ANATOMY OF THE OLFACTORY ORGAN 35 



to the olfactory cells, free-nerve endings occur in the ol- 

 factory epithelium of vertebrates. 



The source of the nerve-fibers from which the free- 

 endings of the olfactory epithehum arise is not definitely 

 settled. The fact that these endings may be very near the 

 outer surface of the olfactory epithehum shows that they 

 are not due to the incomplete impregnation of fibers from 

 the olfactory cells as was suggested by Van Gehuohten 

 (1890). Free-endings like those in the olfactory region 

 also occur in the respiratory region and here the only pos- 

 sible source for them is the trigeminal nerve ; hence it is 

 probable that this nerve is also the source of the free- 

 nerve endings of the olfactory region. This opinion is sup- 

 ported by the observations of B-ubaschkin (1903) who has 

 shown that in certain portions of the olfactory epithelium 

 of the developing chick the two sets of fibers, those from 

 the olfactory nerve and those from the trigeminal nerve, 

 take somewhat different courses and that the trigeminal 

 fibers are the fibers that give rise to the free-endings. 

 (Fig. 10). Thus such evidence as there is favors the 

 opinion first expressed by von Brunn and subsequently 

 reiterated by a number of investigators, that the free- 

 nerve endings of the olfactory region are from the tri- 

 geminal fibers. The vertebrate olfactory epithelium, 

 therefore, has two types of nerve terminations, olfactory 

 cells as the exclusive receptors for the olfactory nerve 

 and free-nerve endings as the probably exclusive endings 

 for the trigeminal nerve. 



8. Development of Olfactory Nerve. Since the fibers 

 from the olfactory cells pass as olfactory nerve-fibers 

 to the olfactory bulb and terminate there without direct 

 connections with any other cells, the olfactory cells in the 



