i2 4 8 THE SENSE OF SMELL. 



upon by a tumour, anosmia may follow. The olfactory nerves are occasionally 

 absent, and in these cases we find congenital anosmia, though this is said not 

 to be invariably the case. 1 In the very exceptional cases in which sensation 

 is present, it has been suggested that fibres of the fifth nerve officiate as nerves 

 of smell. 



In the case of man, the monkeys, and some cetacea, the olfactory sense- 

 organ is evidently one which has undergone involution. This is evident, not 

 only from a study of the nose itself, but also from a study of the central nervous 

 system. The olfactory bulbs and tracts are but rudimentary organs, when 

 compared with the olfactory lobes of macrosmatic mammals. In the nose of 

 man, the three turbinals represent the chief remnants of the much more 

 complex organ which must have existed in ancestral forms. 2 The sensory 

 surface must have greatly diminished in extent, and now occupies, as has 

 already been described, but a small patch of the upper region of the nasal 

 cavity. It is probable, therefore, that the sense of smell was more acute in 

 the ancestral forms, and that it played a relatively great part in the psychology 

 of man's ancestry, a condition which we find to-day among the macrosmatic 

 mammalia. 



The passage of air through the nose. The organ of smell is 

 stimulated by odorous particles which reach the nose, and enter it 

 either by the anterior or by the posterior nares. If a bottle containing 

 a volatile oil be unstoppered in a room, the oil volatilises, and the 

 particles gradually diffuse into the room. The dispersion of these 

 particles by diffusion may be aided by agitating the air, as with a fan, 

 or by a draught, and by displacements of air due to differences of tem- 

 perature, etc. 3 



The outside air in the neighbourhood of the face is drawn through the 

 nose, either in the course of ordinary inspiration, or by sniffing. If this air 

 contains a sufficiency of odorous particles, we are sensible of the odour. 



The air stream was supposed by the older observers to pass chiefly 

 along the lower region of the nose on the upper surface of the palate, 

 and to take, in fact, an almost straight course from the anterior to the 

 posterior nares. More recent observers have shown that the main 

 air-stream on entering the nostrils at first ascends almost vertically, due 

 probably to the horizontal position of the inlet ; it then bends round, 

 and sweeps backwards and downwards at the level of the middle 

 turbinals, and leaves the nose by the posterior nares. 



The earliest experiments were made by E. Paulsen. 4 He divided 

 the head of a cadaver in the mesial plane, and placed tiny squares of 

 red litmus paper upon the mucous membrane of the nasal cavity. The 

 two halves were then replaced in juxtaposition, and air saturated with 

 ammonia vapour was drawn through the nose by means of a pump 

 attached to the trachea. On separating the halves of the head once 

 more, and looking into the exposed nasal cavities, he could see the 

 direction which the main air-current had taken, for this was indicated 

 by the blueness of the litmus paper. H. Zwaardemaker 5 performed a 



1 See Gowers' "Diseases of the Nervous System," vol. ii. p. 140. 



2 Schwalbe, Schrift. d. phys-okonom. Gesellsch. zu Konigslerg, 1882, Bd. xxiii. ; Zucker- 

 kandl, "Das periphere Geruchsorgan der Saugethiere," Stuttgart, 1887 ; Seydel, Morphol. 

 Jahrb., Leipzig, Bd. xvii. 



3 See an excellent article on the " Diffusion of Volatile Substances," by Justus Gaule, 

 "Physiol. des Nase und ihrer Nebenhohlen," Wien, 1896. 



4 SUzungsb. d. k. Akad. d. Wissensch., Wien, 1882 (3), Bd. Ixxxv. S. 348. 



5 "Die Physiologic des Geruchs," Leipzig, 1895, S. 49. 



