THE MIDBHAIN. 



119 



]oins anteriorly with the transverse fibers, which, as Commissura posterior, 

 pass along the frontal end of the midbrain-roof (Fig. 69). But, through the 

 narrower caliber of its fibers and their somewhat more dorsal position, the 

 bundle is always readily distinguished from the posterior commissure. 



The optic nerve arises, in all lower vertebrates, chiefly from the roof of 

 the midbrain. Figs. 64 and 70 give a good view of this origin. 



Only in the higher mammals does the opticus origin in the ganglia of 

 the thalamus appear to play a greater role. The roof of the midbrain is a 

 segment of a sphere. From the ventral chiasma a curving tract of manifold 

 bundles encircling the whole mass of the midbrain passes to the Opticus. 



Kg. 70. — Transverse section of lihodeus amarus througli the region of the 

 cHasma. Note that the Tectum mesencephali covers the Opticus. The median 

 portion of the figure belongs to the thalamencephalon. 



Most of the bundles lie near the surface, and, like the partially flexed fingers 

 of a hand, inclose the spherical mass; but a smaller number of bundles, 

 especially those which are destined for the more posterior portion of the 

 Opticus, turn inward Just before reaching the roof of the midbrain, and 

 pass toward their terminus, breaking through the basis of the midbrain- 

 roof, thus, in a measure, passing under it. 



These different bundles have been described as different "roots": viz., 

 as "median, lateral," etc.; but this has little to recommend it, for, though 

 some of the bundles vary their course somewhat, they all pass to the same 

 end. 



