388 SPECIAL PHYSIOLOGY. 



continuous lachrymal secretion. These fluids also serve to charge the 

 inspired air-current wkh a due proportion of moisture. The necessity 

 for the moist condition of the olfactory membrane, is shown by the 

 absence of smell when this is more than ordinarily dry, as in certain 

 stages of a cold. In perfectly dry air, odors are perceived with diffi- 

 culty. The opposite condition, of too great an amount of moisture, as 

 in other stages of a cold, is also unfavorable to smell. The deficiency 

 of smell noticed after certain surgical operations, in which the nerves 

 regulating the secretion of the nasal mucous membrane have been 

 divided, may also be due to the unusual state of dryness of the mem- 

 brane, which ensues under those circumstances. In all these abnormal 

 conditions, however, the loss of smell may be owing to inflammation, 

 or some other changes in the membrane or its nerves. 



In reference to the necessity for the presence and co-operation of 

 oxygen for the exercise of the olfactory sense, and for some oxidation 

 of the odorous substance on the surface of the nasal mucous membrane, 

 it has been stated, that odors are imperceptible, when inspired mixed 

 with carbonic acid or nitrogen; but, in the former case, the excitability 

 of the olfactory nerves may be suspended by the specific action of the 

 carbonic acid; and, in the latter case, careful observation has shown, 

 that some sense of smell is still retained. This, however, may be due 

 to oxygen already dissolved in the fluids of the nose; and it is difficult 

 to inhale pure nitrogen sufficiently long, to avoid this source of fallacy 

 without risk to life. The remarkable pungency of ozone, suggests the 

 possibility, that, even if odorous substances be not oxidized at the sur- 

 face of the membrane, and so, whatever their nature, be rendered 

 soluble in the nasal mucus, yet ordinary oxygen may operate as a 

 ispecial stimulant to the excitability of the olfactory nerves, and thus 

 increase the intensity of the sensation. 



Whether the odorous substances are first oxidized or not, some 

 chemical reactions probably occur between them and the substance of 

 the olfactory nerves; and these reactions, of the nature of which we 

 ;are entirely ignorant, are probably, moreover, of some special nature, 

 in the case of each distinct odor. In this way, we might account for 

 the differences between different smells. Various irritants, such as 

 mustard, and acetic acid, produce quite similar impressions upon the 

 nerves of common sensation, as, for example, when they are applied 

 to the denuded cutis; but these two substances, and all others having 

 different odors, produce distinct impressions of smell. Hence, not 

 only do different odors, most likely, produce different chemical effects 

 on the olfactory nerves, but these nerves, and their nervous centres, 

 must experience special physiological reactions in the discrimination 

 of different odorous substances. Our knowledge concerning this ap- 

 parently very simple sense is, however, extremely limited. 



It has been said by some, but is doubted by others, that mechanical 

 irritation can produce odorous impressions on the olfactory nerves. 

 Direct mechanical irritation of the olfactory nerves, causes neither 

 pain nor reflex movements; but irritation of the nasal branches of the 

 fifth pair, is followed by pain and reflex movements. The galvanic or 

 other form of electrical stimulus, produces a sensation of tickling, and 



