CHAP. XVI.] CONDITIONS OF SMELL. 11 



terior palatine canal, and then spread over the three turbinated bones 

 and the septum nasi, anastomosing at several points with the olfac- 

 tory filaments., and with the nasal branch of the ophthalmic (figs. 106 

 and 108). When the fifth nerve is diseased, so that sensation is 

 lost generally in the parts supplied by it, a brush may be introduced 

 into the nostril, and rubbed over the surfaces usually so extremely 

 sensitive, without the slightest discomfort to the patient. Similar 

 effects follow division of the nerve. Hence it may be concluded 

 that the fifth gives common sensibility to the nose, in common with 

 most of the other parts which it supplies. 



Conditions of Smell. In addition to the essential conditions of 

 integrity of the nervous apparatus, and the presence of the requisite 

 stimulus, a healthy condition of the epithelial investment of the 

 papillae seems necessary for perfect smell. If the mucous surface is 

 dry, or if it is in the raw irritable state, attended with watery dis- 

 charge, induced by cold, smell is impaired or lost. This is ex- 

 plained by considering the manner in which the nerves are ordi- 

 narily brought under the influence of the stimulus. As in taste, a 

 solution of the stimulus in the surface of the membrane is requisite 

 in order that the odorous substance may actually reach the nerve. 

 Insoluble substances cannot be smelt. Hence, whether the mem- 

 brane be too dry, or an inordinate excretion of fluid be going on 

 from its surface, the necessary penetration of the stimulus to the 

 nerves is alike interfered with. In the latter case, the effect may 

 partly depend also on a change produced by the inflammatory 

 action, in the excitability of the nerves themselves. 



Since odorous substances must undergo solution before they can 

 affect the olfactory nerves, why, it may be asked, cannot such sub- 

 stances, if dissolved in water and injected into the nose, be recog- 

 nized by their smell? In answer to this it may be stated, that 

 there is no reason to deny the possibility of their being so recog- 

 nized, as far as the excitability of the nerves is concerned. But 

 the ciliated epithelium of the nose, and the nerves of common sen- 

 sation supplying the lining membrane, instantly resent the contact 

 of all other fluids than the film which moistens the surface, and 

 which is naturally furnished by it in due proportion to the exigen- 

 cies of the part; and when the membrane is thus irritated, and 

 its texture altered by the water, it need not excite surprise that its 

 special sensibility should be altered or disguised. The organ of 

 smell in fishes resembles that of air-breathing animals in every es- 

 sential point of structure, and differs mainly in the habitual contact 

 of its sentient surface with the surrounding water. It may there- 



