SMELL. 491 



producing an unnatural tumefaction of the mucous membrane, 

 which prevents the free passage of the air through the nasal fossae. 



As in the case of the tongue, also, we must distinguish here 

 between the perception of true odors, and the excitement of the 

 general sensibility of the Schneiderian mucous membrane by irri- 

 tating substances. Some of the true odors are similar in their nature 

 to impressions perceived by the sense of taste. Thus we have 

 sweet and sour smells, though none corresponding to the alkaline 

 or the bitter tastes. Most of the odors, however, are of a very 

 peculiar nature and are difficult to describe ; but they are always 

 distinct from the simply irritating properties which may belong to 

 vapors as well as to liquids. Thus, pure alcohol has little or no 

 odor, and is only irritating to the mucous membrane ; while the 

 odor of wines, of cologne water, &e., is communicated to them by 

 the presence of other ingredients of a vegetable origin. In the 

 same way, pure acetic acid is simply irritating; while vinegar has 

 a peculiar odor in addition, derived from its vegetable impurities. 

 Ammonia, also, is an irritating vapor, but contains in itself no 

 odoriferous principle. 



The sensations of smell, like those of taste, remain for a certain timt 

 after they have been produced, and modify in this way other less 

 strongly marked odors which are presented afterward. As a 

 general thing, the longer we are exposed to a particular odor, the 

 longer its effect upon our senses continues ; and in some cases it 

 may be perceived many hours after the odoriferous substance has 

 been removed. Odors, however, are particularly apt to remain 

 after the removal or destruction of the source from which they 

 were derived, owing to their vaporous character, and the facility 

 with which they are entangled and retained by porous substances, 

 such as plastered walls, woollen carpets, and hangings, and woollen 

 clothes. It is supposed to be in this way that the odor of a post- 

 mortem examination will sometimes remain so as to be perceptible 

 for several hours or even an entire day afterward. But this alone 

 does not fully explain the fact. For if it depended simply on the 

 retention of the odor by porous substances, it would afterward be 

 perceived constantly, until it gradually and continuously wore off; 

 while in point of fact, the physician who has made an autopsy of 

 this kind does not afterward perceive its odor constantly, but only 

 occasionally, and by sudden and temporary fits. 



The explanation is probably this. As the odor remains con- 

 stantly by us, we soon become insensible to its presence, as in the 



