External circumstances doubtless produce definite effects on the living body, but these effects are frequently very difficult to determine. Climate, temperature, condition of the atmosphere as regards moisture or purity, the character of the soil, the dwellings, the food and drink, the clothing, the occupations of men, all have influence on their susceptibility to disease. The geographical distribution of disease and the effects of. occupations have been made especially the subject of observations, but a more particular examination of these points scarcely lies within our province.

External circumstances will affect the living tissues chiefly by rendering them less vigorous in their resistance to influences calculated to produce disease. As an illustration of this we may cite the effect of exposure to cold in the causation of disease. It is an admitted fact that exposure to cold, especially when the body is fatigued and the general vitality lowered, is frequently followed by attacks of disease. But the exposure can scarcely be the cause of the disease, as it produces in different persons most diverse results. Thus a similar exposure will lead in one person to acute catarrh of the air-passages, in another to acute rheumatism, in a third to acute inflammation of the kidneys, in another to diarrhoea, and so forth. It is clear that the effect of the exposure is, in most cases by reflex nervous influences, to render some part unduly susceptible to the exciting causes of disease, the locality which is thus rendered susceptible being determined often by the individual characteristics of the person. In many cases the disease, indeed, is due to an infective agent, which is able to make good its entrance when the structures are weakened. In what is specifically designated a common cold, the exposure to a low temperature is merely one of the predisposing conditions. Colds frequently arise without any such exposure. The fact that such colds are often obviously infectious is an indication that they are due to infective agents, which latter must be very prevalent in the community.

Occupations will influence the production of disease according as they tend to weaken particular tissues. Employments which imply severe muscular exertion, for instance, cause considerable strain on the vascular system, as every strong muscular effort is accompanied by a closure of the glottis, with suspension of the respiratory movements, and a rise in the blood-pressure. Such efforts frequently repeated will render the heart and large vessels peculiarly susceptible to disease. Hence we find that affections of the heart and arteries, more particularly aneurysm, are much more common in men than in women, and in men who are engaged in occupations which require strong and sustained muscular exertion, such as blacksmiths, engineers, sailors, etc. Again, the frequent respiration of impure air, while it leads to a general weakness of the body, produces a special susceptibility to disease in the lungs.