196 THE SENSES OF ANIMALS 



upper part of the nose cavity. The elephant has a very acute 

 sense of smell yet the scroll bones in its nose are almost 

 rudimentary and the long trunk performs the function of 

 warming, moistening and filtering the air breathed in. The 

 lower part of the nose-cavity is the passage used in breathing 

 and odorous particles diffuse from it into the upper part 

 where they produce the sensation of smell when they touch 

 the receptor cells. If we consciously try to smell a scent we 

 sniff, to draw air up into the part where the receptors lie. 

 So, too, do other animals — we have all seen a dog scenting 

 the air, holding its head up and turning this way and that 

 as it sniffs about to pick up smells wafted to it. 



The receptor cells for smell are nerve cells lying in the 

 membrane lining the nose-cavity with their outer ends reach- 

 ing the surface where they are divided into several minute 

 hair-like threads ; the lining surface of the membrane is kept 

 moist by the secretion of numerous small glands. Odorous 

 substances are those that give off a vapour which can be 

 drawn into the nose with the air and that will dissolve in 

 water or oil, so that they can come into contact with the 

 nerve endings and act upon them chemically to start a 

 message on its way to the brain. Things that produce no 

 vapour, or which produce a vapour that is insoluble, give 

 no sensation of smell. It is not necessary for any great 

 quantity of vapour to be given off, for some substances con- 

 tinue to be strongly scented for years with no measurable 

 loss of weight and others are perceptible even to our noses 

 in extremely high dilution — it is said that we can detect 

 musk diluted one part in eight million parts of air, and the 

 evil-smelling substance mercaptan diluted one in twenty-five 

 thousand million. One wonders just how many, or rather 

 how few, molecules are needed to stimulate a single nerve- 

 ending of smell. 



Recent research has shown that in mammals some of the 

 cells in the lining of the nose contain free vitamin-A and 

 protein-bound carotenoids. The latter are probably the sub- 

 stances that receive energy from molecules of odorous matter. 

 The visual purple of the eye, rhodopsin, is also a protein- 

 bound carotenoid, and the action of light upon it causes a 

 geometrical isomeration or rearrangement of the shape of 



