306 Some of the Decussating Tracts of the Brain, fyc. [June 21, 



Meynerfs Commissure. This commissure is also invariably found 

 degenerate, but the author has been unable to determine its exact 

 mode of origin and termination. It would appear that the commis- 

 sure had a wide field of origin, numerous fibres either passing 

 through or arching round the pes pedunculi on its dorsal aspect to 

 form it. The fibres pass to the opposite side behind the chiasma, 

 and then descend slightly, and appear to diminish in number ; they 

 do not appear to enter the corpus Luysii ; a few of the fibres may 

 penetrate with the optic tract into the thalamic region, and inter- 

 mingle with the superficial fibres of the superior fillet (compare 

 Darkschewitch and Pribytkow, and more recently Bechterew in " Die 

 Leitungsbahnen "). 



2. Posterior Commissure. The degenerate fibres which cross in the 

 commissure or in the roof of the Aqueduct of Sylvius, and which 

 result from a complete unilateral lesion of the quadrigeminal area, 

 have not a long course, but terminate, for the most part, in the 

 opposite corpora quadrigemina, dorsal and lateral aspects of the 

 Sylvian grey matter, or posterior portion of the tegmentum. 



Degenerate fibres have never been traced into the posterior longi- 

 tudinal bundles, as has been asserted by some authors. A special 

 group of large superficial degenerate fibres in the anterior portion of 

 the roof of the aqueduct have been traced from the internal capsule 

 across the thalamus into the stalk of the superior corpus quadri- 

 geminum and then across the commissure. These fibres alone are 

 found degenerate in the commissure when the anterior one-third of 

 the cat's hemisphere is removed. 



3. In cases where the motor region is completely removed in the 

 cat, degenerate fibres are found which leave the pyramidal system in 

 the pes pedunculi, crusta, pons, and medulla. The fibres which leave 

 the pes pedunculi and crusta pass backwards to the quadrigeminal 

 region of the same side, those which leave the pyramid in the medulla 

 decussate across the raphe to the opposite side, and lose themselves 

 in the tegmentum ; they have not been traced directly ending in the 

 motor nuclei of the cranial nerves. Muratoff has described a group 

 of these fibres in the medulla, and supposes that they are the cortical 

 motor fibres of the Vllth ; the author, on the other hand, has not 

 found the fibres limited alone to this region. The decussation of the 

 pyramid is thus not confined to the upper cervical region, but is 

 gradually taking place during the descent of the pyramid through 

 the bulbar segments. 



