Observe that society has also multiplied to a great extent the injurious excitements to which the skin is subjected. These excitements consist especially in the rapid passage from heat to cold, which makes the latter act very powerfully upon the cutaneous sensibility, which like that of all the other systems, answers so much the more to excitements made upon it, as they are different from those, whose action they had previously experienced. In the natural state, there is only the succession of the seasons; nature knows how to connect insensibly heat with cold, and to make the transition but rarely abrupt. But in society, the different garments, the artificial degrees of temperature of our apartments, degrees differing at first from that of the atmosphere, then varying greatly from each other, so that the same man who in winter enters thirty apartments, is often subjected to thirty different temperatures; the hard labour in which most men are engaged, and which makes them sweat copiously, every thing incessantly presents numerous causes which make the vital forces of the dermoid system vary rapidly. Thus the bronchial mucous surface is constantly in contact in cities, with a thousand excitements that are continually renewed, and with which the air is not charged in a natural state. Thus the alimentary substances, continually varying in their composition, temperature, &c. change the excitement of the gastric mucous surface, and are the source of many affections, from which most animals are exempt by the uniformity of their food. If the skin and the mucous surfaces were always kept at the same degree of excitement by the constant uniformity of the stimuli, they would certainly be a much less fruitful source of diseases, as is clearly proved by the fœtus, which is hardly ever sick, because all the external causes which act upon its mucous and cutaneous sensibility, as the heat, the waters of the amnios and the parietes of the womb, do not vary until birth. At this period, animals brought into a new medium, find many more varieties in the stimuli which act upon them, even in a natural state and far from society; thus their diseases are naturally much more frequent after than before birth. In society, in which man has increased four, six and even ten times the number of the stimuli which affect the surfaces destined to be in contact with the external bodies, is it astonishing that the diseases should be so disproportioned to those of animals? IV. _State of the Dermoid System in Old Age._