36 Journal of Comparative Neurology. 



names that, after long effort to unravel the maze, it has been 

 decided to describe the commissures as they exist in the fish, 

 under such terms as seem least ambiguous, without attempting 

 to reconcile therewith the discordant statements of Forel, Ganser, 

 Meynert, Edinger and others. Mayser has given the most ac- 

 curate account of these structures from which, however, we are 

 obliged to differ in several particulars. It should be observed 

 that teleosts afford the simplest connections and here, if any- 

 where, the problems are to be solved. Moreover the great vari- 

 ation of form in fish brams is of much assistance, because the. 

 same tracts are thus brought into very different topographical 

 relations while, of course, the real connections are constant. 



1. The commissura ventralis — one portion of the inferior 

 commissure, Gudden's commissure (?). It is the most ventral 

 of the system and in the drum is entirely distinct from the optic 

 nerves. It is in fact separated by a large part of the geniculu- 

 latum at the base. Fig. 5, Plate IV, gives an especially clear 

 idea of its position. Compare also Plate VI, Fig i. This has 

 been considered a decussation and for a long time we entertained 

 this opinion. Fig. 4, Plate IV, seems to substantiate it, but a 

 very careful study shows that there is no decussation. The 

 fibres which arise, as elsewhere described, in the tuber near the 

 mammillare, collect on the cephalo-lateral aspects of the tuber 

 and associate themselves with the fibres of the commissura ven- 

 tralis of that side, passing with them into and through the genic- 

 ulum to the lateral aspects of the colliculus and brachium 

 caudale. 



2. The commissura transversa (Hallen) is the most closely 

 associated with the chiasm and was regarded by Professor Fritsch 

 as a separate arch from the chiasm.^ Obviously Fritsch has com- 

 bined in his account the two systems, transversa and ventralis. 

 We have been unable to find any connection between the optic 

 fibres and those of this commissure. The probability of mistake 

 is rendered less likely by the difference in calibre of the fibres. 



1" Die Com. transversa . gilt ais ein Convolut zum Tra^tus op- 



ticus hehoriger, abgezvveigter, zum Tlieil in dass Tuber cinereurn einschlics- 

 ender Biindel . . . wenn ich auch nicht dafiir eintreten mochte, dass 

 ihre Fasern lediglich Opticus Fassern sind." L. c, p. 57. 



