DISEASES OF THE HOESE. 485 



breathing-. The term congestion, however, usuallj^ indicates a morbid 

 condition, with more or less lasting effects. Congestion is active or pas- 

 sive. The former is produced by an increased siippl}^ of blood to the 

 part, the latter by an obstacle preventing the escape of blood from the 

 tissue. In either case there is an increased supply of blood, and as 

 a result increased combustion and augmented nutrition. 



ACTIVE CONGESTION. 



Active congestion is caused by — 



(1) Functional activity. — Any organ which is constantly or exces- 

 sivel}^ used is habituated to hold an unusual quantity of blood; the 

 vessels become dilated; if overstrained the walls become weakened, 

 lose their elasticity, and any sudden additional amount of blood en- 

 gorges the tissues so that the}^ can not contract, and congestion results. 

 Example: The lungs of a race horse, after an unusual burst of speed 

 or severe work, in damp weather. 



(2) Irritants. — Heat and cold, chemical or mechanical. Any of 

 these, by threatening the vitality of a tissue, induce immediately an 

 augmented flow of blood to the part to furnish the means of repair— a 

 hot iron, frostbites, acids, or a blow. 



(3) Nerve influence. — This ma}-^ produce congestion either by acting 

 on the part reflexly or as the result of some central nerve disturbance 

 affecting the branch which supplies a given organ. 



(4) Plethora and sanguinary temperament. — Full-blooded animals are 

 much more predisposed to congestive diseases than those of a lymphatic 

 character or those in an anemic condition. The circulation in them is 

 forced to all parts with much greater force and in larger quantities, 

 A well-bred full-blooded horse is much more subject to congestive dis- 

 eases than a common, coarse, or old worn-out animal. 



(5) Fevers.- — In fever the heart works more actively and forces the 

 current of blood more rapidl}^; the tissues are weakened, and it requires 

 but a slight local cause at any part to congest the structures already 

 overloaded with blood. Again, in certain fevers, we tind alteration of 

 the blood itself, rendering it less or more fluid, which interferes with 

 its free passage through the vessels and induces a local predisposition 

 to congestion. 



(0) War7n climate and summer heat. — Warmth of the atmosphere 

 relaxes the tissues; it demands of the animals less blood to keep up 

 their own body temperature, and the extra quantity accumulates in 

 the blood-vessel system. It .causes sluggishness in the performance 

 of the organic functions, and in this wa}^ it induces congestion, espe- 

 cially of the internal organs. So we find founders, congestive colics 

 and staggers more frequent in summer than in winter. 



(7) Previous congestion. — Whether the previous congestion of any 

 organ has been a continuous normal one— that is, a repeated func- 



