SECTION III 

 SENSATIONS OF SMELL AND TASTE 



EVERY living organism shows a susceptibility, i.e. a power of reac- 

 tion, to chemical stimuli. Thus the plasmodium of myxomycetes, 

 placed on a strip of filter-paper of which one end is immersed in an 

 infusion of dead leaves and the other in distilled water, will crawl 

 along the paper towards the infusion of leaves. If the infusion of 

 dead leaves be replaced by a weak solution of quinine, the plasmodium 

 will be repelled and will travel along towards the vessel of water. 

 These movements of attraction and repulsion are spoken of as positive 

 and negative chemiotaxis respectively. A similar chemical sensi- 

 bility accounts for the clustering of aerobic bacteria towards the surface 

 of a fluid, i.e. where the density of oxygen is greater, or around chloro- 

 phyll-containing algae which are giving off oxygen in the sunlight. 

 The aggregation of leucocytes round microbes or other foreign particles 

 in the tissues is also determined by their chemiotactic sensibility. 

 Chemiotaxis then represents the faculty by means of which these 

 minute organisms are able to adapt themselves to chemical changes 

 in their environment and to react to chemical substances at a con- 

 siderable distance from themselves. If we could endow these ele- 

 mentary organisms with consciousness and with a sense of their 

 surroundings, we should have to say that they became aware of the 

 presence of some harmful or attractive material at some distance from 

 themselves. The sensation they received from these distant objects 

 would be therefore a projected sensation. 



On the other hand, a chemical sensibility of the body surface or 

 part of it furnishes the criterion by which particles are accepted and 

 ingested as food or rejected as useless or harmful. Consciousness in 

 this case would be of something affecting and in contact with some 

 part of the organism itself. The sensation would not be projected 

 further than the periphery of the body. 



These two kinds of chemical sense the projected and the surface 

 sense are found throughout almost all classes of the animal kingdom, 

 and in the higher animals at least are known as the senses of smell and 

 taste. The former sense in many animals attains a high degree of 

 complexity and is prepotent in determining the behaviour of an animal 

 in response to the changes in its surroundings. In the elasmobranch 



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