invaded individual has coped successfully with the invaders at the very point of attack—­has repulsed the attacking party without appreciable impairment of its own force—­and no illness results.  For example, practically all of us inhale the germ of consumption repeatedly, but most of us suffer no harm from it simply because the fluids which bathe the surface on which the germ effects a lodgment are endowed with properties which either kill the germ or rob it of its power for harm; but these properties suffice only when the general health is unimpaired.

In case the attack is not successfully repelled at the outset, what happens?  There begins a struggle between the invaders and what may be called the reserves of the organism, consisting of the white blood corpuscles, which undergo a great augmentation in number.  These corpuscles are endowed with the faculty of amoeboid movement; that is to say, they may shoot out projections from their substance, and even convert themselves for the time being into traps, seizing upon the pathogenic bacteria, incarcerating them within their own mass, and carrying them away to be thrust out of the system by organs whose function it is to eliminate extraneous matter.  These corpuscles are, indeed, said figuratively to eat the malign micro-organisms, whence they have been termed phagocytes (from [Greek:  phagein], to eat, and [Greek:  kutos], a cell); also because they carry away refuse and noxious material, they have been called “the scavengers of the system.”  By means of their amoeboid movement they are enabled to worm themselves through inconceivably minute apertures in the blood vessels, and attack and devour peccant matter wherever it may have effected a lodgment.  These white corpuscles are also known as leucocytes, and their increase in number when they are called upon to resist bacterial invasion is spoken of as hyperleucocytosis.  The discovery of their protective function is to be credited to Metchnikoff, a Russian physician now teaching in Paris.  When they migrate from the blood vessels in great numbers they finally, after having fulfilled their office as phagocytes, degenerate into the corpuscular elements of pus, which is the creamy liquid contained in an abscess.  Their migratory power was discovered by Cohnheim.

But as a general thing the phagocytes do not succeed in making away with all the pathogenic germs, or even with enough of them to prevent the illness which they tend to produce.  The further combat is between the poisonous products, termed toxines, engendered by the bacteria and certain antidotal substances, called antitoxines, newly created in the watery portion of the blood by some wonderful provision of Nature that is not yet well understood.  Each infective disease has its special toxine, and for the destruction of each the blood prepares its particular antitoxine; possibly, however, some of the antitoxines may be efficacious against more than one kind of toxine, for there are physicians who are convinced that vaccination is a temporary preventive of whooping-cough.  But the elaboration of an antitoxine takes time, and the result in any given case, whether in recovery or in death, seems to be settled by the ability or inability of the vital powers of the individual to hold out until they are relieved by the evolution of the necessary amount of antitoxine.