THE CONDITIONS OF SMELL. 387 



motion, and takes place in the direction of that motion. Secondly, 

 the solution of the odorous particles in the moisture of the olfactory 

 mucous membrane. Thirdly, their passage, by means of diffusion, into 

 the substance of the extremities of the olfactory nerves, which, as 

 already mentioned, approach to, or even reach, the epithelial surface. 

 Fourthly, the particles must exercise a chemical action upon the ex- 

 tremities of the nerves ; and there are reasons for supposing that the 

 presence and action of oxygen, in conjunction with the odorous sub- 

 stance, are necessary to this process. (Graham.) Fifthly, the nerve 

 itself must be endowed with the property of receiving such substance, 

 or its resulting compounds, and of being chemically acted upon by 

 them. Sixthly, the olfactory lobes must be capable of discriminating 

 the effects of the special impressions excited at the extremities of the 

 nerves. Lastly, the nasal cavities must contain air ; for, in man, and 

 probably in all air-breathing animals, the sense of smell fails in the 

 water ; and according to Weber, when water, or even solutions of odor- 

 ous substances, are poured into the nose, smell is temporarily sus- 

 pended. These effects have been referred to abnormal changes in the 

 epithelial and other cells of the nasal mucous membrane. 



The physical conditions necessary to the exercise of smell, are influ- 

 enced by the respiratory movements, and by the state of moisture of 

 the nasal mucous membrane. Inspiration is obviously necessary to 

 smell, as the means of drawing the odorous particles into the nose. 

 In ordinary expiration, smell is almost entirely absent ; but if the 

 mouth be filled with tobacco-smoke, or the vapor of chloroform, and 

 this be forced from the pharynx, through the posterior nares, into the 

 nasal fossae, its characteristic odor is at once, though less distinctly, 

 perceived. The act of inspiration is, however, essential as a mechan- 

 ical aid ; for holding the breath prevents the exercise of the sense of 

 smell. In gentle inspiration, odors are faintly perceived ; in strong, 

 quick inspirations, as in sniffing, to appreciate the odor of wines and 

 flowers, or very faint odors, they are most acutely perceived, because 

 a larger quantity of the odorous substance, passing by the middle and 

 lower regions, impinges, in a given time, with a certain force, upon 

 the olfactory region ; the upward direction of the stream of air thus 

 inhaled, carries it at once to that part of the nose. Hence, too, clos- 

 ing one nostril diminishes the force of odorous impressions. The pres- 

 ence of large polypi in the nose, destroys smell. In quiet breathing, 

 the air passes chiefly through the lower and middle regions of the nose, 

 viz., through its respiratory portion, and only a limited quantity 

 reaches the upper or olfactory region ; whilst, in stronger inspirations, 

 the stream of air is drawn upwards, a movement said to be favored by 

 the form of the inferior turbinated bones. 



The natural moisture of the mucous membrane is indispensable to 

 the exercise of smell. The secretion of the nasal glands is abundant; 

 it is slightly alkaline, but otherwise its nature is unknown. From the 

 depth, protected character, and position of the olfactory region, out of 

 the course of the respiratory air-current, its moisture is preserved 

 from evaporation ; this is also checked by the copious supply of mucus 

 from the nasal fossae and their communicating sinuses, and by the 



