Turner, Morpholoi^y of tJic Avian Brai)i. 123 



hemispheres and finally disappears after bending around into 

 the outer portion of the mantle. The second bundle descends 

 directly along the outer wall of the thalami. These bundles 

 are clearly seen when the commissure is well developed. 

 One fact militates against our considering the 

 commissure as a purely decussational system; that is, the 

 bundle entering the hemispheres is much larger than that 

 entering the thalamic In the avian brain it has not been 

 possible to find more than one tract leading from the superior 

 commissure. The taenia thalami, the tract leading to the 

 prosencephalon, is present, but the other is, apparently, 

 absent. I say apparently, for it must be kept in mind that 

 the avian taenia thalami is a very small tract; hence if in the 

 avian brain the ratio of the tract going to the prosencephalon 

 to the tract leading to the diencephalon is the same as it is in 

 the amphibian brain, the tract passing to the thalamus would 

 be so minute that its discovery would be next to impossible. 



Tcpiiia thalami. — This tract passes from the vicinity of 

 the superior commissure cephalo-ventrad to the crura cerebri. 

 As has been stated,(') near the meson and dorsad to the 

 peduncular tracts these fibres enter the prosencephalon. 

 Immediately they turn dorsad, and, if I have traced them 

 correctly, after passing dorsad for a short distance, they turn 

 laterad. After traversing about half the width of the hemi- 

 sphere, the tract again turns dorsad. Always keeping near 

 the caudad extremity of the hemisphere, it continues dorsad 

 and disappears near the dorsal surface of the brain. 



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

 commissure lies in the dorsal portion of the diencephalon at 

 a short distance caudad to the superior commissure. Com- 

 pared with the latter commissure, this fasciculus is more than 

 ten times as large. In longitudinal-perpendicular sections 

 the posterior commissure resembles an inverted horse-shoe. 



In the amphibia, according to Professor Osborn,(^) " the 



I Journal of Comi'Aratue Neurology, Vol. I, p. 76. 

 g Op. cit. p. 79-80. 



