Turner, Morphology of the Aviati Brain. 1 1 ;; 



growths of the same type as the predominant cells. In addi- 

 tion to these undoubted nerve cells, the nidulus contains a 

 large number of Deiter's corpuscles. A commissure connects 

 this nidulus with its fellow. 



Nidulus of the fourth nerve (Plate XIV, Fig. 12; Plate 

 XV. Fig. 5). — As has been mentioned above, this cell cluster 

 lies near the meson in the caudo-dorsad portion of the mesen- 

 cephalon. Unlike the nidulus of the third nerve, it is located 

 near the surface. Histologically, it resembles the nidulus of 

 the motor-oculi nerve. 



In birds, as in reptiles and lower vertebrates, each optic 

 lobe contains a ventricle. In the aves, this ventricle is a 

 narrow expansion, which passes from the third ventricle into 

 the body of the optic lobe (Plate XIV, Figs. 1-6; Plate XV, 

 Figs. 3-5, 7-1 1 ). This cavity is convex, the convexity being 

 directed cephalad. From the body of each optic lobe a dome- 

 shaped expansion of the mesencephalon projects cephalad 

 into the ventricle. This expansion has been called the 

 "colliculus" (Plate XV, Figs. 2,4). In some bird brains 

 this feature is more pronounced than in others. 



The histology of the mesencephalon is very pretty. Each 

 optic lobe may be said to consist of an interior core or body, 

 which is surrounded on all sides except the mesad by a 

 stratified shell. This shell, the tectum opticum, consists 

 chiefly of alternate layers of neuroglia and Deiter's cor- 

 puscles; while the core consists, in the main, of nerve 

 niduli and their concomitant fibre tracts. With one ex- 

 ception, all the niduli of the body of each optic lobe are 

 situated caudad to the mesencephalic ventricle (Plate XV, 



Figs- 5' 7-9)- 



Tectu7n opticum. — The tectum, as here considered, con- 

 sists of all that portion of the optic lobe which lies ectad to 

 the fasciculus internus.(') Passing from the periphery entad 

 through the tectum opticum of Swainson's thrush {I/ylo- 



I For a tlescriptloii of this tract see p. 129. 



