CEREBRAL NERVES 



237 



founded mainly on the conditions existing in Elasmobranch 

 embryos. 



TABLE SHOWING THE SEGMENTAL ARRANGEMENT OF THE CEREBRAL NERVES, 

 WITH THEIR RELATION TO THE METAMERES OF THE HEAD. 



!*< Metamere (superior, in- 

 ferior, and anterior rec- 

 tus, and inferior oblique 

 muscle). 



2nd Metamere (superior 

 oblique). 



3rd Metamere (posterior 

 rectus). 



\th Metamere (muscles 

 which are early aborted). 



5/h Metamere (muscles 

 which are early aborted). 



Ventral branch. 



Oculomotor (III). 



Pathetic (IV). 



Abducent (VI). 

 Wanting. 

 Wanting. 



Dorsal branch. 



Ramus ophthalmicus pro- 

 fundus of the trigeminal 

 ( V), together with the 

 ciliary ganglion. 



Trigeminal (with its gang- 

 lion, mini(-t the ramus 

 ophthalmicus profnn- 

 dus). 



Facial (VII), and audi- 

 tory ( VIII), with their 

 ganglia. 



Glossopharyngeal (IX ), 

 with its ganglion. 



Nerves of the Eye-muscles. The oculomotor (III) pathetic 

 or trochlear (IV) and abducent (VI) nerves (Figs. 179181) 

 supply the muscles which move the bulb of the eye (cf. table 

 above). 



The oculomotor arises from the base of the mid-brain, and is 

 in intimate relation with an oculomotor or ciliary ganglion which 

 primarily belongs to the sympathetic system, and from which oculo- 

 motor fibres pass out to the iris and ciliary muscles of the eye. 1 



The pathetic nerve, 2 although actually arising in the interior 

 of the ventral part of the mid-brain, near the nucleus of the 

 oculomotor, appears externally on the dorsal side of the anterior 

 margin of the hind-brain (valve of Vieussens, p. 203). 



The abducent nerve arises far back on the ventral side of the 

 medulla oblongaia : in addition to the posterior rectus, it supplies 

 the retractor bulbi and the muscles of the nictitating membrane in 

 the Sauropsida. In the Anura it becomes intimately connected 

 with the Gasserian ganglion of the trigeminal. 



1 Further researches are desirable with regard* to the ciliary ganglion of 

 Anamnia and Sauropsida. It appears to be made up of two distinct ganglia 

 (eerebro-spinal and sympathetic), one of which, in Mammals, fuses with the 

 Gasserian ganglion during development, the other remaining as the ciliary gang- 

 lion of the adult. 



- Before emerging on the roof of the brain, the fibres of each trochlear nerve 

 cross those of its fellow (c/tiitxnia trorh/nii-t), so that the right nerve supplies the 

 left superior oblique muscle, and ri<-i /< /:/. Originally this muscle may be a 

 derivative of muscles which once supplied the parietal eye. 



