THE SENSE OF SMELL. 497 



of the fifth nerve. Hence the sensations of cold, heat, itching, 

 tickling, and pain ; and the sensation of tension or pressure in 

 the nostrils. That these nerves cannot perform the function 

 of the olfactory nerves is proved by cases in which the sense 



FIG. 177. 



Left olfactory nerve, with its distribution on the septum narium (from Wilson). 



1, Frontal sinus ; 2, nasal bone ; 3, crista galli of ethmoid bone ; 4, sphenoidal sinus 

 of left side ; 5, sella turcica; 6, basilar process of sphenoid and occipital bone ; 7, pos- 

 terior opening of the right naris ; 8, Opening of the Eustachian tube in the upper 

 part of the pharynx ; 9, soft palate divided through its middle ; 10, cut surface of the 

 hard palate ; a, olfactory nerve; b, its three roots of origin ; c, the olfactory bulb; d, 

 nasal nerve (ophthalmic division of 5th) ; e, naso-palatine nerve (from the spheno- 

 palatine ganglion); /, the anterior palatine foramen ; g, branches of the naso-pala- 

 tine nerve to the palate ; A, anterior and posterior palatine nerves; i, septum narium. 



of smell is lost, while the mucous membrane of the nose re- 

 mains susceptible of the various modifications of common sen- 

 sation or touch. But it is often difficult to distinguish the 

 sensation of smell from that of mere feeling, and to ascertain 

 what belongs to each separately. This is the case particularly 

 with the sensations excited in the nose by acrid vapors, as of 

 ammonia, horseradish, mustard, &c., which resemble much 

 the sensations of the nerves of touch ; and the difficulty is the 

 greater, when it is remembered that these acrid vapors have 

 nearly the same action upon the mucous membrane of the eye- 

 lids. It was because the common sensibility of the nose to 

 these irritating substances remained after the destruction of 

 the olfactory nerves, that Magendie was led to believe the 

 fifth nerve might exercise the special sense. 



Animals do not all equally perceive the same odors; the 



