Turner, Morphology of tJic Avian Ih-ain. 12^ 



true, then the homology between the avian and amphibian 

 posterior commissure is sufficiently close, for the commissura 

 Sylvii is essentially a commissure of the tectum opticum. 



In the avian diencephalon two other tracts appear to be 

 associated with the posterior commissure. One of these 

 tracts comes from the epencephalon, the other from the 

 metencephalon. 



Tract from the epencephalon (Plate XVI, Fig. 4). — This 

 tract arises in the cephalad portion of the epencephalon, and 

 passes cephalad through the valve of Vieussens into the 

 mesencephalon. There it loses itself in the vicinity of the 

 posterior commissnre. Apparently this tract decussates in 

 that commissnre. This tract is not the anterior peduncle of 

 the epencephalon. 



Fasciculus cuneatus (Plate XVI, Fig. 4). — The tract that 

 comes from the metencephalon is probably a continuation of 

 the fasciculus cuneatus. It passes from the medulla cephalad 

 into the optic lobes. In company with the fibres from the 

 cerebellum, it loses itself near the posterior commissure.(') 



Commissura Sylvii. — From the posterior commissure 

 almost to the origin of the fourth nerve, the aqueduct of 

 Sylvius is occupied by a long band of commissural fibres. 

 As long ago as 1S6S Stieda recognized this band as distinct 

 from the posterior commissure. He christened it the " com- 

 missura Sylvii." He included under that name not only the 

 commissural fibres, but also the large flask cells which 

 laterad lie ventrad to them.(') In this paper the name is 



1 In the "Applied Anatomy ot the Nervous System," p. 230, Ranney, in describing 

 the tracts of the valve of Vieussens, writes: " Certain longitudinal fibres may be demon- 

 strated which can be traced into the superior vermiform process of the cerebellum. The 

 course of these fibres is peculiar. They decussate before leaving the superior vermiform 

 process ; they then traverse the valve of Vieussens almost to the lower portion of the 

 corpus quadrigeminum ; at this point they double upon themselves, describing curves 

 whose convexity looks upward; finally, they join the inferior lamina of the lemniscus at 

 its posterior bundle, and pass onward with the latter, in the posterior division of the pons 

 Varolii to the spinal cord " This quotation seems to indicate that the two tracts described 

 above are one, and, further, that that one is the homologue of this tract described by 

 Ranney. In defense, I aver that my sections do not warrant such a conclusion. 



2 Op. cit., p. 44. 



