74 



CLASS PISCES. 



The sjDinal nerves have two roots, which unite, and the dorsal 

 of which has a ganghon. The ganghon may, however, be placed 

 at the junction of the two roots. The cranial nerves are in ten 

 pairs, but there are often some small nerves — the spino- 

 occipital nerves (miscalled ventral vagus roots) — arising from 

 the ventral side at the hind end of the medulla. They pass 

 out through foramina in the skull, but are perhaps better 

 regarded as anterior spinal nerves (see above) the dorsal roots 

 of which are not developed. 



analysis of the cranial nerves, which is due to Gaskell and is there referred 

 to, was fully developed. The later work, which is still incomplete, may 

 very possibly necessitate a new scheme of cranial segmentation, and the 

 groups of cranial nerves expressed by the terms fifth, seventh, ninth and 

 tenth may possibly be found to be connected with the ventral segmenta- 

 tion (pseudo-segmentation of the text) of the branchial pouches, and to be 

 independent of the mesoblastic segmentation which is so conspicuous 

 a feature in the trtmk. 



