OLFACTORY ORGAN 



259 



centripetally towards the olfactory lobes and pass into the fore-brain, 

 when they become connected with the olfactory centre. The 

 individual olfactory cell and fibre thus form an organic unit a 

 primitive condition such as occurs in certain integumentary sense- 

 organs of Worms and Molluscs, but not in any of the other sensory 

 cells of Vertebrates. The olfactory cells thus constitute the only 

 true neuro-epithelium in Vertebrates, as the nerve arises in connec- 

 tion with the cell itself, with which it remains continuous (primary 

 & n*ory^ cell) : in other secondary nerve-cells, the relation of cell and 

 nerve is one of apposition merely. 



In their final form, the olfactory cells are elongated, swollen in 

 the region of the nucleus, and bear hair-like processes on their free 

 ends, while proximally each is continuous 

 with a nerve-fibre (Fig. 189). Between 

 them are isolating or supporting cells, 

 which have a similar origin, and ciliated 

 cells may also be present. 



The olfactory organs in all Fishes are 

 of a simple sac-like form, but from the 

 Dipnoi onwards they come to communicate 

 with the cavity of the mouth as well as 

 with the exterior. In consequence of this, 

 anterior or external nostrils, and posterior or 

 internal nostrils (choance) can be distin- 

 guished : as a free passage is thus formed 

 through which air can pass, the olfactory 

 organ takes on an important relation to 

 the respiratory apparatus, and in it olfac- 

 tory and respiratory regions can be dis- 

 tinguished. 1 From the Amphibia onwards FlG - 189. EPITHELIUM OF 

 glandular elements are present, the secre- 

 tion of which serves to keep the nasal 

 cavity moist. 



In Amphioxus, the ciliated pit sup- 

 plied by a nerve and situated above the 

 anterior end of the central nervous system 

 probably represents an unpaired olfactory organ. 



Cyclostomes. In these forms the oltactory organ consists of a 

 sac enclosed by a fibro-cartilaginous capsule containing numerous 

 radial folds of the mucous membrane enclosing tun-shaped sense- 

 buds, and is unique in being unpaired (Fig. 190). It lies just in 

 front of the cranial cavity, arid opens on the dorsal surface of the 

 head by a chimney-like tube, which in Myxine is long and is 



1 The mode of formation of the primitive choanae is already indicated in 

 many Elasmobranchs as well as in the embryo of Ceratodus, in which a groove, 

 bounded by folds of the skin, extends backwards from each external nostril to 

 the mouth (Fig. 191, A), through which water passes. In Mammals there is no 

 naso-oral funnel, and the development of the choana? in the higher Vertebrates is 

 accompanied by a secondary perforation of the primarily blind nasal sac. 



s 2 



THE OLFACTORY Mucous 

 MEMBRANE. A, oiPetro- 

 myzon planeri ; B, of 

 Salamandra alra. 



E, interstitial epithelial 

 cells ; ft, olfactory cells. 



