THE SENSES 139 



stances seem tasteless which under normal conditions have a 

 marked flavour. 



YI. Smell. 



Mechanism. Over the upper part of the nasal cavity the 

 columnar epithelial cells are devoid of cilia, and between them 

 are placed spindle-shaped cells (Fig. 74, 01. c.\ which send 

 processes through the mucous membrane, and through the 

 cribriform plate of the ethmoid into the olfactory bulb. Here 



C.T*. 



FIG. 74. The Connections of the Olfactory Fibres. 01. c., olfactory cells ; 01. Gl. t 

 glomeruli; M.G., mitral cells; 01. T., olfactory tract; C.M., corpus 

 mammilarium ; Hipp. , hippocampus ; Op. Th. , optic thalamus ; F. , fornix 

 (modified from SCHAFER). 



they form synapses with other neurons (01. GL), the axons of 

 which pass to the base of the olfactory tracts(6). Some fibres (a) 

 pass straight on to the optic thalamus, while others (6) form 

 synapses with other neurons, the fibres of which, passing 

 round the fornix, end by arborising round cells in the corpora 

 mammilaria (C.M.), which send processes on to the thalamus. 

 There is some evidence that the fibres to the cortex end in 

 the hippocampus. 



Physiology. To act upon the olfactory mechanism the 

 substance must be volatile, and must be suspended in the air. 

 In this condition infinitesimal quantities of such substances 

 as musk are capable of producing powerful sensations. The 



