FOOTNOTE:  [1] Flexner has recently succeeded in isolating and cultivating the organism of poliomyelitis, but the organism is so small that its classification is not possible.

CHAPTER VII

THE NATURE OF INFECTION.—­THE INVASION OF THE BODY FROM ITS SURFACES.—­THE PROTECTION OF THESE SURFACES.—­CAN BACTERIA PASS THROUGH AN UNINJURED SURFACE.—­INFECTION FROM WOUNDS.—­THE WOUNDS IN MODERN WARFARE LESS PRONE TO INFECTION.—­THE RELATION OF TETANUS TO WOUNDS CAUSED BY THE TOY PISTOL.—­THE PRIMARY FOCUS OR ATRIUM OF INFECTION.—­THE DISSEMINATION OF BACTERIA IN THE BODY.—­THE DIFFERENT DEGREES OF RESISTANCE TO BACTERIA SHOWN BY THE VARIOUS ORGANS.—­MODE OF ACTION OF BACTERIA.—­TOXIN PRODUCTION.—­THE RESISTANCE OF THE BODY TO BACTERIA.—­CONFLICT BETWEEN PARASITE AND HOST.—­ON BOTH SIDES MEANS OF OFFENSE AND DEFENSE.—­PHAGOCYTOSIS.—­THE DESTRUCTION OF BACTERIA BY THE BLOOD.—­THE TOXIC BACTERIAL DISEASES.—­TOXIN AND ANTITOXIN.—­IMMUNITY.—­THE THEORY OF EHRLICH.

As has been said, infection consists in the injury of the body by living organisms which enter it.  The body is in relation to the external world by its surfaces only, and organisms must enter it by some one of these surfaces.  It is true that the bacteria in the intestine—­either those normally present or unusual varieties—­may, under certain circumstances, produce substances which are injurious when absorbed; but this is not infection, and is analogous to any other sort of poisoning.  Each surface of the body has its own bacterial flora.  Organisms live on the surface either on matter which is secreted by the surface or they use up an inappreciable amount of body material.  Many of these bacteria are harmless, some are protective, producing by their growth such changes in the surface fluids that these become hostile to the existence of other and pathogenic forms.  The surfaces also frequently harbor pathogenic organisms which await some condition to arise which will permit them to effect entrance into the tissues.

The surfaces of the body protect from invasion to a greater or less degree.  The skin protects by the impervious horny layer on the outside, the external cells of which are dead and constantly being thrown off.  Bacteria are always found on and in this layer, but the conditions for growth here are not very favorable and the surface is constantly cleansed by desquamation.  The new cells to supply the loss are produced in the deepest layer of the epidermis, and the movement of cells and fluids takes place from within outwards.  The protection is less perfect about the hairs and the sweat glands.  Infection by the route of the sweat glands is, however, uncommon, for the sweat is a fluid unfavorable for bacterial growth and the flow acts mechanically in washing away organisms which may have entered the ducts.  Infection by the route of the hair follicles is common.  There is no mechanical cleansing as by the sweat,