444 HISTORY OF THE HUMAN BODY 



of several other spinal elements originally belonging to the 

 primary anterior end of the cord, which do not survive in 

 the higher forms as definite cranial nerves. These are desig- 

 nated as the spino-occipital nerves, and are first met with in 

 the selachians, where they appear as 1-5 pairs, placed very 

 far back, along the medulla. They are spinal in character 

 and not associated with the other cranial nerves, although 

 they are all included within the skull. As this latter part 

 ends abruptly with the otic region in cyclostomes, and is im- 

 mediately followed by the successive pairs of true spinal 

 nerves, it seems reasonable to suppose that when, in the sela- 

 chians, the cranial cavity became enlarged by an addition at 

 the posterior end, several of the original spinal nerves were 

 included, forming the nerves in question. In the higher car- 

 tilaginous fish (Holocephali), and in ganoids, this set of 

 nerves becomes reduced to two pairs, yet a second set, also 

 of 1-5 pairs, has been taken in, presumably in the same way. 

 To distinguish between these two sets of spino-occipital nerves, 

 the first are termed occipital, the second occipito-spinal. Rep- 

 resentatives of both sets occur in varying proportions in other 

 fishes, but in the amphibians they seem to have wholly 

 disappeared, and are never seen again as distinct nerves. Al- 

 though nothing has as yet been definitely proven in the matter, 

 it is probable from other evidence that above the fish the 

 occipital region suffers considerable reduction, during which 

 many of these elements may have become lost, while others 

 may have become established among the root elements of the 

 twelfth nerve, the hypoglossal, since this nerve appears first 

 as a cranial element in the reptiles and continues throughout 

 Sauropsida and Mammalia. 



For purposes of description and with reference to their 

 morphology the cranial nerves fall naturally into groups which 

 are best considered separately. These may now be taken up 

 in detail. 



i. THE ANTERIOR GROUP. (Olfdctorius and Opticus.) 

 These, the two first in the list, are nerves of special sense, 

 the fibers of which are distributed respectively to the nasal- 



