THE ORGANS OF SENSE. 113 



thirds and the glossopharyngeal to the posterior third of the tongue. 

 The taste fibres have their central termination in each case in the 

 column of grey matter which forms the sensory nucleus of the fifth 

 nerve, the nervus intermedius and the glossopharyngeal nerve. 



The Sense of Smell. The end-organs for the sense of smell are 

 limited in their distribution to the upper part of the nasal cavities. 

 The membrane lining this region is yellow in colour, and consists of a 

 characteristic epithelium lying on a connective-tissue layer. The 

 epithelial layer is formed of a superficial layer of columnar, supporting 

 cells and several layers of nerve cells ; the latter are elongated in shape, 

 and each possesses a nucleus. The prolongation of the cell peripheral 

 to the nucleus lies between the columnar cells, and terminates at the 

 surface in six to eight hair-like processes ; the central prolongation is 

 continued as a non-medullated nerve fibre to the olfactory lobe, where 

 it arborises in the manner already described (p. 92). The connective- 

 tissue layer contains small alveolar glands, known as Bowman's glands, 

 the secretion of which moistens the surface of the membrane. 



The adequate stimulus for the sense of smell must be in the gaseous 

 form, or in the condition of excessively minute particles. The gases or 

 particles are conveyed to the lower or respiratory portion of the nasal 

 cavities by the air currents due to the respiratory movements. From 

 the respiratory passage the gases reach the olfactory region by diffusion. 

 No satisfactory classification of odours has been arrived at, and it is 

 usual to describe them as pleasant or unpleasant. Other sensations 

 excited in the nasal mucous membrane, such as the irritation produced 

 by ammonia, are due to stimulation of the endings of the fifth nerve. 



The chief characteristics of the sense of smell are (1) its extreme 

 delicacy, and (2) the ease with which it may be fatigued. (1) The 

 delicacy of the sense of smell is indicated by the dilution of a substance 

 which can still be perceived. Musk can be detected in a dilution in 

 air of one in eight millions ; mercaptan in a dilution of one in twenty- 

 five billions. (2) The fatiguability of the olfactory sense is shown by 

 the insensibility of a person sitting in a closed room to the fact that 

 the air has become vitiated, and also by absence of sensation from a 

 particular perfume after it has been inhaled for a short time. 



Certain odours are antagonistic, that is, one will neutralise the 

 effect of another. Thus the odour of iodoform is annulled by mixing 

 it with balsam of Peru, and carbolic acid neutralises to some extent 

 the odour of putrefaction. 



The estimation of the threshold stimulus for the sense of smell is 

 made by means of the olfactometer of Zwaardemaker. This instrument 

 consists of a porous cylinder which is impregnated with the odorous 



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