886. This sense, with that of taste, aids man as well as the inferior animals, in selecting proper food, and it also gives us pleasure by the inhalation of agreeable odors. The sense of smell, like that of taste and touch, may be improved by cultivation. It likewise varies in different persons.
Observation. Sometimes this sense seems to possess a morbid degree of acuteness in respect to odors, which is highly inconvenient and even dangerous. With some individuals, the smell of certain fruits, flowers, cheese, &c., produce nausea and even convulsions.
887. In the inferior animals generally, the sense of smell is more acute than in man. Thus the bloodhound will track the hare over the ground for miles, guided only by the odor that it leaves in its flight. He also traces the progress of his master through thickly-crowded streets, distinguishing his footsteps from those of a thousand others, and amidst the odorous particles emanating from a thousand sources.
Observation. In some of the higher orders of the inferior animals, there is an astonishing acuteness of smell in regard to effluvia that come from living animals. To these animals, it possesses an importance in them far beyond what it has in man, by making them acquainted with the presence of their enemies or their prey, when the eye and ear are incapable of acting. It is related by travellers in Africa, that they were always apprised of lions in their vicinity during the night, by the moans and tremblings of their horses.
888. Smell is somewhat under the control of the will. That 393 is, we have the power of receiving or rejecting odors that are presented; thus, if odors are agreeable, we inspire forcibly, to enjoy them; but, if they are offensive, our inspirations are more cautious, or we close our nostrils. This sense is likewise modified by habit; odors which, in the first instance, were very offensive, may not only become endurable, but even agreeable.
886. What is the use of the sense of smell? Can this sense be improved by cultivation? What is said respecting this sense in some individuals? 887. What is said of this sense in the bloodhound? Mention an instance of astonishing acuteness of smell in some of the higher orders of animals. 888. Show that smell is somewhat under the control of the will.
889. Acuteness of smell requires that the brain and nerve of smell be healthy, and that the membrane that lines the nose be thin and moist. Any influence that diminishes the sensibility of the nerves, thickens the membrane, or renders it dry, impairs this sense.
Observations. 1st. Snuff, when introduced into the nose, not only diminishes the sensibility of the nervous filaments, but thickens the lining membrane. This thickening of the membrane obstructs the passage of air through the nostrils, and thus obliges “snuff-takers” to open their mouths when they breathe.
2d. The mucous membrane of the nasal passages is the seat of chronic catarrh. This affection is difficult of removal, as remedial agents cannot easily be introduced into the windings of these passages. Snuff and many other articles used for catarrh, produce more disease than they remove.
889. On what does acuteness of smell depend? What effect has snuff when introduced into the nose? What is said of chronic catarrh?