Virchow fixed his attention upon the forms and activities of the cells, their multiplication and degradation, and how they build up tissues, both healthy and morbid.

To another matter with which, both literally and metaphorically, the air is filled, we must also make allusion.  The existence of micro-organisms in countless numbers is no new fact, but the influence they may exert over living tissues has only lately become the subject of earnest attention.  So long as they were not known to have any practical bearing upon human welfare, they interested almost nobody, but when, however, it was shown that putrefaction of meat is due to the agency of the bacterium termo, and the decomposition of albumen to the bacillus subtilis; when anthrax in cattle and sheep was found to depend on the bacillus anthracis, and that in human beings it caused malignant pustules; when suppuration of wounds was found to be associated with micrococci; and when it was announced that by a process of inoculation cattle could be protected against anthrax, and that by carbolic spray and other well known precautions the suppuration of wounds could be prevented—­all the world lent its ears and investigation at once began.

Because labors in bacteriology promised to be fruitful in practical results, the workers speedily became innumerable, and we are accumulating a wondrous store of facts.  How long now is the list of diseases in which germs make their appearance—­in pneumonia, in endocarditis, in erysipelas, in pyaemia, in tuberculosis, and so on and so on.  One of the most striking illustrations is the gonococcus of gonorrhoea, whose presence in and around gives to the pus cells their virulent properties, and when transferred to the eye works such lamentable mischief.  Without their existence the inoculation of pus in the healthy eye is harmless; pus bearing the gonococci excites the most intense inflammation.  Similar suppurative action in the cornea is often caused by infection of cocci.  The proof of causation may be found in the fact that the most effective cure now practiced for such suppuration is to sterilize them by the actual cautery.  Rosenbach says that he knows six distinct microbes which are capable of exciting suppuration in man.  Their activity may be productive of a poison, or putrefactive alkaloid, which is absorbed.

There are at present two prominent theories in regard to the infections which produce disease.  The first is based upon chemical processes, the second upon the multiplication of living organisms.  The chemical theory maintains that after the infectious element has been received into the body it acts as a ferment, and gives rise to certain morbid processes, upon the principle of catalysis.  The theory of organisms, or the germ theory, maintains that the infectious elements are living organisms, which, being received into the system, are reproduced indefinitely, and excite morbid processes which are characteristic of