366 THE BODY AT WORK 



Probably even such figures as these would be thrown into the 

 shade if we could estimate the minimum amount of human 

 effluvium which will enable a dog to follow his master's trail. 

 Explanations have been sought in alterations in the vibrations 

 of molecules of air caused by the presence amongst them of 

 relatively heavy molecules of volatile substances ; but the diffi- 

 culty of accounting for the generation of nerve-impulses in the 

 sensory cells remains as great as ever. The hairs borne by 

 olfactory cells are so short that it is impossible that they should 

 project beyond the film of moisture on the surface of the mem- 

 brane. This seems to preclude an answering vibration. Yet 

 an increase in the thickness of this layer and in its density, 

 due to the presence in it of mucus secreted during a catarrh, 

 renders the sense-cells incapable of responding to odorous 

 particles. 



Smell in an animal is not a test of the quality of the air it 

 is breathing, but a source of information as to the direction 

 in which it may seek its prey ; or, although far more rarely, as 

 to the direction from which the advance of a foe is to be feared. 

 Hunting animals depend for the most part on the nose. Hunted 

 animals rely chiefly on the eye. 



If we attempt to analyse our smell-sensations, we find that 

 we can pick out a number of varieties which appear so unlike 

 as to have nothing in common : Putrid meat, burning india- 

 rubber, sulphuretted hydrogen, ammonia, roses, onions, lemon 

 verbena, methylated spirit. Everyone can make for himself a 

 list of typical odours which seem to have specific qualities 

 odours so distinct that he never confuses one with another. 

 He can also class together scents about which he is often un- 

 certain. The type- odours he can distinguish when present in 

 a mixture ; whereas odours which are less distinct reinforce or 

 modify one another. It has been found, by careful experiment, 

 that certain type-odours even tend to neutralize each other. 

 Musk and bitter almonds, for example, if present in small 

 quantities and properly proportioned, produce a very dim 

 sensation, whether supplied as a mixture to both nostrils, or 

 the one assertive odour to one nostril and the other to the other. 

 This last observation is of great importance. It proves that 

 their mutual destruction does not occur on the olfactory mem- 

 brane. It is not due to physical interference. The sensation 



