THE ORGANS OF SPECIAL SENSE 



591 



peculiar olfactory cells there described are not neuro-epithelium but 

 are analogs of the spinal ganglion cell, being the only example in man 

 of the peripherally placed ganglion cell found in certain lower animals. 

 Each cell sends to the surface a short dendrite which ends in several 

 short, stiff, hair-like processes. From its opposite end each cell 

 gives off a longer centrally directed process (axone), which as a fibre 

 of one of the olfactory nerves passes through the cribriform plate 

 of the ethmoid (Fig. 391, elhm) to its terminal nucleus in the olfac- 

 tory bulb (Fig. 391). 



The Olfactory Bulb. — This is a somewhat rudimentary structure 

 analogous to the much more prominent olfactory brain lobe of some 



Fig. 39 1 . — Diagram of Structure of Olfactory Mucosa and Olfactory Bulb. (Ram6n 

 y Cajal.) be, Bipolar cells of olfactory raucosa; sm, submucosa; ethm, cribriform plate 

 of ethmoid; a, layer of olfactory fibres; og, olfactory glomeruli; mc, mitral cells; ep, 

 epithelium of olfactory ventricle. 



of the lower animals. It consists of both gray matter and white 

 matter arranged in six fairly distinct layers. These from below 

 upward are as follows: (a) The layer of olfactory fibres; (b) the layer 

 of glomeruli; (c) the molecular layer; (d) the layer of mitral cells; 

 (e) the granule layer; (/) the layer of longitudinal fibre bundles. 

 Through the centre of the last-named layer runs a band of neuroglia 

 which represents the obliterated lumen of the embryonal lobe. The 

 relations of these layers to the olfactory neurone system are as follows: 



