It would be difficult to assign a satisfactory reason why this accident is more frequent in hot than in cold climates. The effect of external heat upon the living body is not to raise its temperature even when the heat of the air exceeds that of the body[128]; so that we are to seek for the effects of it in some of those affections peculiar to animal life. And as the outward temperature of the air does not affect the general mass of the body, all the effects produced by it must depend on impressions made on the external surface of the body and lungs; and the skin, which may be considered as a large expanded tissue of nervous fibres endowed with universal sympathy and great sensibility, affects every organ and every function of the body, according to the state of the air in contact with it, whether cold or hot, moist or dry, pure or vitiated. This sympathetic sensibility of the skin is chiefly affected by the state of the perspiring pores on its surface; for it is only when these are open that the impression of the air on the skin produces catarrhs, rheumatisms, and internal inflammations in cold climates; and the external temperature in hot climates being such as to keep the pores almost always open, this seems to be a principal reason of that universal irritability prevailing there, and of the general sympathy that prevails between every part, particularly as connected with the organs of perspiration[129]. This readiness of one part to be affected by another in hot climates is well illustrated by the sudden translation of certain diseases. I have seen, for instance, a catarrh cease, and be converted, as it were, into a diarrhœa, and this as quickly disappearing, a pain in the foot would arise, like an attack of the gout. All this would happen in the space of a few hours. But, in cold climates, wounds are by no means exempt from the locked jaw; for it sometimes occurs in England, where I have seen it even in the winter season[130]. Since my return to England I have received some new and useful information on this subject in conversing with Dr. Warren, physician to the King; and as any observations derived from so much acknowledged skill and sagacity must be valuable, I shall here relate what he was so kind as to communicate to me.