CEREBELLUM OPTIC LOBES. l8 



Just in front of the cerebellum is the valvula cerebelli, containing 

 the decussating fibres of the fourth pair of cranial nerves. These nerves 

 have their origin in a group of cells beautifully illustrated by Stieda, as 

 they are found in the turtle. 



THE OPTIC LOBES. 



IN Plates LXX. LXXXV. I have represented not only the outline of 

 these important parts of the encephalon, but have also by the use of high- 

 er powers, endeavored to give a more satisfactory idea of their minute 

 structure. The arrangement of the cells of the cortex or roof "decke" 

 into separate zones, which is always to be observed, in saurians, che- 

 lonians, and batrachians, does not appear in sections from ophidians. 

 Plates LXXIIL LXXVI. 



Although these plates were made from three different species, and 

 by the method generally employed throughout, it is possible that this 

 absence of linear arrangement of the cells is due to faults in preparation. 

 It is more probable however, that it signifies a peculiar structure, and 

 chiefly from the fact that sections of the cerebral lobes from the same in- 

 dividuals, and by the same method, show the usual arrangement of nu- 

 clei. Compare Plates LXXIII. & LXXIV. with LXXXIX. 



In the cerebellum of the same class of reptiles, the nerve cells are 

 seen, both in transverse and longitudinal vertical sections, to be more 

 widely scattered through the white layer than they are in the other class- 

 es, where they are limited to the zone between the white and gray layers. 



The "roof" over the ventricular cavity of the optic lobes, in the 



