Johnston, 1 Jie Brain of Petroniyzon. 51 



4. The N. lateralis is made up probably wholly of lateral 

 line fibers derived from the post-auditory root and from the 

 VII— X anastomosis. 



5. The end-buds are comparatively few and the fasciculus. 

 communis system is consequently small both centrally and peri- 

 pherally. 



6. The communis VII root has essentially the same re- 

 lations as in other fishes. It consists of fine fibres with very 

 large ganglion cells which are characteristic of this root in fishes. 



As the peripheral organs and nerves do not belong to the 

 main subject of this study I shall not burden this part of the 

 paper with an extended discussion of the literature of the cra- 

 nial nerves of Cyclostomes. Langerhans ('73), as reported by 

 Ahlborn, has described both lateral line organs and end-buds. 

 Ahlborn ('83, '84) has given the fullest description of the 

 nerve roots but has overlooked two important roots, the lateral 

 line X (post-auditory) and the communis VII. There is ap- 

 parently considerable difference in the number and arrangement 

 of the roots of the vagus group between Petromyzon planeri 

 and Lampetra. The roots are fewer in number in Lampetra 

 and the IX is single and not the most cephalic. It is pre- 

 ceded by the post-auditory lateral line root, while in P. planeri, 

 according to Ahlborn, the root which gives rise to the lateralis 

 nerve follows the four rootlets of the glossopharyngeus. It 

 seems certain, considering the position of the lateral line root 

 in all fishes, that Ahlborn has made an error in analyzing the 

 ganglia and roots, and that he did not properly identify the root 

 or roots which give rise to the "R. lateralis vagi." The fact 

 that he assigns the whole of the lateral limb of the lateral line 

 ganglion to IX shows that the analysis of the ganglia and roots 

 is not accurate. Since it is not possible that he should have 

 overlooked so large a root as the post-auditory lateral line root, 

 it is probable that it is represented by the first two of his IX 

 roots, which arise higher up on the wall of the medulla than the 

 others and enter the acusticus nucleus. Ahlborn does cor- 

 rectly state that the VII-X anastomosis joins the ramus lateraHs 

 and gives part of its fibers to the vagus proper. 



