276 THE DOG-FISH. 



The ophthalmic branches of the fifth and seventh nerves, 

 which run through the orbit close to the skull-wall, and dorsal to 

 all the eye-muscles, should be identified at once ; and care must 

 be taken not to injure them (see pp. 278, 279, and fig. 59, p. 272). 



A paired pre-olfactory nerve arises at the extreme front 



end of the brain, and, resting upon the olfactory lobe, 



passes to the nasal septum. The olfactory * nerves ' 



arise on each side in two large bundles from the 



anterior surface of the olfactory lobe, and pass at 



once through the sieve-like membrane separating 



the cranial cavity from the nose. They end in the 



folds of the large olfactory organ. 



Dissect the olfactory nerves from the dorsal surface, taking 



care to preserve the ophthalmic branches of the fifth and seventh 



nerves, which run forwards across the olfactory lobe, but separated 



from this by the cartilaginous cranium. 



2. The optic " nerve " enters the orbit through a foramen in 



the side wall of the skull, near its ventral surface, 

 and about midway between the origins of the recti 

 and of the obliqui muscles. In the orbit it is en- 

 closed in a very tough connective-tissue sheath. 

 It is not a nerve but a portion of the brain. 

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



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



will be seen when the brain is removed. 



3. 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 first supplies the rectus internus, and 

 the middle one the rectus superior. The hindmost 

 branch passes downwards between the rectus superior 

 and rectus externus close to their origins, and then 



