FTJliirDAMENTAL PEINOIPLES OF DISEASE. 31 



(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.— Wh&the,r 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 

 always leaves the walls of the vessels weakened and more predisposed 

 to recurrent attacks from accidental causes than are perfectly healthy 

 tissues. Thus a horse which has had a congestion of the lungs from 

 a severe drive is liable to have another attack from even a lesser cause. 



The alterations of congestion are distention of the blood vessels, 

 accumulation of the cellular elements of the blood in them, and effu- 

 sion of a portion of the liquid of the blood into the fibrous tissues 

 which surround the vessels. When the changes produced by conges- 

 tion are visible, as in the eye, the nostril, the mouth, the genital 

 organs, and on the surface of the body in white or unpigmented ani- 

 mals, the part appears red from the increase of blood; it becomes 

 swollen from the effusion of liquid into the spongelike connective 

 tissues ; it is at times more or less hot from the increased combustion ; 

 the part is frequently painful to the animal from pressure of the effu- 

 sion on the nerves, and the function of the tissue is interfered with. 

 The secretion or excretion of glands may be augmented or dimin- 

 ished. Muscles may be affected with spasms or may be unable to con- 

 tract. The eyes and ears may be affected with imaginary sights and 

 sounds. . 



PASSIVE CONGESTION. 



Passive congestion is caused by interference with the return of the 

 current of blood from a part. 



Old age and debility weaken the tissues and the force of the circu- 

 lation, especially in the veins, and retard the movement of the blood. 

 We then see horses of this class with stocked legs, swelling of the 

 sheath of the penis or of the milk glands, and of the under surface of 

 the belly. We find them also with effusions of the liquid parts of the 

 blood into the lymph spaces of the posterior extremities and organs 

 of the pelvic cavity. 



Tumors or other mechanical obstructions, by pressing on the veins, 

 retard the flow of blood and cause it to back up in distal parts of the 

 body causing passive congestion. 



The alterations of passive congestion, as in active congestion, con- 

 sist of an increased quantity of blood in the vessels and an exudation 



