THE MIDBEAIN. 



115 



to parts beyond. Then there are fibers which pass into the base from their 

 origin in the roof. Finally, a number of nuclei have been found there, from 

 which arise bundles which pass, in part, into the cerebellum, in part, to 

 the brain-surface as peripheral nerves: Oculomotorius, Troehlearis (see 

 Fig. 68). 



From the roof of the midbrain passes from one side to the other of 

 the brain a large commissure: Commissura posterior cerebri. It lies in the 

 roof -plate itself, and borders entirely upon the posterior wall of the epiphysis 

 (see Figs. 18 and 89). 



The minute structure of the roof of the midbrain is exactly known only 



Fig. 66. — Showing the minute structure of the midbrain-roof. Two sections 

 placed side by side for comparison of the layers. After Pedro R. y Cajal. Right- 

 hand section from a frog. Note opticus fibers entering and ramifying in different 

 layers. Sagittal section (compare Kg. 64). Left-hand section from a, lizard. 

 Note the cells. 



through the researches of E. y Cajal, of Fusari, and especially of von 

 Gehuchten and P. R. y Cajal, in representatives of the different classes of 

 vertebrates. 



It becomes evident that the different layers into which the roof-plate 

 may, in all animals, be subdivided arise in a relatively simple manner (see 

 Fig. 66). 



Into the outer layer enter the fibers of the optic nerve with innumer- 



