AFTER -SMELLS. 389 



sneezing. It has been said that galvanism excites an ammoniacal 

 smell, when the negative pole is applied to the olfactory membrane, 

 but an acid smell, when the positive pole is employed. (Miiller.) These 

 effects may be owing to chemical decomposition of the mucus; and, 

 according to general authority, the electrical stimulus does not directly 

 excite the special properties of the olfactory nerves. 



The acuteness of smell, in regard to certain substances, is very re- 

 markable, gg.Jflflth part of ammonia, soo^ou^h P art f bromine, 

 T.soiJ.TJSTjthpart of sulphuretted hydrogen, and even ^^^^th part 

 of musk, being perceptible when mixed with common air. Odorous 

 impressions are quickly blunted, and are of very short duration. It 

 has been conjectured, that we do not perceive double sensations of 

 smell from the two nostrils, because of the plexiform arrangement and 

 non-medullated structure of the olfactory nerve-fibres ; but, to this, it 

 may be objected, that we can, by attention, discriminate the sensation 

 conveyed through each nostril, even when the same odor is presented 

 to both; and, moreover, when different odors are presented to the 

 two nostrils, we do not perceive a combined impression or sensation ; 

 but each smell is alternately, or separately perceived. The olfactory 

 sense, like the other senses, varies in different persons, both as to its 

 quantitative and qualitative character, some having the sense obtuse, 

 and others acute, in regard to all odors, some discriminating par- 

 ticular faint odors, as those of certain flowers or fruit, and others not 

 being able to perceive them. The sense may be trained, by exercise 

 and attention, alternating with due intervals of rest and abstinence 

 from the action of the same odor. On the other hand, it may be 

 blunted, by the habitual presence of any one odor, at least, as regards 

 that odor itself. Why certain odors are agreeable, and others dis- 

 agreeable, to persons generally, is not known; in the case of individ- 

 uals, the utmost diversity prevails in this respect, habit especially, be- 

 ing a second nature. 



Whether after-smells due to states of the nerves, occur similarly to 

 after-tastes, is difficult to decide, for particles of odorous substances 

 retained on the mucous membrane, may cause prolonged sensations. 

 Subjective sensations, due to changes in the composition of the blood, 

 or to disturbances in the circulation through the olfactory nervous 

 centre, have been less commonly observed in regard to smell than to 

 the other senses. Maniacal persons, however, often complain of dis- 

 agreeable odors. 



The sense of smell is allied, not only by its chemical and physical 

 phenomena, but also physiologically, to the sense of taste. It ministers 

 but little to the intellectual faculties of man, but rather to his bodily 

 wants, aiding him in determining his choice of food, and of such pro- 

 ducts of the animal, vegetable, or mineral world, as are agreeable to 

 this particular sense; and often teaching him to avoid injurious or dis- 

 agreeable substances, especially gases and vapors. It is, in particular, 

 the sanatory sense, serving to test the air we breathe, and, if duly 

 attended to, warning us of the deleterious emanations from decompos- 

 ing organic matter. 



Smell is very acute in certain uncivilized tribes, as amongst the 



