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body is occupied by the insertion of the muscles of the two eyes, these muscles bein<T 

 separated from the interior cavity of the skull by a tough membrane, continuous with 

 the dura mater posteriorly, and anteriorly with the membrane which completes the 

 septum between the orbits. This membrane is pierced for the exit of the optic, 

 olfactory, and other nerves, and on it rest the olfactory lobes. 



The Cranial Nerves. 



The olfactory nerves pass from the olfactory lobes to the olfactory capsules, termi- 

 nating in the olfactory epithelium of these organs. Owing to the rotation of the left 

 eye and orbit the position of these nerves in the sole is peculiar. Each olfactory nerve 

 in all fishes passes over the dorsal side of the recti muscles of the eye. Tlie relations 

 of the nerves to surrounding structures are not altered in the sole, but the rotation of 

 the eyes has brought the olfactory nerves into asymmetrical positions. Thus both of 

 these nerves are apparently on the upper (right) side of the head, though morphologi- 

 cally they are on opposite sides. Both of them, like other structures connected with 

 the orbits, are on the coloured side of the head, to the right of the anterior part of the 

 dorsal fin. The left nerve passes above the recti muscles of the left eye close to the 

 interorbital septum, and in front of these muscles bends downwards and passes through 

 the large foramen in the left ectethmoid bone (PI. XI, G, I) to reach the left olfactory 

 capsule on the lower side of the head. 



The right nerve has a perfectly straight course along the ventral (right) siile of the 

 interorbital septum, dorsal to the recti muscles of the ventral (right) eye. It passes 

 through the foramen in the right ectethmoid without any bending, and then bends 

 slightly downwards to the olfactory capsule of the upper (right) side. 



Each of these nerves, though spoken of in the singular, consists of a bundle of 

 separate nerves, whieli are separate tliroughotit their course, and not united into a 

 single cord. 



The optic nerves, as usual in bony fishes, cross one another at their origin without 

 mingling. The dorsal (left) nerve is very slightly longer than the ventral (right). The 

 left arises from tlie lower side of the right optic lobe in front of the lobiis inferior or 

 optic thalanms, the right from a corresponding position on the left side. The optic 

 nerve passes between the internal and superior 7-ecins close to their origin into the 

 space enclosed by the recti muscles, and so reaches the eye-ball. 



The third, fourth, and sixth pairs of cranial nerves are motor nerves all distributed to 

 the nmscles of the eyes. The third pair are called the motores oculorum and split up 

 into branches which enter ths superior, inferior, and internal recti, and the inferior 

 oblique nmscles. The fourth pair of nerves are called trochleares and supply the 

 superior obliciue muscles exclusively. The sixth pair are called nbduccntes, and enter 

 only the external recti maicles. These nerves arise from the sides of the lower part of 



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