Pemreuton, Structure of the Optic Tlialanii. 143 



one of the first bundles to become medullated. In the human 

 brain [Edinger '' p. 73, 74] the fibres take their origin inside 

 of the thalamencephalon near the median ganglion, and 

 running caudad, decussate just cephalad to the corpora 

 quadrigemina, and soon pass lower down and run along the 

 base of the tegmentum. Running parallel to this, and being 

 reinforced by fibres from it, they finally reach the meten- 

 cephalon. The investigations of Spitzka and of Darkschew- 

 itsch confirm this statement of Edinger. The fibres that lie 

 nearest to the median line probably terminate in the nidulus 

 of the oculo-motor nerve. 



The Supra-conmilssjira. — Osborn, [" Preliminary Obser- 

 vations upon the Brain of Menopoma and Rana," 1S84, p. 

 26S], says: "In the forward portion of the roof of the 

 diaccelia (in amphibians) , just above the optic chiasma, is 

 the supra-commissura. It is closely connected with, and is 

 just in front of the ganglia habenulaa. It passes across the 

 posterior ends of the thalami. The distribution is similar to 

 that of the fibres of the tsenia of the thalami. It divides 

 into two bundles, one going down to the inner mantle of the 

 hemispheres, the other descending directly along the outer 

 wall ot the thalami. Thus is occupies the same relative 

 position as the commissure of the pineal stalk of the mam- 

 malian brain. Osborn [-" p. So] does not consider this 

 commissure a purely decussational system, as the bundle 

 entering the hemisphere is much larger than that entering 

 the thalami; consequently, it forms either partly a com- 

 missural system between the posterior portions of the hemi- 

 spheres and between the thalami, or partly a decussational 

 system between the hemispheres and the thalami. Bellonci 

 lays especial stress upon the presence, in this commissure as 

 in the other cerebral commissures, of decussational fibres in 

 addition to the commissural ones. Herrick says ['^ p. 26]: 

 "The supra-commissure (of reptiles) lies entirely cephalad 

 to the habena at a level considerably ventrad to the com- 

 missure of the habena, which lies caudad to it, and passes, by 



