NERVOUS SYSTEM OF VERTEBRATA. 391 



as well as the relative size of its several parts, vary much in 

 the different classes and families of vertebrated 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 im- 

 mediate continuation of the spinal marrow (s,) is termed the 

 medulla oblongata. The single tubercle (c,) arising from 

 the expansion of the posterior columns of the spinal mar- 

 row, is termed the cerebellum, or little brain. Next follow 

 the pair (T) which are termed the optic tubercles, or lobes* 

 and appear to be productions from the middle columns of 

 the spinal marrow. These are succeeded by another pair of 

 tubercles (H,) which are called ti\Q cerebral hemispheres, and 

 the origin of which may be traced to the anterior columns 

 of the spinal marrow. There is also generally found, in 

 front of the hemispheres, another pair of tubercles (o,) which, 

 being connected with the nerves of smelling, have been 

 called the olfactory lobes, or tubercles.] These are the 

 principal parts of the cerebral mass to be here noticed, for I 

 purposely omit the mention of the minuter divisions, which, 

 though they have been objects of much attention to anato- 

 mists, unfortunately furnish no assistance in understanding 

 the physiology of this complicated and wonderful organ. 



On comparing the relative proportions of the brain and 

 of the spinal marrow in the four classes of vertebrated ani- 

 mals, a progressive increase in the size of the former will be 

 observed as we ascend from Fishes to Reptiles, Birds, and 

 Mammalia. This increase in the magnitude of the brain 

 arises chiefly from the enlargement of the cerebral hemi- 

 spheres (H,) which, in the inferior orders of fishes, as in the 



* In the Mammalia, and in Man, they have been often designated by the 

 very inappropriate name of Corpora quadrigemina. 



-j- Several cavities, termed Ventricles, are occasionally found in the inte- 

 rior of the principal tubercles of the brain; but their use is unknown. 



