CEREBRAL NERVES 



181 



ganglion consists of a primary " spiiial" ganglion and of a more peripheral 

 Lntvi'id ganglion in connection with the nerve, from which latter an epi- 

 branchial ganglion arises from the epiblast dorsally to the region of the gill- 

 clefts, and takes part in the formation of the terminal branches of the nerve. 

 The presence of an epibranchial ganglion on the trigeminal may indicate 

 the former presence of a gill-cleft in this region. 



It must be remembered that the head is primitively composed 

 of a series of metameres (p. 66), and it is therefore important to 

 ascertain, as far as is possible in the present state of our knowledge, 

 to which individual metameres the different cranial nerves belong. 

 The olfactory and optic nerves present certain peculiarities which 

 bring them under another category, and they will be treated of 

 later in connection with the corresponding sensory organs. 



The following general summary gives a scheme of the prob- 

 able primitive relations of the head-segments and cerebral 

 nerves, founded mainly on the conditions existing in Elasmo- 

 branch embryos. 



TABLE SHOWING THE SEGMENTAL ARRANGEMENT or THE CEREBRAL NERVES, 

 WITH THEIR RELATION TO THE METAMERES OF THE HEAD. 



Ventral branch. 



Dorsal branch. 



\*t Metamere (superior, in- 

 ferior, and anterior rec- 

 tus, and inferior oblique 

 muscle). 1 



'2nd Metamere (superior 

 oblique). 1 



Oculomotor (///). 



Trochlear (IV). 



3rd Metamere 

 rectus). 1 



(posterior Abducent (VI). 



4th Metamere (muscles 

 which are early aborted). 



oth Metamere (muscles 

 which are early aborted). 



Qth and 1th Metamere* (part 

 of the most anterior 

 region of the large trunk- i 

 muscles). 



8th and 9th Metameres (an- 

 terior part of trunk- 

 muscles). 



Wanting. 



Wanting. 



Appears to be 

 wanting. 



Ventral roots of the 

 hypoglossal. 



Ramus ophthalmicus pro- 

 fundus of the trigeminal 

 (V), together with the 

 ciliary ganglion. 



Trigeminal (with its gang- 

 lion, minus the ramus 

 ophthalmicus profun- 

 dus). 



1 Facial ( VII), and audi- 

 tory ( VIII), with their 

 ganglia. 



1 Glossopharyngeal 

 with its ganglion. 



Vagus (X), with its gang- 

 lia. 



Vestigial dorsal roots of 

 the hypoglossal (XII), 

 usually only present in 

 the embryo. 



Figures 148 and 149 illustrate the distribution of the cerebral 

 nerves in adult aquatic and terrestrial Vertebrates respectively (comp. 



1 It is possible, however, that these eye-muscles belong, not to the somites, 

 as stated on pp. 133 and 143, but to the visceral muscles. 



