CHAPTER XXV. THE MODERN THEORY OF MICRO-PARASITES. (276) Micro-Strife. MODERN research has amply shown that the greatest proportion of the maladies, which are more or less preventable, belong to the domain of parasitism. Life preying upon life. The micro-tragedy which may be watched in a highly-magnified drop of pond-water, a voracious monad swallowing up a weaker, is repeated within the body of the mammal. It is scarcely necessary to call to mind, that all the higher animal bodies are but communities of monads or living points, some floating free, others stationary, these last attached to their neighbours by protoplastic bonds of great tenuity (just as adjoining households may have telephonic connection), but with their individuality and automatism unimpaired. A colony of inimical microbes obtaining access to this republic is similar to a hostile armed band entering a city-strife at once commences, the strangers attack and are attacked. If the invaders are all killed, no disturbance of health is produced; in any other event, the strangers increase and multiply at the expense of the normal inhabitants, the latter being rather destroyed by some special toxic substance excreted by the enemy than in any other way. The analogy of invasion and defence is rendered closer by a knowledge of the fact that the mammalian body has its fighting cells, a soldier-like community, one of whose special offices is to fight; these have been called "macrophages." Under appropriate conditions the fight between macrophage and microbe has been actually witnessed, the former swallowing, as it were, the latter and digesting it. The details of the process have also been followed by removal of the affected tissues, appropriate staining and observation of the combatants in the several stages-the free microbe and macrophage, the microbe within the substance of the macrophage, but still preserving its outline intact, the microbe becoming cloudy, and final disappearance by complete absorption or digestion. The macrophages themselves are nothing more than leucocytes, cells endowed with automatism and powers of movement, capable of amoeba-like pushing out and retracting of portions of their protean bodies. Each micro-organism seems to have a particular rate of multiplication and those that are pathogenic excrete some toxic substance. We may presume that when a sufficient quantity of the toxic substance is excreted, then and then only the phenomena of general systemic disturbance are experienced. So far as experimental research has gone, there is no true but only an apparent incubation; there is no mysterious localisation for days or weeks of the enemy in lymphatic gland or other tissue. The battle at once commences, and when the victory inclines to the invader then there is sufficient disturbance of function to produce fever, or other symptoms. In some cases there is a general infection, everywhere the microbe can be found, slices of liver, of kidney, of lung, reveal countless myriads of the foreign host, in others, as, for instance, in diphtheria, a portion only of the body is affected, but the poison thus locally developed is carried to every part of the circulation in the same way as when an alkaloid in solution is injected subcutaneously. (277) Acquired Immunity. By the same theory the curious fact that an animal body having once recovered from one of these micro-parasitic diseases obtains a shorter or longer immunity is capable of a plausible explanation. The macrophages have a rapid cycle of existence, a few hours may represent several generations, so that acquired properties are rapidly transmitted; those poisoned by the excretion of pathogenic microbes perish, those that more or less effectually resist, continue to live and propagate, until by a repetition again and again of this process, the body may be full of resistant living particles, and the foreign tribe is annihilated or expelled. If now a second inimical colony of the same kind obtains access to the body, it |