FUNDAMENTAL PRINCIPLES 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.—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 
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 
