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PART VI 

 SENSATIONS OF SMELL AND TASTE 



'ERY living organism shows a susceptibility, i.e. a power of reaction, 



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 sensibility accounts for the clustering of aerobic bacteria 

 towards the surface of a fluid, i.e. where the density of oxygen is greater, or 

 around chlorophyll-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. 

 Chemiofaxis then represents the faculty by means of which these minute 

 organisms are able to adapt themselves to chemical changes in their environ- 

 ment and to react* to chemical substances at a considerable distance from 

 themselves. If we could endow these elementary organisms with con- 

 sciousness 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 com- 

 plexity and is prepotent in determining the behaviour of an animal in 

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

 the olfactory lobes form the greater part of the higher brain, and extirpation 

 of them produces a loss of spontaneity and of delayed reactions similar to 

 that which can be brought about in higher types by extirpation of the whole 

 of the cerebral hemispheres. 



