Il6 ELEMENTARY BIOLOGY. [CHAP. 



Lay bare the cerebro-spinal axis from the ventral 

 aspect, as directed in Sect. K. § 3, and expose 

 the Gasserian ganglion with the utmost care. 



a. The yd (oculomotor) nerve ; arising, on each 

 side, from the ventral surface of the brain on 

 a level with the pituitary body. It passes up- 

 wards and outwards to leave the skull in 

 front of the 5th nerve (cf. p. 67, vi.). 



p. The 6th nerves (n. abducentes) ; extremely 

 delicate, arising close together in the middle 

 line from the floor of the medulla, on a level 

 with the 9th and 10th. Each passes upwards 

 and outwards along with the trunk of the 

 5th to enter the Gasserian ganglion. 



y. The 4th (n. patheticus) ; best seen on re- 

 moving one optic nerve and turning the brain 

 a little to one side. It leaves the dorsal sur- 

 face of the brain in front of the cerebellum ; 

 and passes upwards and forwards to leave the 

 skull above and in front of the optic nerve 

 (cf. p. 67, vii.). 



iii. The structure of the eye-ball. Obtain a freshly- 

 killed frog and remove the head from the trunk ; 

 bisect the former longitudinally and examine the 

 eye-ball while still in position, as under. 



Make two sections; that of one side (a.) to 

 bisect the eye-ball transversely to the long axis of 

 the body (to pass through the crystalline lens if 

 possible) ; that of the other (b.) to bisect it equa- 

 torially, at right angles to a. Examine under a 

 hand-lens. 



