2 54 Animal Life and Intelligence. 



is true that the stimulation of the end-organs is effected by 

 actual contact with the odoriferous vapour. But since this 

 vapour may be given off from an odoriferous body at some 

 distance from the organism, such as a flower or a decom- 

 posing carcase, it is clear that the sense gives information 

 of the existence of such bodies before they themselves come 

 in contact with us. Primitively, we may suppose that it 

 was developed in connection with that sense of taste with 

 which, as we have seen, it is so closely associated. In this 

 respect smell is a kind of anticipatory taste. But it has 

 now other ends, apart from those which are purely aesthetic. 

 In us it may serve as a warning of a pestilential atmo- 

 sphere ; in many organisms, such as the deer, it gives 

 warning of the presence of enemies ; in many again, and 

 some insects among the number, it is the guiding sense in 

 the search for mates. 



The organ of smell in ourselves and in all the mammalia 

 is the delicate membrane that covers the turbinal bones in 

 the nose. It contains cells with a largish nucleus, around 

 which the protoplasm is mainly collected. A filament 

 passes from this to the surface, and ends in a fine hair or 

 cilium (or a group of hairs or cilia in birds and amphibia) ; 

 a second filament runs downwards into the deeper parts of 

 the tissue, and may pass into a nerve-fibril. 



In us and air-breathing creatures, the substance which 

 excites the sensation of smell must be either gaseous or in 

 a very fine state of division ; but in water-breathers the 

 substance exciting this sensation — or, in any case, one of 

 anticipatory taste — may be in solution. The sensitiveness 

 of the olfactory membrane is very remarkable. A grain of 

 musk will scent a room for years, and yet have not sensibly 

 lost in weight. Drs. Emil Fischer and Penzoldt found 

 that our olfactory nerves are capable of detecting the 

 460^0000 P^^^ ^^ ^ milligramme of chlorophenol, and the 

 4^(Jo'o oooo P^^* ^^ ^ milligramme, or about one thirty- 

 thousand-millionth of a grain, of mercaptan. It may be 

 that to such substances our olfactory sensibility is especially 

 delicate. 



