558 THE SENSORIAL FUNCTIONS. 



Perch (Fig. 457) ; for the medulla oblongata (m) 

 is doubled underneath the cerebellum (c), push- 

 ing it upwards, and rendering it more prominent 

 than the other tubercles. This folding inwards, 

 and shortening of the whole mass, proceeds to a 

 greater extent as we trace the structure upwards, 

 as may be seen in the brain of the Green Turtle 

 (Fig. 458). In that of Birds, of which Fig. 459 

 presents a vertical section, the optic tubercles 

 have descended from their former place, and 

 assumed a lateral position, near the lower sur- 

 face of the brain, lying on each side of the 

 medulla oblongata, at the part indicated by the 

 letter t. In Mammalia, as in the Lion (Fig. 

 460), they are lodged quite in the interior of the 

 organ, and concealed by the expanded hemi- 

 spheres (h) ; their position only being marked 

 by the same letter (t). These changes are con- 

 sequences of the increasing developement of the 

 brain, compared with that of the cavity in which 

 it is contained, requiring every part to be more 

 closely packed ; thus the layers of the hemi- 

 spheres in Mammalia are obliged, from their 

 great extent, to be plaited and folded on 

 one another, presenting at the surface curious 

 windings, or convolutions, as they are called 

 (seen in Fig. 456), which do not take place 

 in the hemispheres of the inferior classes. The 

 foldings of the substance of the cerebellum pro- 

 duce, likewise, even in birds, transverse furrows 



