126 ZOOTOMY. 



t.s, tori semicirculares : v.t,, third ventricle : v.^, fourth ventricle : x, 

 valve-like flap covering anterior part of aqueduct of Sylvius : /, 

 peduncles of olfactory lobes : //, optic nerve : ///, third nerve : IV, 

 fourth nerve : V, fifth nerve : K\ orbitonasal nerve : V-"", maxillary 

 nerve : V ''^, palatonasal nerve : V'', mandibular nerve : V^, cutaneus 

 quinti : VI, sixth nerve : VII, seventh nerve : VIII, auditory nerve : 

 IX, glossopharyngeal ; X, vagus : X', commissure between fifth and 

 tenth nerves. 



cords are single in the anterior, double in the posterior part 

 of their course. 



190. The large strap-like optic nerves (II), arising 

 from the ventral surface of the brain, immediately in front of 

 the optic lobes : they pass at first directly forwards, then 

 cross one another, the right nerve going to the left eye, and 

 vice versa . they make their exit from the cranial cavity 

 through foramina in the membranous side-walls of the 

 skull. 



191. The third pair of nerves (oculomotor, III), 

 arising from the ventral surface of the mid-brain, external to 

 the lobi inferiores and passing to their foramina in the mem- 

 branous wall of the skull some distance posterior to the 

 optic foramina. 



192. The delicate fourth pair of nerves (pathetic, IV), 

 arising from the dorsal surface of the brain, between the 

 optic lobes and the cerebellum, and leaving the skull a little 

 above the optic foramen. 



193. The fifth (trigeminal, V) and seventh (facial or 

 portio dura, VII) nerves, arising together from the anterior 

 part of each side of the medulla oblongata, and leaving the 

 cranial cavity by the trigeminal notch in the prootic (§ 27). 



The fifth nerve early divides into the three characteristic divisions : 

 the first of these (orbitonasal, V^) passes forwards and upwards along 

 the inner wall of the orbit, over the optic nerve, and supplies the parts 

 about the snout : the second division (maxillary, V-") passes, outwards 



