CHAPTER XYI. 



THE BRAIN OF QUADRUPEDS AND SOME OTHER MAMMALS. 



A GREAT advance is to be met with in the cleyeloi^ment 

 of the Brain in passing from Birds to Mammals, and from 

 lower to higher forms of the latter. There are obvious 

 differences in external conformation, and also internal 

 differences only to be detected by dissection of the 

 organ. 



External Differences. — The first and most important 

 of these peculiarities is the increasing size of the 

 Cerebral Lobes or Hemispheres. In lower Quadrupeds 

 these parts scarcely extend far enough back to cover the 

 Optic Lobes, whilst in higher terms of the series they not 

 only hide these bodies completely, but also in part hide 

 the more developed Cerebellum. The Cerebral Hemi- 

 spheres in Quadrupeds also tend to become more and 

 more plainly indented by certain primary depressions or 



* fissures,' by which they are divided into what are called 



* lobes.' The Hemispheres are also, to an increasing 

 extent, marked by various smaller secondary fissures or 

 'sulci,' by which, together with the primary fissures, certain 

 foldings of the surface of the brain, known as * convolu- 

 tions ' or * gyri,' are produced. 



The second of the external peculiarities, above referred 

 to, is the gradually increasing size of the lateral lobes of 

 the Cerebellum— parts which, like the Cerebral Hemi- 



