474 Dynamic Theory. 



smell. The ends of the olfactory nerve are distributed to this part of 

 the membrane only. They come directly down through the roof bone 

 of the inner nose called the cribriform plate of the ethmoid or sieve 

 bone, which also at that point forms the base of the skull. On top of 

 this plate, inside the cranium, lies the olfactory bulb or lobe, which in 

 man constitutes a large ganglion or expansion of the olfactory nerves. 

 From the under side of this olfactory lobe the fibres of nerve pass 

 through the holes of the cribriform plate and spread themselves over 

 the pituitary membrane. 



In the nasal cavities the membrane appears formed of two layers in- 

 timately united. The one in contact with the bone is fibrous ; the other 

 is free at one surface. The latter is a mucous membrane on which there 

 are to be found small papillae and mucous follicles that open into the 

 cavity of the nostrils. On penetrating the maxillary, frontal, sphenoidal 

 and ethmoidal sinuses, the membrane becomes very thin, transparent, 

 less vascular, and seems reduced to its mucous layer. (Dunglison.) 



FIG. 220. Rod-shaped sense cells from the 

 Organ of Smell. 



a. From the Frog. 



6. Man. 



c.- " " Pike-fish. 



Sz.A supporting cell between two ciliated 

 rod cells which probably are the active agents 

 to convey the stimulus to the cells and nerve 

 fibres below. The top ends in all three are the 

 parts directly exposed to the irritation of odor- 

 ous particles. ( Todd & Bowman.) 



The inferior meatus or channel be- 

 low the inferior turbinal bone in each 

 nostril is an air passage, but not con- 

 cerned in the olfactory sense. The 

 mucous membrane in this part of the 

 nasal cavities is covered with epi- 

 thelial cells which are furnished with 

 cilia or minute hairs. In the olfac- 

 tory part of the cavities the cells of 

 the membrane present a broad end to 

 the surface, but become attenuated 

 FIG. 220. when traced inwards toward the un- 



derlying network. Between them we find long, rod-like filaments, which 

 in their lower part swell out into a nut-shaped expansion and then are 

 prolonged into a fine fibre toward the interior. ( Bernstein. ) These 

 fibres are lost in the network of nerve fibres and probably connect with 

 them and thus with the olfactory nerve. Very fine hairs have been 

 found upon these rods in some animals. The cells receive the contact 

 of the odorous substance as the organs of smell, and the nerve fibres are 

 the vehicles of the irritation as in the other senses. 



