126 JoiRNAI. OK Co.MI'.\KA'n\'K N Kl' KOI.OtJ Y. 



restricted to the commissural fibres. The cells have been 

 described elsewhere as the mesencephalic niduliis of the fifth 

 nerve. This commissure is identical with what some authors 

 have named the " commissure of the optic lobes." It appears 

 to be a true commissure of the mesencephalon. The fibres 

 composing it arise from both the entad and the ectad sides of 

 the mesencephalic ventricle. At the meson they converge 

 and form a conspicuous commissure. 



Inferior coiii/i/issiire (Plate XV, Fig. lo). — In the ventral 

 part of the thalamus, between the optic chiasm and the tuber 

 cinereum, there is a well-defined commissure. This has been 

 named by Bellonci the " inferior commissure." In well- 

 stained sections the fibres of this commissure can be traced 

 into the interior of the optic lobe, where they apparently 

 intermingle with the fibres of the fasciculus internus. The 

 majority of its fibres pass mesad to the central nidulus of the 

 diencephalon; a few, however, pass undisturbed through 

 that nidulus. 



Fibrce ansu/atce.—r-K few fibres arise in the ventral part 

 of the diencephalon, decussate in the region of the chiasm, 

 and then pass cephalo-dorso-lateiad, through the optic 

 chiasm, into the prosencephalon. Bellonci thinks these 

 fibres are homologous with the fibrie ansulatae of higher 

 brains. 



Optic chiasm (Plate XIV, Figs, ii, 13; Plate XV, 

 Figs. 6, 10). — In all the specimens examined the avian 

 chiasm differs in some essential features from that of the 

 human subject. In the human brain some of the optic fibres 

 decussate in the chiasm and others do not. (') In the avian 

 brain all of the optic fibres decussate in the chiasm. There 

 is also another difference. In the avian brain there do not 

 appear to be any homologues of the two commissures found 

 in the human chiasm. 



Optic tracts. — After decussation the majority of the fibres 



I " Applied Anatomy of tlic Nervous System," by Amurose L. Kannev, A.M., 

 M.D. Second edition. D. Appleton & Co. Page 351, Fig. Si. 



