486 DISEASES OF THE HOESE. 



ways 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 apt to have another attack from even a lesser cause. 



The alterations of congestion are distention of the blood vessels, 

 accumulation of the cellula"r 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. Where 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 diminished. 

 Muscles may be affected with spasms or may be unable to contract. 

 The eyes and ears may be ^.ffected 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 iorce 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 

 of its fluid into the tissues surrounding them, but in passive conges- 

 tion we have a dark thick blood which has lost its oxygen, instead of 

 the rich combustible blood rich in oxygen which is found in active 

 congestion. 



The termination of congestion is by resolution or inflammation. In 

 the first case, the choked-up blood vessels find an outlet for the exces- 

 sive amount of blood and are relieved ; the transuded serum or fluid 

 of the blood is reabsorbed, and the part returns almost to its normal 

 condition, with, however, a tendency to weakness predisposing to fu- 

 ture trouble of the same kind. In the other case further alterations 

 take place, and we have inflammation. 



