1 64 THE FROG CHAR 



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 (III) is a small nerve arising 

 from the crura cerebri beneath the optic lobes. It passes 

 through a small hole in the side of the skull behind the 

 optic foramen, and supplies four out of the six muscles 

 by which the eyeball is moved, and is purely motor. 



The fourth QI 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 above 

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

 the muscles of the eye the superior oblique. 



The fifth 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 ganglion, the Gasserian ganglion, 

 and leaves the skull by the large aperture noticecT 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, F 1 ), going to the skin of the snout; another, the maxillary 

 ( V 1 ), to the upper lip and lower eyelid ; and the third, or 

 mandibular ( F 3 ), to the muscles and skin of the lower jaw. 

 The trigeminal is a mixed nerve. 



The sixth or abducent (Fig. 49, VI) 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. 



