24 'Journal of Comparative Neurology and Psychology. 



In this neighborhood, according to Goldstein, it gives some 

 fibers to the nucleus thalami anterior; this, however, I cannot con- 

 firm in the material which I have examined. The tractus then 

 more and more assumes a circular outline on transverse section, 

 a point which also struck Herrick. Directly laterally of it there 

 is a group of cells that can be followed backward up to a point a 

 little before the nucleus rotundus (nucleus praerotundus, see 

 below). Here, however, no fibers end, though perhaps they give 

 off collaterals (Figs, xliii, xliv), since the thickness of the tract 

 remains the same during this part of its course. Medially and at 

 first a little above it there are here the two tractus olfacto-lobares, 

 which now soon turn ventrally to their endings. Then the tractus 

 strio-thalamicus of each side bends at a sharp angle in a lateral 

 direction and ends (Figs, xlviii, il, Plate III) in the back part of 

 the lobi inferiores laterally under and probably in the nucleus 

 rotundus proprius (see below). In Gadus, where these points are 

 very clear, one can easily be convinced that the fibers end here. 

 Van Gehuchten writes that he has observed in Golgi prepara- 

 tions that its fibers extend farther backward through the mid- 

 brain mto the medulla oblongata, which, however, I must decidedly 

 contradict as far as Gadus is concerned. 



Having now treated of the tracts which at least in part run 

 through the praethalamus, I can pass to one of the more difficult 

 parts of the 'tween-brain: the post-optic commissures. There is a 

 great variety of opinion regarding the commissures and decussa- 

 tions of the 'tween-brain, differences, which are partly caused by 

 the confusion of nomenclature, partly undoubtedly by differences 

 in the animals described. For the moment passing over the com- 

 missura horizontalis, I shall give a condensed account of the 

 literature of the other commissures, after which I shall describe 

 these as they are found in the fishes which I have examined, 

 together with the course of the nervi optici. 



Following the example of his predecessors Fritsch called the 

 most ventral commissure of the praeinfundibular part of the 'tween- 

 brain com. transversa or com. of Gudden, and he thought he saw 

 m It a direct opticus decussation, an opinion which was rectified 

 some years later by Mayser. Both of them described as its end- 

 nucleus the so-called stratum zonale (antero-lateral part) of the 

 tori semicirculares. Bellonci called this commissure, com. of 

 Gudden or com. inferior and he, too, found its end-nucleus in the 



