rious degrees of inflammation of the skin may
be produced by the action of the sun, the injury being due not to the
heat but to the actinic rays. In a mild degree of exposure only
redness and a strong sense of heat are produced, but in prolonged
exposure an exudate is formed which causes the skin to swell and
blisters to form, these being due to the exudate which passes through
the lower layers of the cells of the epidermis and collects beneath
the impervious upper layer, detaching this from its connections. If a
small wad of cotton, soaked in strong ammonia, be placed on the skin
and covered with a thimble and removed after two minutes, minute
blisters of exudate slowly form at the spot.

The pain in an inflamed part is due to a number of factors, but
chiefly to the increased pressure upon the sensory nerves caused by
the exudate. The pain varies so greatly in degree and character that
parts which ordinarily have little sensation may become exquisitely
painful when inflamed. The pain is usually greater when the affected
part is dense and unyielding, as the membranes around bones and teeth.
The pain is often intermittent, there being acute paroxysms
synchronous with the pulse, this being due to momentary increase of
pressure when more blood is forced into the part at each contraction
of the heart. The pain may also be due to the direct action of an
injurious substance upon the sensory nerves, as in the case of the
sting of an insect where the pain is immediate and most intense before
the exudate has begun to appear.

When an inflamed area is examined, after twenty-four hours, by
hardening the tissue in some of the fluids used for this purpose and
cutting it into very thin slices by means of an instrument called a
microtome, the microscope shows a series of changes which were not
apparent on naked eye examination. The texture is looser, due to the
exudate which has dilated all the spaces in the tissue. Red and white
corpuscles in varying numbers and proportions infiltrate the tissue;
all the cells which belong to the part, even those forming the walls
of the vessels, are swollen, the nuclei contain more chromatin, and
the changes in the nuclei which indicate that the cells are
multiplying appear. The blood vessels are dilated, and the part in
every way gives the indication of a more active life within it. There
are also evidences of the tissue injury which has called forth all the
changes which we have considered. (Fig