268 THE DOG-FISH. 



Dissect the nerves on the right side, leaving the left side 

 for the dissection of the sense-organs. 



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 thejn (see pp. 270 and 271). 



1. 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 mem- 

 brane 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, ivhioh run forioards across the olfactory lobe, 

 but separated from this by the cartilagiiious 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 oblique muscles. In the orbit it is enclosed 

 in a very tough connective-tissue sheath, and runs 

 straight outwards to the eyeball. 



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 luhen 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 



