IV. VERTEBRATA. 539 



phibia, lizards, snakes, and birds the choanais far forward, behind 

 the upper jaw; in alligators, turtles, and mammals it is carried far 

 back, in crocodiles and some mammals 

 (edentates) nearly to the vertebral col- 

 umn. This position is brought about by 

 the development of the hard palate, a 

 parting wall which divides the primitive 

 mouth cavity into two portions, a lower, 

 the persistent or secondary mouth cavity, 

 and an upper, which, as secondary nasal Fl <?- 573. Diagram of nose of 



lizard. (After Wiedersheim.) 



cavity, contributes to the air passages. AN, outer nasal cavity; c, 



.m, ^ ,1 -11 -i i f olfactory sac ; O, canal from 



The bones of the maxillary and palatine Jacobson's organ to mouth : 



, ., ,-, i -, i , . C7i, choana; IN, inner nasal 



Aeries contribute to the hard palate, since cavity; MS, roof of mouth; 



11 -n j i j.i -P, Jacobson's organ ; t, con- 



premaxillanes, maxillanes, and rarely the nexion between nasal cavi- 

 pterygoids send out horizontal processes 



"which meet in the middle line. In the mammals this partition 

 is continued back by the muscular soft palate. In crocodiles there 

 is a fibrous palate. 



In the olfactory organ of the chordates two constituents must be 

 recognized, an unpaired and two paired portions. The unpaired portion 

 alone occurs in Amplrioxus, this being supplied by the lobus olfactorius 

 impar ; in all vertebrates there are paired sacs with paired olfactory lobes. 

 The unpaired sac of the cyclostomes has apparently arisen from a union of 

 paired and unpaired parts, hence the double olfactorius. 



A further increase in the nasal cavity is brought about by complicated 

 folds in the walls supported by special skeletal parts, the turbinal bones, 

 .and also by the outgrowth of chambers, lined with mucous membrane 

 which extends into the neighboring bones. Thus are formed the sinus 

 frontalis in the frontal bone ; behind, the sphenoid sinus in the sphenoid, 

 and the antrum of Highmore in the maxillary. Again, a part of the primi- 

 tive chamber lined with olfactory epithelium can be cut off from the rest 

 and form an accessory nose, Jacobson's organ, which opens into the 

 mouth behind the premaxillaries by 'Stenson's duct' (fig. 573, P). This 

 organ is best developed in lizards, mbnotremes and ungulates, but often 

 occurs in a reduced condition in other terrestrial vertebrates. 



In all vertebrates with the exception of Myxine and a few forms 

 living in the dark the eyes are composed of all the principal con- 

 stituents which occur in the human eye and which have already 

 been briefly described (p. 131, fig. 83). In most vertebrates it is a 

 nearly spherical body with, the optic nerve entering it from behind, 

 with its interior occupied by transparent, refractive substances 

 (lens, vitreous body), and its walls of three concentric layers. 

 The outer of these is the tough protecting sclera (sclerotic), a 



