CEANIAL NERVES 243 



and obliqui muscles. In the orbit it is enclosed 

 in a very tough connective-tissue sheath, and runs 

 straight out to the eyeball. 

 Press the eye-muscles aside to see the optic nerve betiveen 



the recti and obliqui. The course of the nerve within the 



skull will be seen when the brain is removed. 



(ty The third nerve, or motor oculi, is a rather slender nerve 

 which, arising from the ventral surface of the brain 

 below the optic lobes, runs outwards and slightly 

 backwards to the skull- wall, which it perforates 

 about a quarter of an inch behind the optic nerve, 

 and immediately in front of the origins of the recti 

 muscles. 



In the 'orbit it divides at once into three branches, 

 of which the most anterior supplies the rectos 

 internus, and the middle one the rectus superior. 

 The posterior branch passes downwards between 

 the rectus superior and rectus externus close to their 

 origins, and then forwards, ventral to the rectus 

 inferior which it supplies, to the obliquus inferior 

 in which it ends. As it crosses the rectus inferior 

 it is joined by a branch from the fifth nerve. 



Gently press away the optic lobe from the skull-wall so as 

 to see the nerve within the skull, and also the point at which 

 it enters the orbit. In the orbit the branches to the rectus 

 internus and rectus superior are readily dissected from above. 

 To trace the branch to the obliquus inferior place the fish on 

 its side, and turn the eyeball upwards so as to expose the 

 nerve from below. 



4. The fourth or pathetic nerve is a very slender nerve 

 which arises from the dorsal surface of the brain, in 

 the angle between the optic lobe and the cerebellum, 

 and almost in the median plane. From the brain 

 it runs forwards and outwards to the skull-^sall, 

 which it perforates a little in front of the middle of 

 the orbit, dorsal to the optic nerve^ and immediately 

 ventral to the ophthalmic branches of the fifth and 



K2 



