THE MIDBBAIN. 



117 



lated layer of the roof-plate: the stratum medullare profundum. Still be- 

 yond that one recognizes cells and thin non-mednllated fibers. What be^ 

 comes of the mediillated layer may be easily understood (see Fig. 67). A 

 part passes direct to the side of the midbrain and down to the base, thence 

 posteriorly: a second part goes the same way, except that it first crosses 

 the median line before it turns downward. This decussation was formerly 

 called Commissura ansulata. These two parts together represent the lateral 

 division of the deep medullary layer. The fibers of the median division 

 which lies next to the ventricle do not turn toward the base of the midbrain. 



Valvula eerebelli 



Gangl. mesenceph. lat. _ 

 Median fasciculus of thel 

 deep medullary layer J 

 Nucl. N. oculo-mot. . 



Tr. lobi inf. ad cerebell. 



Lateral portion of the") 

 deep medullary layer J 



Lobi inferiores 



Fig. 68. — Frontal section through the midbrain of a teleost, Rhodeus amarus. 



For a short distance they pass parallel to the ventricular wall and then 

 divide, like the lateral portion, into a direct and crossed bundle. 



The direct portion ends mostly in a ganglion, — Ganglion laterale 

 mesencephali (see Fig. 68); the crossed portion encircles the floor of the 

 ventricle, which presents here only a narrow cleft, and forms under it in 

 beautifully plaited lines the tegmental decussation {Haubenkreutzung). 

 Afterward it passes posteriorly close to the middle line, ventral to the fibers 

 of the posterior longitudinal bundle (see Fig. 91). 



Thus, all of these are bundles which connect the deep layers of the 

 midroof with more posterior segments. Most of them end in the medulla, 



