SMELL AND^TASTE 365 



relies. We describe it as looking, because in ourselves the eye 

 has so far outdistanced the other senses as a channel of informa- 

 tion that we speak of " looking " when we mean seeking, and 

 say that " we see " when we wish to imply that we under- 

 stand. 



The difference between smell and taste is, in fishes, a differ- 

 ence in the quality of the sensation, and not in its " modality " 

 or kind ; but in terrestrial animals the olfactory membrane of 

 the nose has become specialized for the recognition of particles 

 suspended in air, the tongue for substances dissolved in water. 

 The olfactory membrane, which lines the upper two of the three 

 chambers of the nose, is covered with elongated cells of two 

 kinds : (a) Columnar cells, fairly thick ; and (b) fusiform cells, 

 each carrying at its free extremity a bunch of exceedingly 

 minute hairs. The fusiform cells are neuro-epithelial cells of 

 the most primitive type. Before nerve-cells, properly so called, 

 appeared, certain favourably-placed epithelial cells were con- 

 nected by protoplasmic bridges with muscle-fibres, to which 

 they delivered the impulses which were generated in them by 

 external forces. Later some of the neuro-epithelial cells sank 

 beneath the surface, where, as ganglion-cells, they served as 

 intermediaries between groups of sensory cells on the surface 

 and the nerve-net which lay more deeply in the tissues. The 

 olfactory membrane perpetuates the earlier stage ; in so far as 

 it consists of elements which are combinations of sense-cells 

 and nerve-fibres. Each of its fusiform cells sends inwards a 

 nerve-filament, which, traversing the submucous tissue of the 

 nose and the bone (cribriform plate) on the base of the skull, 

 between the orbits, enters the olfactory bulb. The olfactory 

 bulb is a part of the local nervous mechanism of smell. It is 

 the ganglion of the nerve of smell plus nerve-elements which in 

 all segments behind the eye have been withdrawn from the 

 neighbourhood of the sense-organ into the central nervous 

 system (cf. p. 333). 



The way in which odorous particles in air stimulate the 

 fusiform cells is unknown. The quantity which suffices as a 

 stimulant is so small as to put chemical stimulation out of the 

 question. A few grains of musk will scent a room for years. 

 0-0000000 4 milligramme of mercaptan (sulphur-alcohol) is recog- 

 nizable in a litre of air. This is a dilution to 1 in 50,000,000,000. 



