1 64 THE FROG chap. 



their origin the right and left optic nerves have their 

 fibres intermingled, forming a structure something like a 

 St. Andrew's Cross and called the optic chiasma {opt. ch), 

 the other limbs of the cross passing upwards and back- 

 wards to the optic lobes. The optic nerve makes its 

 exit from the brain-case through the optic foramen, and is 

 distributed to the retina, a delicate membrane which 

 lines the eyeball, and is, as we shall see, the actual organ 

 of sight. This nerve also is purely sensory. 



The third or oculomotor (Fig. 49, III) is a small nerve 

 arising from the crura cerebri beneath the optic lobes. It 

 passes through a small hole in the cartilaginous side of 

 the skull and supplies four out of the six muscles by 

 which the eyeball is moved, and is purely motor. 



The fourth ox pathetic (IV) is a very small nerve leaving 

 the dorsal surface of the brain between the optic lobes and 

 the cerebellum, and making its exit from the skull just above 

 the optic nerve. It is also purely motor, supplying one of 

 the muscles of the eye — the superior oblique. 



The ^fth or trigeminal {Figs. 49 and 53, V) is a large nerve 

 arising from the side of the medulla oblongata. Its root 

 dilates to form a large ganglion, the Gasserian ganglion, 

 and leaves the skull by the large aperture noticed in the 

 pro-otic bone. It owes its name to the fact that it soon 

 divides into three main branches, one, the ophthalmic (Fig. 

 53, V^), going to the skin of the snout, another, the maxillary 

 { F^) to the upper lip and lower eyelid, and the third or 

 mandibular { V^) to the muscles and skin of the lower jaw. 

 The trigeminal is a mixed nerve. 



The sixth or abducent (Fig. 49, fT') is a very small motor 

 nerve arising from the ventral aspect of the bulb and 

 supplying one of the muscles of the eyeball called the 

 posterior rectus. 



