Notes upon the Brain of the Alligator. 139 



course of the fibres in these bundles. Orr (Journal of Morphology, 

 Vol. I., 1887), says of the commissures in the lizard: "The 

 superficial position of these commissures, anterior, superior and 

 posterior, their similar connection with the lateral bands and their 

 relations to the constrictions of the brain, suggest at this period a 

 striking homology between them." This remark grows out of a 

 conjecture that the anterior, as well as other commissures are con- 

 tinuations of the primitive lateral longitudinal fibres. Osborn 

 (Journ. Morph., Vol. II.,) observes : "It is true that the fibres 

 beneath the fore-brain branch from the lateral longitudinal band 

 much as do those passing to the region of the other commissures ; 

 but I can not at present adopt his view that they represent the 

 anterior commissure, because the development of this commissure, 

 as I have found it in the Amphibia and Mammalia, indicates that it 

 is strictly commissural, and not the decussation of a longitudinal 

 tract. Immediately beneath the anterior commissure in the 

 amphibian fore-brain, I have observed fibres decussating from the 

 longitudinal bundle to the opposite hemisphere, which probably 

 represent those attributed to the anterior commissure by Orr." 



From what has been already said, it is plain that the alligator 

 gives us a clue to the discordant statements above quoted. All 

 three of the transverse systems are present, and sustain a relation 

 entirely similar to that in mammals, making the necessary allow- 

 ances for topographical modifications. The comparative perfection 

 of the corpus callosum is especially important, and indicates the 

 impossibility of making hard and fast lines of distinction between 

 vertebrate classes upon the basis of the presence or absence of 

 major structures in the brain. 



(d) The superior commissure and taenia thalami optici. This bun- 

 dle is of great importance, from the direct connection between the 

 superior surface of the diencephalon and the cortex of the hemi- 

 spheres. It is doubtless of sensory function, and may be traced from 

 the postero-lateral regions of the cerebral cortex medianly and 

 upward, thence across the interval between the hemispheres and 

 the thalamus, to the latter. Its fibres stain deeply and are com- 

 paratively large. They finally enter the ganglion habenulae or its 

 apparent homologue, while other portions pass upward and cross 

 in a well-defined commissural band. In the alligator, no evidence 

 of the further extension of the fibres of this region to a pinneal 

 body could be discovered. . It appears probable that some fibres 



