270 THE DOG-FISH. 



Gently press away the medulla from the skull-wall until 

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

 the shull-wdll and the auditory capsule until the roots are 

 fully exposed. 



a. The fifth or trigeminal nerve has three main 

 branches, 

 i. 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 

 and the cranium, crosses the olfactory lobe, 

 and continuing its course forwards divides into 

 branches, which supply the skin of the dorsal 

 surface of the snout. 



Gently press away the brain from the skull-ivall to see the 

 root and the part of the nerve within the cranium, talcing care 

 not to confound it with the ophthalmic branch of the seventh 

 nerve which lies immediately dorsal to it. In the orbit dissect 

 from the side, and in front of the orbit from above. 



The main stem of the fifth nerve on entering 

 the orbit (accompanied by the buccal branch of 

 the seventh) runs forwards and outwards across 

 its floor as a broad ribbon-like band, easily 

 mistaken in spirit specimens for a muscle. Near 

 the outer margin of the orbit this separates 

 into the maxillary and mandibular branches. 

 The buccal branch of the seventh nerve lies 

 between and dorsal to these. 



