CONGESTION. 485 



active or passive. The former is produced by an increased sup- 

 ply 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- 

 sively 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 they 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 

 liot iron, frostbites, acids, or a blow. 



(3) Nerve influence. — This may produce congestion either by act- 

 ing on the part reflexly or aS the result of some central nerve dis- 

 turbance affecting the branch which supplifes 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 circula- 

 tion in them is forced to all parts with much greater force and in 

 larger quantities. A well-bred full-blooded horse is much more sub- 

 ject to congestive diseases 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 rapidly; 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 find 

 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. 



(6) Warm 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 way it induces congestion, 

 especially 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- 

 tional activity — or has been a morbid temporary overloading, it al- 



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