CEREBRAL NERVES 



181 



■gangHon consists of a primary "spinal" ganglion and of a more peripheral 

 lateral ganglion in coiuiection ^Yith the nerve, from which latter an ejji- 

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

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

 The presence of an epibranchial ganghon on the trigeminal may indicate 

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



It must be remembered tbat 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- 

 brahch embryos. 



Table showing the Semiextal Arrangemext of the Cerebral Nerves, 

 WITH their Relation' to the Metameres of the Head. 



Ventral branch. 



Dorsal branch. 



\st Mi'amtre (superior, in- 1 

 ferior, and anterior rec- 

 tus, and inferior oblique 1 

 muscle).' 



Oculomotor (III)- 



Ramus ophthalmicus pro- 

 fundus of the trigeminal 

 (F), together with the 

 ciliary ganglion. 



•2ivl Miitamtr 

 oblique).' 



(superior i Trochlear {IV). 



Srd Metamere (posterior ' Abducent |( TT). 

 rectus).' 



ith Metartiere (muscles 

 which are early aborted). 



bih Jlttnmere (muscles 

 which are early aborted). 



6th and Itli Mdameres (part 

 of the most anterior 

 region of the large trunk- 

 muscles). 



Slh and Qth Metameres (an- 

 terior part of trunk- 

 muscles). 



Wanting. 



Wanting. 



Appears to be 

 wanting. 



Ventral roots of the 

 hypoglossal. 



Trigeminal (with its gang- 

 lion, miniix the ramus 

 ophthalmicus profun- 

 dus). 



[ Facial ( VII], and audi- 

 tory ( VIII), with their 

 ganglia. 



Glossopharyngeal {IX), 

 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. 



' 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. 



