Turner, Morphology of tJic Avian Brain. 137 



in going to the optic tracts pass caudo-laterad and spread 

 out over the ventral and lateral surfaces of each optic lobe. 

 Although this is a continuous sheet of fibres, yet Bcllonci(') 

 has divided it into two portions, a cephalo-dorsal (vorderes- 

 oberes) and a caudo- ventral (unteres hinteres) portion. 

 Thus he obtains two tracts , which are homologous to corre- 

 sponding tracts found in the brains of fishes ( Tc/eosfci) and 

 amphibians. The cephalo-dorsal root passes ectad of the 

 co-rpus geniculatum externus (Plate XV^, Fig. 7). 



All of the fibres of the chiasm do not pass into this exter- 

 nal optic tract. A few meagre bundles lose themselves in 

 the vicinity of the third ventricle. 



Intimately associated with the external optic tracts are 

 three fasciculi which demand our attention. Two of these 

 tracts come from the mesencephalon and one from the pro- 

 sencephalon. These tracts are: the median optic fasciculus, 

 an unnamed tract, and tractus Bummi. 



Tract Hs Biimmi (Plate XV, Fig. 7). — Permit me to 

 repeat('^) that " this tract originates in either the frontal or 

 fronto-median lobe of the prosencephalon and passes caudo- 

 ventrad through the intra-ventricular lobe. After passing 

 beneath the anterior commissure, the tract turns and passes 

 ventro-latero-caudad to the crura cerebri. Penetrating the 

 crura, it passes to the outer fibre layer of the tectum." Be- 

 yond its union with the external fibi'e layer of the tectum the 

 course of this tract becomes obscure. 



A short distance caudad to the fusion of tractus Bummi 

 with the external optic fibres, a tract from the interior of the 

 optic lobe unites with the external optic fibres (Plate XIV, 

 Fig. 11). This seems to indicate that Bumm's tract passes 

 into the interior of the mesencephalon. One serious objec- 

 tion militates against such a conclusion, viz., the tract pass- 

 ing to the interior of the optic lobe has a much smaller 

 diameter than tractus Bummi. 



1 Op. cit., p. 17. 



2 Journal of Comparative Nei rolocv, p. 76. 



