250 THE DOG-FISH 



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 swface, taking 

 care to 'preserve the ophthalmic branches of the fifth and 

 seventh nerves, which run forwards across the olfactory lobe, 

 but separated from it by the cartilaginous cravAum. 



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



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

 and about midway between the origins of the recti 

 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 ietioeen 

 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 most anterior supplies the rectus 

 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 



