NERVOUS SYiJTEM OF VERTEBRATA. 501 



in a straight line, continuous with the spinal cord, 

 of which, as they scarcely exceed it in diameter, 

 they appear to be mere enlargements. As the 

 skull expands more considerably than the brain, 

 this organ does not fill its cavity, but leaves a large 

 space, filled with fluid. Some degree of shortening, 

 however, may be perceived in the brain of the 

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

 doubled underneath the cerebellum (c), pushing 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 surface 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 ex- 

 panded hemispheres (h) ; their position only being 

 marked by the same letter (t). These changes are 

 consequences 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 hemispheres 

 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 



