72 THE ANATOMY OF VERTEBEATED ANIMALS. 



nienceplialon, and eacli goes to the eye of its own side. 

 In other Vertebrata, the nerves cross one another at the 

 base of the brain (TeleosteiJ, or are fused together into a 

 chiasma fGanoidei, EJasmobrancliii, and all the higher Verte- 

 hrataj . In the higher Vertebrata, again, the fibres of the optic- 

 nerves become connected chiefly with the mesencephalon. 



All the other cerebral neiwes differ from these in arising, 

 not as diverticiTla of any of the cerebral vesicles, but by 

 histological difi'erentiation of the primitive brain-case, or 

 lamince dorsales of the skull. 



The third (motores oculorum) and fourth fpathetici) pairs 

 of nerves are distributed to the muscles of the eye; the 

 third to the majority of these muscles, the fourth to the 

 superior oblique muscles. The third paii- of nerves issues 

 from the crura cerebri, or inferior division of the meten- 

 cephalon, upon the base of the brain ; the f om-th pair, from 

 the forepart of the upper division of the metencephalon, 

 immediately behind the optic lobes, upon the superior 

 surface of the brain. This region is known as the Valve of 

 Vieussens in the Mammalia. 



All the other cerebral nei-ves originate in the posterior 

 division of the hind-brain — the myelencepbalon. The great 

 fifth pair ftrigeminij passes out from the sides of the meten- 

 cephalon, and supplies sensory nerves to the integument of 

 the head, and motor nerves to most of the muscles of the 

 jaws, by its three divisions — the ophthalmic, the superior 

 maxillary, and the inferior maxillary, nerves. 



Of these divisions the two latter are, very generally, 

 closely connected together, while the ophthalmic division 

 remains distinct. The ophthalmic di\ision passes to the 

 cleft between the trabecula and the maxillaiy process (which 

 nearly corresponds with the orbit, and might be termed the 

 orbito-nasal cleftl, and is distributed to the inner and the 

 outer side of that cleft. Hence its main branches are nasal 

 and lachi-ymal. The two maxillary nerves, on the other 

 hand, are distributed to the inner and outer sides, or ante- 

 rior and posterior boundaries, of the buccal cleft. Hence 

 the superior maxillary belongs to the posterior, or outer, 



