THE SEGMENTAL VALUE OP THE CRANIAL NERVES. 



167 



greatest number of clefts supplied by it in vertebrates above Am- 

 phioxus is met with among the Marsipobranchii and in Notidanus, 

 where it supplies the six posterior branchial clefts, and must therefore 

 be equivalent to at least six segmental nerves. Whether this is the 

 full number, however, is a point not yet decided. 



XI. and XII. The Eleventh, or Spinal Accessory, and Twelfth, 

 or Hypoglossal Nerves. — Neither of these nerves is constant as a 

 cranial nerve througout the vertebrate series, a fact which renders 

 it very doubtful whether the claim of either of them to segmental 

 value could be entertained. For this reason, and partly because I am 

 at present engaged in investigating their development, about which 

 we know as yet very little, I do not propose to deal further with them 

 in the present paper. Forming, as they do, the connecting links 

 between cranial and spinal nerves, they may be expected to yield 

 valuable evidence concerning the validity of the hypothesis propounded 

 above concerning the relations between these two groups of nerves. 



Summary. — The conclusions arrived at concerning the segmental 

 value of the cranial nerves may be expressed in a tabular form thus 

 (c./., fig. 17):- 



Segment. 



Nerve. 



Visceral Cleft. 



Visceral Arch. 



1. Prseoral. 



I. Olfactory. 



Olfactory. 





2. Do. | 



III. Oculomotor, ) 



IV. Trochlear. j 



Lachrymal. 



Maxillary. 



3. Oral. 



V. Trigeminal. 



Buccal. 



Mandibular. 



4. Postoral. j 



VII. Facial, 

 VI. Abducent. 



Spiracular or *) 

 hyomandibular, ) 



Hyoid. 



5. Do. 



IX. Glossopharyngeal. 



1st Branchial. 



1st Branchial. 



6. Do. 



X. Vagus, 1st branch. 



2nd „ 



2nd „ 



7. Do. 



„ 2nd „ 



3rd 



3rd 



8. Do. 



„ 3rd „ 



4th 



4th 



9. Do. 



„ 4th „ 



5th „ 



5th 



10. Do. 



„ 5th „ 



6th 



6th „ 



11. Do. 



„ 6th „ 



7th 





