SENSATIONS OF SMELL AND TASTE 559 



little satisfied with the simple sensations^derived from his four sets 

 of gustatory end-organs. 



THE SENSE OF SMELL 



The psychical analysis of olfactory sensations is rendered difficult 

 by the fact that this sense in man plays but a small part in his usual 

 adaptations. We have thus to deal with a sense which is in many 

 respects vestigial. We see traces of great complexity in its possibilities 

 of performance, but are baffled in our endeavours to reduce the whole 

 of the phenomena to the simpler factors of which they are composed. 

 Moreover, like all vestigial functions, the extent to which the sense is 

 developed varies from one individual to another. Many, for instance, 

 are unable to appreciate the smell of vanilla, of hydrocyanic acid, 

 or of violets. On the other hand, in animals, such as the dog, the 

 olfactory sense seems to play a great part in determining behaviour, 

 and the nervous associations, which are the physiological basis of 

 ideas, must in these animals be largely connected with olfactory 

 impressions. Another factor which diminishes the importance of olfac- 

 tory sensations in man is the ease with which the sense-organ becomes 

 fatigued. It often happens that the inmates of a room are perfectly 

 comfortable and may perceive no fault in the ventilation, although a 

 new-comer from the outside at once remarks that the air is foul. 



The organ of smell is situated at the upper part of the nasal cavities. 

 Here the mucous membrane covering the superior and middle tur- 

 binate bones and the corresponding part of the septum is different 

 from that covering the rest of the nasal passages. Over the lower parts 

 of the nasal cavities the mucous membrane is of the ordinary respira- 

 tory type, and is composed of ciliated columnar epithelium contain- 

 ing a number of goblet-cells. In the olfactory part the epithelium 

 is much thicker, of a yellow colour, and apparently composed of a 

 layer of columnar cells resting on several layers of nuclei. These 

 nuclei belong to the olfactory cells proper, true spindle-shaped nerve- 

 cells with one process extending towards the mucus covering the free 

 surface, while the other is continued along channels in the bone, and 

 through the cribriform plate as one of the non-medullated olfactory 

 nerve fibres. These nerve fibres dip into the olfactory lobes, where 

 they terminate by a much-branched arborisation or end basket in 

 the so-called olfactory glomeruli, in close connection with a similarly 

 branched dendrite of the large ' mitral ' cells of the olfactory lobe. 

 The axons from these latter carry the olfactory impulse towards the 

 rest of the brain. In the connective tissue basis (dermis) of the 

 mucous membrane are a number of small mucous or serous glands 

 (Bowman's glands) whose office it is to keep the surface of the 

 membrane constantly moist. 



