736 



SMELL 



[CH. LIII 



neuroglia); below this four layers are distinguishable; they are 

 shown in the accompanying diagram from Eamon y Cajal's work, 

 the histological method used being Golgi's. 



(1) A layer of white fibres containing numerous small cells, or 

 "granules" (D). 



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



FIG. 538. Nervous mechanism of the olfactory apparatus. A, Bipolar cells of the olfactory apparatus 

 (Max Schultze's olfactorial cells) ; B, olfactory glomeruli ; c, mitral cells ; D, granule of white 

 layer; E, external root of the olfactory tract; F, grey matter of the sphenoidal region of the 

 cortex ; a, small cell of the mitral layer ; b, basket of a glomerulus ; c, spiny basket of a granule ; 

 e, collateral of the axis-cylinder process of a mitral cell ; /, collaterals terminating in the molecular 

 layer of the frontal and sphenoidal convolutions ; g, superficial triangular cells of the cortex ; 

 . h, supporting epithelium cells of the olfactory mucous membrane. (Ramon y Cajal.) 



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 (ef). 



(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, or as we have 

 already termed them, the olfactorial 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, rnonotremes, and 

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

 of the olfactory membrane is only 0'06 mm. Most mammals are in 

 contra-distinction macrosmatic, the thickness of the membrane being 

 01 mm. or more, and its area larger. 



