154 



COMPARATIVE ANATOMY. 



The following general summary gives a scheme of the primitive 

 relations of the head segments. It is to be noticed that the first 

 and second cranial nerves the olfactory and the optic are not 

 mentioned in the list, for reasons to be explained later. 



TABLE SHOWING; TUP. SEKMENTAL ARKAXOKMEXT <>F THE CRAXIAL NERVES, 

 WITH THEIR RELATION TO THE METAMEKES OF THE HEAD. 



1st Met am ere (superior, 

 inferior, and internal rec- 

 tus, and inferior obliijue 

 muscle). 



2nd Me tain ere (superior 

 oblique). 



Ventral branch. 



Dorsal branch. 



3rd Metainere 

 rectus). 



(external 



4th Metamere (muscles 

 which are early aborted). 



5 th Metamere (muscles 

 which are early aborted). 



6th Metamere (very rudi- 

 mentary muscles). 



7th to 9th Metameres 

 (muscles extending from 

 the skull to the pectoral 

 arch, including the an- 

 terior portion of the 

 sterno-hyoid). 



Oculomotor (///). 



Troehlear (//'). 



Abducent ( VI}. 

 Wanting. 

 Wanting. 



Appears to be want- 

 ing. 



Hypoglossal (XII). 



Ramus ophthalmicus pro- 

 fundus of the trigemin il 



Trigeminal (with the excep- 

 tion of its ram us ophthal 

 micus profundus). 



Facial (VII}, and audi- 

 tory ( VIII). 



Glossopharyngeal (IX). 



Vagus (A'). 



The cranial nerves may be divided into four main groups, 1 quite 

 apart from their metameric signification. The first consists of the 

 olfactory, or first, and the optic, or second cranial nerve; the 

 second of the nerves of the eye-muscles, i.e. the oculomotor, 

 trochlear, and abducent nerves, the third of the trigeminal 

 with the auditory and facial,and the fourth of the glossopharyn- 

 geal and vagus. The eleventh cranial nerve, or spinal accessory, 

 as well as the twelfth, or hypoglossal, although they occasionally 

 (in Mammals for instance) are included within the cranial cavity, 

 come under the category of spinal nerves. 



Olfactory Nerve. The olfactory, when compared with the 

 other cranial nerves, possesses many peculiarities, which seem to 

 give it an isolated position. It grows out secondarily from the 



1 No satisfactory morphological explanation has yet been given of the two pairs of 

 nerves branching out to supply the anterior end of the body in Amphioxus. 



