THE SENSE OF SMELL. 631 



odorous matter should be transmitted in a current through 

 the nostrils. This is effected by an inspiratory movement, 

 the mouth being closed ; hence we have voluntary influence 

 over the sense of smell ; for by interrupting respiration 

 we prevent the perception of odours, and by repeated quick 

 inspiration, assisted, as in the act of sniffing, by the action 

 of the nostrils, we render the impression more intense (see 

 p. 224). 



The human organ of smell is essentially formed by the 

 filaments of the olfactory nerves, distributed in minute 



Fig. 176.* 



arrangement, in the mucous membrane covering the upper 

 third of the septum of the nose, the superior turbinated or 

 spongy bone, the upper part of the middle turbinated bone 

 and the upper wall of the nasal cavities beneath the cribri- 

 form plates of the ethmoid bones (figs. 176 and 177). 

 This olfactory region is covered by cells of cylindrical epi- 



* Fig. 176. Serves of the septum nasi, seen from the right side (from 

 Sappey after Hirschfeld and Leveille). . I, the olfactory bulb ; 

 i , the olfactory nerves passing through the foramina of the cribriform 

 plate, and descending to be distributed on the septum ; 2, the internal 

 or septal twig of the nasal branch of the ophthalmic nerve ; 3, naso- 

 palatine nerves. 



