Chemotaxis is a term implying a peculiar property of _attraction and repulsion between cells_, both animal and vegetable. It mainly pertains to vegetable cells alone, and has been offered as the explanation of the sporulation of ferns, for example; but as it interests us most in this place it is manifested between the animal cells of the human body and the bacteria, which are vegetable cells. As a result the former, _i. e._, the phagocytes, having power of migration, are drawn toward the latter. To be more accurate, this mutual or peculiar attraction is known as _positive chemotaxis_, it being also known that exactly the reverse prevails under certain circumstances, and that mobile cells will move away as rapidly as possible from certain organisms or substances for which they seem to have a repugnance, this being known as _negative chemotaxis_. SPECIFIC IRRITANTS. These are essentially living organisms, bacteria, fungi, and the protozoa, the first named being by far the most frequent. Before a lesion can assume the type of inflammation as here understood some one or more of these organisms must have secured an entrance into the tissues, the circumstances determining such invasion being considered a little farther on. It is these living organisms which, having once invaded the tissues, determine that most active congregation and proliferation of certain cells which we have just described under the head of Phagocytosis. When once the irritants are present there begins that very active conflict which Virchow has so graphically alluded to as the _battle of the cells_. Now the mysterious chemotactic properties of the component substances manifest themselves, and now phagocyte is drawn toward bacterium, or the reverse, while the tiny war goes on with sometimes varying results, it being a question which can prove victor in the conquest. This is no fiction of the imagination, but is a contest which may be seen under the microscope in certain of the lower animals, while its results may be seen in the examination of pus from any human source. In another place I have also likened this conflict to that in which certain of the enemy resort to poisoned weapons, because modern biological chemistry has now shown very evidently that it is a part of the life history of many of these microörganisms to produce, probably as excretory products, albuminoid or other substances having sometimes extremely toxic properties. And so it comes about that in many of the surgical infections, while the local destruction is produced by the actual death of tissues which have been invaded by microörganisms, the general or systemic symptoms, generally referred to as the _toxic_ symptoms, are literally due to poisons generated in the infected area, dispersed throughout the system, and often proving fatal.