Ti'RNEH, A/(>rp//i>/o<yv of tJic Avian Brain. 7^ 



External Capsule. — Near the cephalic boudary of the 

 caudate portion of the striatum there is another sheet of 

 fibres. This may be reijarded as the homologue of the 

 external capsule. 



Anterior Commissure. — Immediately cephalad to the 

 diencephalon and near the base of the prosencephalon the 

 anterior commissure connects the two hemispheres of the 

 avian brain (Plate VII, Fig. 3; Plate VI). 



After leaving the crura the majority of the fibres of this 

 commissure pass caudad and slightly dorsad into the basi- 

 occipital lobe. These fibres keep near to the mesal surface 

 of the hemisphere. In addition to these a few fibres appear 

 to pass directly laterad. I have looked in vain for fibres 

 passing from this commissure to the olfactory lobes. 



Corpus Callosum (Plate \^I, Fig. z\ Plate VII, Figs. 3, 7; 

 C C). — This commissure lies caudad and dorsad to the 

 anterior commissure. It is much smaller than the latter. 

 Between the ventricle and the meson the fibres of the 

 corpus callosum radiate towards the dorsal surface of the 

 brain.(') 



Peduncular Tracts. — These tracts enter the prosence- 

 phalon by the crura cerebri. At the crura they are perpen- 

 dicular to the longitudinal axis of the hemisphere. K few 

 of these fibres pass cephalo-dorsad to the caudate portion ot 

 the striatum. These fibres pass mesad of the striated portion 

 of the striatum, the remainder pass through it. As men- 

 tioned above, at this place the striatum is composed of 

 several layers. Passing entad we meet in succession: (i) A 

 layer containing dense clusters of Deiter's corpuscles; (2) A 

 clear lenticular portion which contains two kinds of nerve 

 cells; (3) A large area which is composed chiefly of Deiter's 

 corpuscles. In each of these portions the course of the 

 peduncular tracts is different. In the first the fibres pass 



I Most writers have said that hirtls do not liave a corpus callosum, Init A. Bumm, 

 the most recent European student of the avian prosencephalon, alVirms its existence in 

 European birds. Op. cit., p. 462. 



