309 



ously greater in some elasmobranchs than in Amia that there is no 

 difficulty in the supposition that a large number of such organs exist 

 as ampullae in elasmobranchs which have wholly disappeared in Amia. 



The presence of fasciculus communis fibres in the ophthalmic 

 nerves in Amia gives a warrant for the attempt to read fasciculus 

 communis characters into the ophthalmic nerves of selachians. The 

 evidence that the ophthalmicus superficialis of Amia is wholly a com- 

 munis nerve has been discussed by the present writer (5) as follows: 

 "Allis (1) finds the following rami apparently innervating end buds 

 in Amia: R. ophthalmicus V, R. maxillaris V, accessory rami of V, 

 r. ghs. and r. glii. of V, and rami of IX and X. The first two of these 

 rami he traced directly to end buds and he describes them as made 

 up of communis fibres. The weight of Allis' evidence is greatly 

 weakened by a curious error in analyzing the ganglionic complex. 

 The posterior dorsal portion of his anterior root in Amia is undoubtedly 

 the dorsal V (Va of Strong). According to Allis, the fibres of this 

 root passed into the anterior portion of the ganglionic complex, into 

 the portion from which the rami ophthalmicus and maxillaris arose, 

 and 'disappeared gradually as they approached the truncus maxillaris' 

 p. 594). He nowhere mentions these fibres in any nerve rami, but 

 derives the two important rami of the V above mentioned wholly from 

 the communis system. All other authors have described these rami 

 as made up largely or exclusively of fibres from the root which Allis 

 allows to be lost in the ganglion; and general cutaneous innervation 

 requires the presence of this component. If the rami in question do 

 contain communis fibres as Allis states, it is undoubtedly true that 

 they contain general cutaneous components also. If they contain both, 

 we must suppose that the communis fibres are destined for the end 

 buds." 



Now the fact that this nerve contains fasciculus communis fibres 

 is an argument for the communis nature of the corresponding ramus 

 in selachians only 1) if that ramus has the same origin in the me- 

 dulla, and 2) if there are end buds to be innervated. Since the pre- 

 sence of the homologue of end buds is the question to be determined, 

 we are driven to find the settlement of the question by a study of the 

 center in the medulla. This leads us to the consideration of the first 

 argument, which is the only important one. 



Allis describes the nerves which innervate the ampullae in se- 

 lachians as having their central endings in the "Lobus trigemini", or 

 tuberculum acusticum, or both. If it is known that the "Lobus 

 trigemini" is actually a part of the acusticum, the possibility of 



