786 SMELL [CH. LV. 



(2) A layer of large nerve-cells called " mitral cells " (c), with 

 smaller cells (a) mixed with them. The axis-cylinder processes of 

 these cells pass up into the layer above and eventually become 

 fibres of the olfactory tract E, which passes to the grey matter of 

 the base of the brain F. They give off numerous collaterals on the 

 way (e,f). 



(3) The layer of olfactory glomeruli (B). Each glomerulus is a 

 basket-work of fibrils derived on the one hand from the terminal 

 arborisations of the mitral cells, and on the other from similar 

 arborisations of the non-medullated fibres which form the next layer. 



(4) The layer of olfactory nerve-fibres. These are non-medullated ; 

 they continue upwards the bipolar olfactory cells, which are placed 

 among the epithelial cells of the mucous membrane. 



Animals may be divided into three classes : those which, like the 

 porpoise, have no sense of smell (anosmatic) ; those which possess it in 

 comparatively feeble degree (man, most primates, monotremes, and 

 some cetacea) ; these are called microsmatic. In man the thickness 

 of the olfactory membrane is only O06 mm. Most mammals are in 

 contradistinction macrosmatic, the thickness of the membrane being 

 O'l mm. or more, and its area larger. 



The mucous membrane must be neither too dry nor too moist ; if 

 we have a cold we are unable to smell odours or appreciate flavours 

 (which are really odours). When liquids are poured into the nose, 

 their smell is imperceptible, as they damage the olfactory epithelium, 

 owing to the difference of osmotic pressure. But even if a " normal " 

 saline solution of an odorous substance be substituted, the sense of 

 smell is still lost so long as air-bubbles are carefully excluded from 

 the nasal cavity. It is therefore necessary that odorous substances 

 should be in a gaseous state in order to act upon the olfactory 

 nerve-endings ; they are normally conveyed to the olfactory surface 

 by the air currents passing through the nose. 



Generally, the odours of homologous series of compounds increase 

 in intensity with increase of molecular weight, but bodies of very low 

 molecular weight are odourless, while vapours of very high molecular 

 weight, which escape and diffuse slowly, have little or no smell. A slight 

 change in chemical constitution may produce marked alteration in 

 the character of the odour of a substance ; certain modes of atomic 

 grouping within the molecule appear to be more odoriferous than 

 others. Attempts have been made to discover the elementary sensa- 

 tions of smell, but hitherto with scant success. Many odours have 

 unquestionably a complex physiological effect. For example, when 

 nitrobenzol is held before the nose, it yields first the smell of 

 heliotrope, next the smell of bitter almonds, and finally the smell 

 of benzene ; just as if different end - organs became successively 

 fatigued. Some substances have a very different smell according 



