138 Cincinnati Society of Natural History. 



oblique course from behind forward over the arch formed by the 

 callosum. It seems to arise in the region of the small olfactory 

 (?) cells along the base of the brain surface near the ventro- 

 posterior angle, passing approximately parallel to the above- 

 mentioned fibres till lost in the same region of the mantle as those 

 of the callosum. It is not quite certain that there is a consider- 

 able decussation of fibres here. The tract is in every way compar- 

 able with the direct fibre zone, ascending from the more anterior 

 portions of the same basal region to the median mantle lobe, except 

 for the apparent decussation. 



(c) A third set of fibres lying at a lower level passes directly 

 across in a horizontal plane from the regions of the thalamus, where 

 the peduncular fibres begin to radiate toward the axial lobe. The 

 fibres are certainly commissural, rather than a decussation, and in 

 sections stained especially for the fibres these can be traced paral- 

 lel to the radiating fibres of the peduncles and not into the descend- 

 ing tract. This tract can be homologized with little hesitation with 

 the anterior commissure, being a true commissure of the thalamus. 



The commissural bands thus described have been variously 

 interpreted. Stieda has called this complex "corpus callosum," to 

 which Rabl-Rueckhard responds: "These fibres can not be con- 

 sidered as the anterior commissure, since that organ is always 

 chiefly a commissure of the axial lobe, and has nothing to do with 

 the mantle. Whatever homologue is sought in more highly organ- 

 ized ^brains must, at least ; be a mantle commissure. Such a struct- 

 ure is afforded by the fornix, on one hand, and the corpus callosum 

 on the other. It is of especial interest that Stieda, in his work, 

 " Neber den Bau des centralen Nervensystems der Schildkroete," 

 found an entirely analogous body in the same region, as well as a 

 second one passing more nearly transversely, which loses itself in 

 the basal portion of the axial lobe. Stieda applies to the former 

 the term " callosal rudiment." If he uses the term as including 

 both the callosum and the fornix, covering the median mantle com- 

 missures of the cerebrum of higher vertebrates, nothing can well 

 be opposed to its employment, although it is, perhaps, an unfortu- 

 nately ambiguous expression. If, however, it is intended by Stieda 

 to homologize it directly with the corpus callosum, to the exclusion 

 of the fornix, I must dissent, for the callosum originates later and 

 higher than the fornix, even though the two are united by the 

 lamina genu. Considerable difference of opinion exists as to the 



