274 C. JUDSON HERRICK 



Commissura posterior 



At the rostral border of the roof of the midbrain this com- 

 missure is a conspicuous massive crossing of heavily myeli- 

 nated fibers, whose composition is only imperfectly revealed in 

 our preparations. Many of these fibers appear to be truly com- 

 missural between the optic parts of the tectum mesencephali; 

 but most of them probably form a decussation between the 

 tectum of one side and the region of the tuberculum posterius 

 of the opposite side. Here are the very large neurons of the 

 nucleus of the posterior commissure and fasciculus longitudi- 

 nalis medialis, which seem to form the chief avenue of discharge 

 for this decussation (p. 271). It is probable that this commis- 

 sure has been differentiated principally to connect the tectum 

 opticum \\dth the fasciculus longitudinalis medialis. Some of 

 its fibers, however, appear to go farther and connect directly 

 with dendrites of the nucleus of the III nerve. 



Commissura tecti mesencephali 



Extending backward from the posterior commissure and 

 quite uniformly developed throughout almost the entire length 

 of the midbrain roof is the commissura tecti of the midbrain 

 (Sylvian commissure of C. L. Herrick), of which the posterior 

 commissure is merely a more highly differentiated portion. 

 What proportion of these fibers are truly commissural has not 

 been determined; but most of them appear to effect a decus- 

 sation between the tectum (chiefly its non-optic portion, or 

 coUiculus inferior) and the motor tegmentum of the opposite 

 side. 



Decussatio veli 



The commissura tecti mesencephali is nearly but not quite 

 continuous with the decussatio veli immediately behind the 

 isthmus, the roof of the recessus posterior mesencephali con- 

 taining no commissural fibers (fig. 62; cf. also Herrick, '14, 

 p. 28, fig. 25). The composition of the decussatio veli in Nec- 

 turus has been given in my paper on the cerebellum ('14, p. 9), 



