644 PHYSIOLOGY 



In ordinary respiration the stream of air never passes higher than the 

 anterior inferior border of the superior turbinate bone, so that it does not 

 come in contact with the olfactory mucous membrane. The sensations 

 of smell which are aroused during ordinary respiration depend on diffusion 

 from the respiratory air into the still air of the upper olfactory portion 

 of the nasal cavity. The direction of olfactory attention is achieved by 

 sniffing ; in this act the nostrils are dilated and the direction of the anterior 

 part of the nasal respiratory chamber altered, so that the stream of entering 

 air is directed towards the upper olfactory portion of the cavity. 



The fact that the air, which enters the nasal cavity during respiration. 

 does not come into direct relationship with the olfactory epithelium has the 

 following advantages : 



(1) The cold inspired air does not come into contact with and cause 

 damage to the sensory "surface. 



(2) Foreign particles carried by the air (including bacteria) do not get 

 deposited there. The position of the epithelium at the very top of the 

 nasal cavity is an additional safeguard. 



(3) The olfactory epithelium is not dried by the rush of dry air 

 across it. 



(4) Noxious vapours only reach it indirectly and therefore do not cause 

 permanent damage as they otherwise might. 



The fact that we are able to perceive smells when breathing normally 

 shows that the odorous substance must be diffusible, i.e. gaseous in form. 

 The amount of substance necessary to excite sensation is extremely minute. 

 Thus '01 mg. of mercaptan diffused in 230 cubic metres of air is still distinctly 

 perceptible. In this case a litre of air would contain only '00000004 nig. 

 of the substance, and the amount actually in contact with the olfactory 

 epithelium would be still smaller. It is possible however to show the 

 presence of these odorous substances in air by physical means. Tyndall 

 pointed out that air containing a small proportion of odorous substances 

 absorbed radiant heat to a much greater degree than did pure air. Thus 

 in one experiment air containing patchouli absorbed radiant heat thirty- 

 two times as strongly as the pure air. Most odorous substances possess 

 large molecules and have therefore high vapour densities. On this account 

 the smell tends to hang about objects, the rate of diffusion of the vapour 

 being only small. 



MODE OF ACTION OF SMELLS. Since the endings of the olfac- 

 tory cells are bathed in fluid, it is evident that the odorous 

 substances must be dissolved by this fluid before they can excite the 

 olfactory nerve fibres, and in the case of aquatic animals we know 

 that the projected chemical sense, which we call smell, can be aroused only 

 by substances in solution. It is difficult to show in man that the nerve 

 endings can be excited by solutions. Most of the experiments have been 

 made with solutions which had an injurious effect upon the olfactory 

 epithelium. According to Aron^olm it is possible to excite sensations 

 of smell if the nasal. cavity be filled with normal saline fluid, containing 



