3. =Hereditary Influences.=--The fact that immunity against certain infections and susceptibility to other conditions are transmitted from parent to offspring is one which admits of no dispute. The explanation, however, is almost as remote from us today as it ever was. But the recognition of the fact is of the greatest importance to all practising surgeons. That bacteria frequently enter through wounds and bruises is self-evident, but we all know that such wounds are more likely to suppurate in some than in others, and the causes of infection in some are, to a certain extent, connected with the hereditary habit of tissues. The same causes influence not merely liability to infection, but its severity and character. There are undoubtedly also local as well as general variations, and it is very certain that among these the results of bruising or contusion are by far the most prominent. There is also undoubted experimental evidence that under certain circumstances bacteria produce only local lesions, whereas under others they produce general and even fatal infection. 4. =Local Predisposition.=--Local predisposition is a factor of almost equal importance. Once given a distinct infection, and hyperemia is sometimes a contributing cause of inflammation. _Per contra_, anemia of tissues seems to be also a favoring condition. In parts involved in chronic congestion the blood flows more slowly, while the vessels are dilated and apparently susceptibility is increased. Infection here produces a type of disease mentioned as _hypostatic inflammation_. Conspicuous exception as to the occasional value of an artificial passive hyperemia is seen, however, in the so-called congestion treatment (Bier’s) of tuberculous joints, where the more or less constant flooding of the tissues with venous blood seems to render them uninhabitable for living bacilli, which apparently die and disappear (by phagocytosis), thus permitting a slow return to the normal condition. General anemia, again, is a predisposing cause, while toxemias, including diabetes, etc., are still more so. The liability of diabetic patients to suppurative and even gangrenous infection is proverbial. The presence of foreign bodies has much to do also, and, infection once having occurred along with its introduction, the presence of a foreign body will nearly always excite suppuration; otherwise it will ordinarily remain inert. The withdrawal of trophic nerve influences also apparently permits infection, as is instanced by the ease with which bed-sores form in paralytic patients. Obstruction to the circulation or to escape of secretions more easily permits infection; for example, in the appendix, in the kidney, in the gall-bladder, the salivary glands, etc. Furthermore, one may formulate a quite comprehensive statement and say that all such lesions as solutions of continuity, hemorrhages, degenerations, vascular stasis produced by strangulation, etc., and all perforations, increase more or