164 THE FROG CHAP. 



verse partition of the girdle-bone. Each is distributed 

 to the mucous membrane of the nasal sac or organ of 

 smell of the same side, and is purely sensory. 



The second or optic (II) is a large nerve which springs 

 from the ventral surface of the 'tween brain. At 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 eye-ball, 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 eye-ball is moved (p. 186), and is purely 

 motor. 



The fourth or 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 

 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, F 1 ), going to the skin of 

 the snout ; another, the maxillary (F 2 ), to the upper lip 

 and lower eyelid; and the third, or mandibular (F 3 ), 



