THE NERVOUS SYSTEM 443 



NAME FUNCTION 



I. Olfactorius special sense, smell. 



II. Opticus special .sense, sight. 



III. Motor oculi motor. 



IV. Trochlcaris [Patheticus] motor. 



V. Trigetmnus [Trifacial] { mainl y sensor y, with a small motor 



( root 



VI. Abducens motor. 



VII. Facialis mixed. 



VIII. Acusticus [Auditprius] special sense, hearing. 



IX. Glosso-pharyngeus mixed. 



X. Vagus [Pheumogastricus] mixed. 



XI. Accessorius [Willisii] mixed, mainly motor. 



XII. Hypoglossus mixed. 



Of these the first two arise from the primary fore-brain, 

 the tel- and di-encephalon respectively ; the remaining ten take 

 their origin from the met- and myelencephala, leaving the 

 mesencephalon without any. It would thus seem that the 

 former may be nerves of the archencephalon or primary brain, 

 laid down in Amphioxus, while the latter belong to the second- 

 ary addition from the anterior end of the original spinal 

 cord. The last ten were thus at first spinal nerves, in which, 

 in spite of their extreme specialization, it might be possible to 

 recognize the original spinal elements, each with its sensory 

 and motor roots, its accompanying ganglion, and so on. That 

 the original elements have in some cases become modified is 

 evidenced by several facts, first, the existence among them of 

 wholly motor nerves without sensory fibers and lacking a gan- 

 glion ; and, again, the fact that some of the nerves in the above 

 list are shown to be composed of several primary nerves by 

 their origin from multiple roots, or from the presence of sev- 

 eral associated ganglia. The twelfth nerve is outside of the 

 cranium in fishes, and becomes later included within it, proba- 

 bly by the fusion with the skull of the vertebra with which it 

 is associated. The eleventh is closely associated with the 

 Vagus and appears as a distinct cranial nerve only in 

 mammals. 



Aside from the elements found in the above there are traces 



