Such perfumes are very generally produced by little pockets or glands in the skin, the secretion having, in the case of insects, birds and mammals, an oily nature. In mammals they are largely produced by both males and females, and serve to attract the sexes to one another. Hairs are situated close to the minute odoriferous glands and serve an important part in accumulating and diffusing the characteristic perfume. Musk and civet are of this nature, and it is a significant fact that these substances are used as perfumes by human beings. It would seem as though mankind had lost either the power of satisfactorily perceiving the perfumes naturally produced by the human skin, or that the production of such perfumes had for some reason diminished. Either condition would account for the use by mankind of the perfumes of other animals and of flowers. There are a variety of odorous substances produced by different parts of the human body, of which some are agreeable and others disagreeable. One of the most curious facts in regard to odorous bodies is the close resemblance between agreeable and repulsive odours, and the readiness with which the judgment of human beings may pronounce the same odour agreeable at one period or place, and disagreeable at another. There also seems to be a "dulling" of the power to perceive an odour which is a consequence of constant exposure to that odour. Thus the Chinese say that Europeans all smell unpleasantly, the odour resembling that of sheep, although we do not observe it; whilst Europeans notice and dislike the smell of the negro, a smell of the existence of which he is unaware. The blood of animals, including that of man, has, when freshly shed, a smell peculiar to the species, which has not, however, any resemblance to that of the skin or of the waxy glands of the same animal.