THE SENSE ORGANS 143 



and that in the sinus fron tails of the embryo (as Professor 

 Killian, who has paid especial attention to this subject, has 

 kindly informed me) even now ridge-like structures sometimes 

 occur, reminding one in the manner of their origin of the eth- 

 moidal system, it seems probable that there was once a still more 

 highly specialised development of the olfactory organ. 



The above remarks apply to the olfactory region proper, i.e. 

 to the ethmoidal labyrinth with its olfactory ridges. I have 

 so far purposely avoided the term turbinal, and have always used 

 instead the word ethmo-turbinal, or Schwalbe's term " olfactory 

 ridge," in order to exclude any suggestion of parallelism with the 

 " turbinal " of the lower Vertebrata. But we now come to the 

 question of the persistence of the latter among the Mammalia. 

 To these animals it has been handed down as the " inferior 

 turbinal," but it now possesses no olfactory epithelium, having 

 evidently undergone a change of function. In animals in which 

 smell is acute, it is folded or more or less branched, i.e. is much 

 more complicated than in animals with less keen scent, in which 

 it is merely singly or doubly scrolled. The latter must be con- 

 sidered as the more primitive condition, from which the former 

 was secondarily developed. 



The conditions which have led up to reduction of the olfactory 

 organ in the vertebrate series are very various. In Man its 

 degeneration is due to the subordinate part played by it. The 

 olfactory apparatus is here, as Broca has rightly remarked, but a 

 modest vassal of the brain, which does not reach the perfection 

 of the other higher sense organs. 



JACOBSON'S ORGAN 



The first indications of this organ appear to occur among the 

 tailed Amphibia, 1 in the form of a small ventral diverticulum 

 of the nasal cavity (jc., Fig. 89, A, B), which either retains 

 its original position throughout life, or in the course of develop- 

 ment becomes shifted so as to lie in the maxillary sinus (Fig. 

 89, E). 



At exactly the same point near the nasal septum, where, in 

 the Amphibia, this organ arises, in the Amniota Jacobson's organ 

 is found, in the form of a diverticulum of the principal nasal 



1 Apparent indications of this apparatus are forthcoming in certain fishes 



(Polypterus). 



