CRANIAL NERVES AND SENSE-ORGANS 



239 



the Invertebrates up to the highest Vertebrates, nothing is 

 known of the constitution of the olfactory perceptions. 



In worms, as we have seen, there are superficial ciliated 

 pits supplied with nerves springing from the brain ganglion. 

 In Molluscs (pteropods, gastropods, cephalopoda) certain pits, 

 at the bottom of which is 

 situated a papilla with a nerve, 

 serve as olfactory organs. As 

 to the site of the olfactory 

 organs in insects (mouth or 

 antennae ?) there have been 

 innumerable conjectures. It 

 is generally supposed that the 

 odorous matter is wafted on 

 the current of air by the an- 

 tennas to the olfactory surface 

 provided with its nerves ; per- 

 haps the so-called olfactory 

 bristles which are in communi- 

 cation with nerve - endings 

 perform this function. 



Similar olfactory bristles 

 exist in the antennae of crusta- 

 ceans ; in myriapods these are 

 developed into olfactory cylin- 

 ders which may be hollow 

 (Leydig). 



It might be assumed that 

 the physiology of smell in Ver- 

 tebrates would be explained 

 beyond all error, but the 

 physiology of the olfactory 



FIG. 1 19. Diagram of the complete 

 olfactory tract. (Munk - Schultz.) 

 i, olfactory cells in the mucous 

 membrane ; 2, glomerulus in the 

 olfactory bulb; 3, mitral cells; 4, 

 cells of the granular layer ; 5, 

 branching cell. 



organs lags far behind the 



anatomy. From the very 



dawn of anatomical knowledge, a " club-like " nerve running 



in a groove in the under-surface of the frontal lobes of the 



brain over the ethmoid bone has been recognised as the nerve 



of smell ; its branches run through the ethmoid cells and end 



in the olfactory cells of the nasal mucosa (regio olfactoria, see 



