244 THE DOG-FISH 



seventh nerves. In the orbit it runs outwards and 

 forwards to the posterior border of the obliquus 

 superior, in which it ends. 



Though very slender, the fourth nerve is easy to trace. 

 Lift up the front end of the cerebellum to see its origin 

 between the cerebellum and the optic lobes ; trace it forwards 

 within the skull, and in the orbit outwards to the obliquus 

 superior. 



The fifth, seventh, and eighth nerves arise very close 

 together from the side of the medulla at its widest 

 part, opposite the posterior part of the cerebellum. 

 The eighth or auditory nerve, which is the hindmost 

 of the three, enters the auditory capsule at once ; 

 while the fifth and seventh nerves pass outwards 

 through the skull-wall, by a foramen at the posterior 

 and inner angle of the orbit, immediately behind the 

 origins of the recti muscles. 



Gently press away the medulla from the skull-wall until 

 the nerve-roots come into view, and then carefully cut away 

 the skull-wall and the auditory capsule until they are fully 

 exposed. 



The fifth or trigeminal nerve has three main 

 branches. 



i. 1 The ophthalmic branch arises from the anterior 

 border of the root of the nerve, close to the 

 brain. It runs forwards and upwards for 

 about a quarter of an inch within the skull, 

 and then perforates the skull-wall to enter 

 the orbit, above and a little in front of the 

 origins of the recti muscles. 



In the orbit it runs forwards close to the 

 skull-wall, alongside of and immediately below 

 the ophthalmic branch of the seventh nerve, 

 and dorsal to all the eye-muscles. At the 

 anterior and inner angle of the orbit it passes 

 through a canal between the olfactory capsule 



