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. 15.)

[Illustration:  FIG. 15—­A SECTION OF AN INFLAMED LUNG SHOWING THE EXUDATE WITHIN THE AIR SPACES.  Compare this with Fig 6.  Fig 15 is from the human lung, in which the air spaces are much larger than in the mouse.]

The microscopic examination of any normal tissue of the body shows within it a variable number of cells which have no intimate association with the structure of the part and do not seem to participate in its function.  They are found in situations which indicate that these cells have power of active independent motion.  In the inflamed tissue a greatly increased number of these cells is found, but they do not appear until the height of the process has passed, usually not before thirty-six or forty-eight hours after the injury has been received.  The numbers present depend much upon the character of the agent which has produced the injury, and they may be more numerous than the ordinary leucocytes which migrate from the blood vessels.