for a long time (cast 

 in stall; azoturia; fracture of leg bones), the blood gravitating 



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