FOOTNOTES:  [1] By cachexia is understood a condition of malnutrition and emaciation which is usually accompanied by a pale sallow color of the skin.

[2] By trauma is understood a wound or injury of any sort.

CHAPTER IV

THE REACTIONS OF THE TISSUES OF THE BODY TO INJURIES.—­INFLAMMATION.—­ THE CHANGES IN THE BLOOD IN THIS.—­THE EMIGRATION OF THE CORPUSCLES OF THE BLOOD.—­THE EVIDENT CHANGES IN THE INJURED PART AND THE MANNER IN WHICH THESE ARE PRODUCED.—­HEAT, REDNESS, SWELLING AND PAIN.—­THE PRODUCTION OF BLISTERS BY SUNBURN.—­THE CHANGES IN THE CELLS OF AN INJURED PART.—­THE CELLS WHICH MIGRATE FROM THE BLOOD-VESSELS ACT AS PHAGOCYTES.—­THE MACROPHAGES.—­THE MICROPHAGES.—­CHEMOTROPISM.—­THE HEALING OF INFLAMMATION.—­THE REMOVAL OF THE CAUSE.—­CELL REPAIR AND NEW FORMATION.—­NEW FORMATION OF BLOOD-VESSELS.—­ACUTE AND CHRONIC INFLAMMATION.—­THE APPARENTLY PURPOSEFUL CHARACTER OF THE CHANGES IN INFLAMMATION.

Injury and repair have already been briefly considered in their relation to the normal body and to old age; there are, however, certain phenomena included under the term inflammation which follow the more extensive injuries and demand a closer consideration than was given in Chapter II.  These phenomena differ in degree and character; they are affected by the nature of the injurious agent and the intensity of its action, by the character of the tissue which is affected and by variations in individual resistance to injury.  A blow which would have no effect upon the general surface of the body may produce serious results if it fall upon the eye, and less serious results for a robust than for a weak individual.

Most of the changes which take place after an injury and their sequence can be followed under the microscope.  If the thin membrane between the toes of a living frog be placed under the microscope the blood vessels and the circulating blood can be distinctly seen in the thin tissue between the transparent surfaces.  The arteries, the capillaries and veins can be distinguished, the arteries by the changing rapidity of the blood stream within them, there being a quickening of the flow corresponding with each contraction of the heart; the veins appear as large vessels in which the blood flows regularly (Fig. 11).  Between the veins and arteries is a large number of capillaries with thin transparent walls and a diameter no greater than that of the single blood corpuscles; they receive the blood from the arteries and the flow in them is continuous.  The white and red blood corpuscles can be distinguished, the red appearing as oval discs and the white as colorless spheres.  In the arteries and veins the red corpuscles remain in the centre of the vessels appearing as a rapidly moving red core, and between this core and the wall of the vessels is a layer of clear fluid in which the white corpuscles move more slowly, often turning over and over as a ball rolls along the table.