124 Journal of Comparative Neurology. 



relation of this commissure is threefold: First, to the oculo- 

 motor nucleus, and probably to the main sensory tract; 

 second, to the pale ganglion behind this nucleus; third, to 

 the tectum opticum. As it descends the fibres divide into 

 two bundles, of which the anterior surrounds the superior 

 processes of the ganglion cells of the oculo-motor nucleus; 

 the connection is so close that some of these fibres seem to 

 be actually continuous with the cells. The posterior bundle 

 has a similar connection with the cell processes of the pale 

 ganglion, which may, in fact, also belong to the oculo-motor 

 nerve. None of the fibres of this commissure can be traced 

 directly into the main (sensory) tracts adjoining these nuclei, 

 as observed by Pawlowsky, although such a connection 

 seems highly probable. Dorsally, the fibres in this com- 

 missure in Rana can be clearly followed into the peripheral 

 white substance of the tectum opticum, as shown in hori- 

 zontal sections." 



In the bird brain there is a tract which might be con- 

 sidered the homologue.of the first two tracts described above. 

 However, it is not connected with the nidulus of the third 

 nerve. This tract is a bundle of fibres which passes caudo- 

 ventrad from the posterior commissure and looses itself in 

 the substance of the diencephalon.( ') Apparently this is a 

 true commissure of the thalamus. 



In the bird brain I cannot detect any fibres passing from 

 the posterior commissure to the tectum opticum. This fact 

 apparently militates against our considering the avian and 

 amphibian posterior commissures as homologous. However, 

 since Professor Osborn does not mention a commissura 

 Sylvii, it seems quite probable that what he has described as 

 the posterior commissure consists of the posterior commissure 

 proper and of the commissura Sylvii. If this surmise be 



I In the mammalian brain, according to Professor Strieker, tliis tract presents the 

 same appearance. " A Manual of Histolology," by Prof. S. Stricker. . . . American 

 trans., edited by Ai.uert H. Buck, New York: Wm. Wood & Company, 1872, p. 693, 

 Fig. 270, Ch. 



