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112 DENTAL MEDICINE.

dilatation of blood vessels are so intimately connected that the
one cannot exist without the other.
It is also necessary that the capillaries as well as the arteries
and veins should be injected, in order that the color of the region
so affected should be increased, as the former constitute a dense
network, which traverses the entire organ, while the latter only
form single branches which occupy limited spaces. There are
two forms of hyperemia, active and passive; in the active form
there is an increased quantity of arterial blood passing into the
part, while in the passive form a partial or complete stagnation
of the blood through the vessels occurs.
The term Plethora^ as distinguished from hyperemia, signifies
a superabundance of blood in the entire circulatory system— an
increased amount of blood in all the vessels of the body. Active
hyperemia in the skin and mucous membranes is characterized
by a diffuse bright-red coloration, the parts so affected having a
higher temperature than those around them. The diffuse color-
ation is caused by the bright-red arterial blood which fills the
capillaries ; and the elevation of the temperature is due to the
presence of the blood, which heats the skin or mucous mem-
branes, and the more quickly the blood circulates the warmer
the tissues become, until at length the highest degree of blood
heat is accomplished.
Passive hyperemia denotes a condition of the tissues character-
ized by a diffuse venous, not arterial, coloration and a relatively
low temperature. In passive hyperemia the capillaries are also
dilated, but the blood-current through them is retarded ; hence
the tissues so affected are of a venous color instead of the bright-
red or arterial color of active hyperemia.
The term Anemia signifies a condition in which there is an
abnormal diminution in the amount of blood, and is the opposite
to that of Plethora, being characterized by a pale face, lips, gums
and mucous membranes. But the normal amount of blood may
be present, and yet the characteristic paleness denote an anemic
condition, for the reason that the number of red blood corpuscles
are decreased and replaced by white blood corpuscles, or by
blood plasma.