NEUVOUS SYSTEM OF VERTEBRATA. 555 



of which the brain and whole extent of the 

 spinal marrow are shown in Fig. 449, the en- 

 largements (w and l) corresponding to the wings 

 and legs respectively, are nearly of equal size. 

 In Quadrupeds, we likewise find the relative size 

 of these enlargements corresponding to that of 

 fore and hind extremities. When the latter are 

 absent, as in the Cetacea, the posterior dilatation 

 does not exist. 



The brain (b) may be regarded as an expan- 

 sion of the anterior or upper end of the spinal 

 marrow ; and its magnitude, as well as the 

 relative size of its several parts, vary much 

 in the different classes and families of ver- 

 tebrated animals. This will appear from the 

 inspection of the figures I have given of this 

 organ in various species, selected as specimens 

 from each class, viewed from above ; and in all 

 of which I have indicated corresponding parts 

 by the same letters of reference. 



The portion (m) of the brain, which appears 

 as the immediate continuation of the spinal 

 marrow (s), is termed the medulla oblongata. 

 The single tubercle (c), arising from the ex- 

 pansion of the posterior columns of the spinal 

 marrow, is termed the cerebellum, or little brain. 

 Next follow the pair (t) which are termed the 

 optic tubercles, or lobes,* and appear to be pro- 



* In the Mammalia, and in Man, they have been often desig- 

 nated by the very inappropriate name of Corpora quadri- 

 gemiria. 



