NERVOUS SYSTEM OF VERTEBRATA. 



393 



T. In the Mammalia, as the Lion (Fig. 460,) they are 

 lodged quite in the interior of the organ, and concealed by 

 the expanded hemispheres (H;) their position only being 

 marked by the same letter (T.) These changes are conse- 

 quences of the increasing development of the brain, com- 

 pared with that of the cavity in which it is contained, re- 

 quiring every part to be more closely packed; thus, the lay- 

 ers of the hemispheres in Mammalia are obliged, from their 

 great extent, to be plaited and folded on one another, pre- 

 senting 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 produce, likewise, even in 

 birds, transverse furrows on the surface; and from the in- 

 terposition of a substance of a gray colour between the la- 

 minsB of the white medullary matter, a section of the ce- 

 rebellum presents the curious appearance (seen in Fig. 459,) 

 denominated, from its fancied resemblance to a tree, the 

 *1rbor Vitae. 



Thus far we have followed an obvious gradation in the 

 development and concentration of the different parts of the 

 VOL. II. 50 



