Animal life being equally dependent upon the salubrious qualities of the air respired, as of the food received, a sense has been provided for discriminating the nature of the former, as well as of the latter. As the organs of taste are placed at the entrance of the alimentary canal, so those of smell usually occupy the beginning of the passages for respiration, where a distinct nerve, named the olfactory, appropriated to this office, is distributed.

The sense of smell is generally of greater importance to the lower animals than that of taste; and the sphere of its perceptions is in them vastly more extended than in man. The agents, which give rise to the sensations of smell, are certain effluvia, or particles of extreme tenuity, which are disseminated very quickly through a great extent of atmospheric air. It is exceedingly difficult to conceive how matter so extremely rare and subtle as that which composes these odorous effluvia can retain the power of producing any sensible impression on the animal organs: for its tenuity is so extraordinary as to exceed all human comprehension. The most copious exhalations from a variety of odoriferous substances, such as musk, valerian, or assafœtida, will be continually emanating for years, without any perceptible loss of weight in the body which supplies them.