122 ANATOMY OF VERTEBRATES. 



orbits, as far as tlie pituitary membrane of the ethmoturbinals, 

 upon wliicli its filaments are distributed in a radiated manner. 

 In Apteryx and Dinornis, the rhinencephalon is of large relative 

 size, and sends off the olfactory nerves by many filaments through 

 a ^ cribriform plate.' 



The optic nerves, fig. 44, a, are in general of remarkable size ; 

 they arise from the whole of the outer surface of the optic lobes, 

 and from the thalami, the two origins forming by their union 

 the ^ radix opticus,' fig. 47, d, which expands into the ' chiasma.' 

 Here a partial decussation, ib. h, takes place. By removal of the 

 firmly adherent neurilemma, the optic nerve is seen to be com- 



47 48 



Chiasma of optic ucrves. Fowl. xxx*. Laminated optic nerve of an Eagle, ecu. 



posed of parallel, longitudinal lamellae, the margins of which are 

 most free on one side, fig. 48, 5. 



The third, or oculomotorial nerve, arises behind the hypophysis 

 from the grey matter that lies here between the crura cerebri : 

 it escapes, usually, by a distinct hole, fig. 5Q, 3, near the foramen 

 opticum, and supplies the superior, inferior, and internal musculi 

 recti, and the obliquus inferior : it also sends off a ciliary branch, 

 which sometimes forms a ganglion before, sometimes after, joining 

 the ramus ciliaris trigemini. 



The fourth nerve arises from the posterior flattened band, 

 extending over the ' valvula Yieussenii ' between the back part of 

 the optic lobes : its course, immediately above the superorbital 

 branch of the fifth pair, is shown at fig. 6Q, 4*, as far as its termi- 

 nation in the superior oblique muscle, ib. f\ to which it is, as in 

 other Vertebrates, exclusively distributed. 



The^fth or trigeminal nerve, fig. 49, 5, 6, 7, has two origins ; 

 the ' portio major,' from the fore part of the base of the crus 

 cerebelli, the ' portio minor,' from the prepyramidal tract in ad- 

 vance of the foregoing, which it joins after the reddish gan- 

 glionic swelling, fig. 49, 3, has been formed. The two origins are 

 less distinct than in Mammals ; but the larger one is more readily 



