133 Journal of Compauative Nkukologv. 



Fig. 3). — In the roof of the aqueduct of Sylvius are found a 

 number of large flask cells.(') These cells rank among the 

 largest in the avian brain. Although I have not been able 

 to trace any connection between these cells and any nerve 

 root, yet their position, their form and their size all combine 

 in indicating that these cells constitute the mesencephalic 

 nidulus of the fifth nerve. 



TRACTS of the DIENCEPHAEON AND MESENCEPHALON. 



In describing the tracts of these regions of the brain, it is 

 thought best to consider the combined diencephalon and 

 mesencephalon as a unit. This method has one decided 

 advantage, it facilitates an intelligent description of the 

 tracts. In discussing these tracts I have pursued the follow^- 

 ing order. First, I have described all the obvious com- 

 missures and decussations and all the tracts that appear to 

 be associated with them; secondly, I have described the 

 various nerve roots and associated fibres; finally, I have 

 described the remaining tracts of this region. 



Excluding the anterior commissure, which has been de- 

 scribed in a previous paper, (') this region of the brain con- 

 tains six well-defined commissures and decussations. Near 

 the dorsal surface the superior, posterior and Sylvian com- 

 missures are found; further ventrad lies the medi-commissure; 

 while near the base of the thalamus we find the inferior com- 

 missure and the " decussatio inferior." 



Superior commissure (Plate XIV, Figs. 8, 9). — This 

 commissure lies in the caudad extremity of the habena. In 

 the bird brain it is very small, and might be easily over- 

 looked. In the amphibian brain, according to Professor 

 Osborn,(*) "the superior commissure divides into two distinct 

 bundles, one of which descends into the inner mantle of the 



1 These cells have been observed by Siieda. Op. cit., p. 44. 



2 JoiRNAL OF Comparative NEUROi.otiv, Vol. I, p. 75. 



3 " A Contribution to the Internal Structure of the Amphibian Brain, " hy Professor 

 Henrv Fajrheld Osborn, Princeton College. Journal of Morphology, Vol. H, p. 80. 



