534 THE NERVOUS SYSTEM. 



of smell. The whole mucous membrane of the nose is not sup- 

 plied with olfactory fibers ; hence only in that part where they are 

 present, the olfactory membrane, does the sense of smell reside. 

 The proof that the function of these nerves is that of smell is 

 derived from experiments upon lower animals and from observa- 

 tions upon man. Animals whose sense of smell is very acute have 

 the olfactory bulbs and tracts more highly developed that is, 

 these structures are larger than in those animals in which the 

 acuteness of the sense of smell is not marked. If the tracts are 

 destroyed, the sense of smell is abolished. Although this experi- 



FIG. 312 . Nerves of the outer walls of the nasal fossse : 1, network of the 

 branches of the olfactory nerve, descending upon the region of the superior and 

 middle turbinated bones ; 2, external twig of the ethmoidal branch of the nasal 

 nerves ; 3, sphenopalatine ganglion ; 4, ramification of the anterior palatine nerves ; 

 5, posterior, and 6, middle, divisions of the palatine nerves ; 7, branch to the region 

 of the inferior turbinated bone ; 8, branch to the region of the superior and middle 

 turbinated bones; 9, nasopalatine branch to the septum cut short (from Sappey, 

 after Hirschfeld and LeveillS). 



mental proof is not applicable in man, still there are cases on 

 record in which an absence of the sense of smell during life has 

 been found after death to have accompanied an absence of the 

 olfactory tracts ; and there are cases also of individuals whose 

 sense of smell has been seriously impaired after injury to the tracts. 

 During ordinary respiration the inspired air does not pass over 

 the olfactory membrane, but only over the lower part of the 

 Schneiderian membrane, the respiratory portion. Hence if odors 

 are faint, they are not detected, unless by a strong inspiration the 

 air is carried up to and over that portion to which the olfactory 

 nerves are distributed. If the nasal passages are closed by a 

 catarrhal condition, the sense of smell is obtunded or may even 

 be abolished temporarily. 



