rs, 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 functional
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 effusion
of a portion of the liquid of the blood into the fibrous tissues which
surround the vessels. When the changes produced by congestion are
visible, as in the eye, the nostril, the mouth, the genital organs, and
on the surface of the body in white or unpigmented animals, 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 effusion 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 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
circulation, 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