THE SENSE ORGANS 143 
and that in the sinus frontalis 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, 7.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, 2.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. 
J ACOBSON’S ORGAN 
The first indications of this organ appear to occur among the 
tailed Amphibia,’ in the form of a small ventral diverticulum 
of the nasal cavity (yc, 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). ; 
