Johnston, The Brain of Petromyzon. 55 



Strong and Cole cautiously and suggests the possibility of this 

 nerve containing both communis and lateral line fibers and 

 showing gradual modifications such that it contains only the 

 one or the other in certain cases. 



The interpretations given by these authors rested upon in- 

 sufificient knowledge of the structures concerned. The origin 

 of the nerve as a post-auditory root (plus fibers from the VII 

 anastomosis) from the acusticum and its distribution to lateral 

 line organs put its homology with the ramus lateralis vagi of 

 higher forms beyond all question. The position of the nerve 

 offers no difficulty, since it arises (Kupffer '95) as an epidermal 

 thickening as in other fishes and sinks into its definitive position 

 later. Its fibers probably reach their sense organs by way of 

 the spinal nerves. These constitute the connections seen by 

 Ransom and Thompson and wrongly interpreted. The nerve 

 has probably no relation whatever with the accessory lateral 

 system. The position which it has secondarily assumed remains 

 to be explained. It appears that the lateral line in Petromyzon 

 has degenerated from a typical ichthyopsid condition, and the 

 position of the nerve is connected with this degeneration. 



As this paper is passing through the press I have had for 

 the first time an opportunity to examine Merkel's work 

 "Ueber die Endigungen der sensibien Nerven in der Haut der 

 Wirbelthiere." His description of the histology and distribu- 

 of both lateral line organs and end-buds in Petromyzon jinviatilis 

 agress in all important points with that given in the present 

 paper. 



c. The sensory centers.- 

 In regard to the centers of these sensory systems in the 

 medulla, the chief interest lies in the light which the brain of 

 Petromyzon throws on the relation of the cutaneous and visceral 

 centers to one another and to the grey columns of the cord. In 

 a paper on the brain of Acipenser ('01 c) the writer has argued 

 that the general and special cutaneous centers are essentially a 

 unit, although already highly developed and differentiated, and 

 that they represent the dorsal horns of the cord, while the com- 

 munis center represents structures in the cord mesial to the dor- 



