502 PHYSIOLOGY 



and may perceive no fault in the ventilation, although a new-comer from 

 the outside at once remarks that the air is foul. 



The organ of smell is situated at the upper part of the nasal cavities. 

 Here the mucous membrane covering the superior and middle turbinate 

 bones and the corresponding part of the septum is different from that 

 covering the rest of the nasal passages. Over the lower parts of the nasal 

 cavities the mucous membrane is of the ordinary respiratory type, and 

 is composed of ciliated columnar epithelium containing a number of goblet- 

 cells. In the olfactory part the epithelium is much thicker, of a yellow 

 colour, and apparently composed of a layer of columnar cells resting on 

 several layers of nuclei. These nuclei belong to the olfactory cells proper, 

 true spindle-shaped nerve-cells with one process extending towards the 

 mucus covering the free surface, while the other is continued along channels 

 in the bone, and through the cribriform plate as one of the non-medullated 

 olfactory nerve fibres. These nerve fibres dip into the olfactory lobes, 

 where they terminate by a much-branched arborisation or end basket in 

 the so-called olfactory glomeruli, in close connection with a similarly 

 branched dendrite of the large * mitral ' cells of the olfactory lobe. The 

 axons from these latter carry the olfactory impulse towards the rest of the 

 brain. In the connective tissue basis (dermis) of the mucous membrane 

 are a number of small mucous or serous glands (Bowman's glands) whose 

 office it is to keep the surface of the membrane constantly moist. 



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' 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 mg. 

 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. 



