370 DR. G. ELLIOT SMITH ON 



These peculiar changes which have bcca just briefly outlined are distinctive of the 

 Mammalia ; they are never found oixtside the class, and there is no mammal in which 

 tliese features are lacking. Moreover, these changes appear to have practically reached 

 their consummation even in the lowliest mammal. 



Although the rhinencephalon is thus much larger and much more highly developed in 

 the mammal than it is elsewhere, it at the same time takes a relatively much greater 

 shai'e in the fonnation of the cerebral hemisj^here in the non-mammalian vertebrates. 

 The simple primordial hippocampus of the reptile forms not only the greater part of the 

 mesial Avail, but also a large part of the dorsal w^all or roof of the hemisphere ; and its 

 lateral margin is thus brought into proximity with the simple representative of the pyri- 

 f orm lobe which forms part of the lateral wall. All that separates tliese two parts of the 

 rhinencephalon in the reptile is an insignificant area of simple cortex on the dorso-lateral 

 aspect of the hemisphere. It is difficult to appreciate the fact that this apparently 

 unimportant and almost undifferentiated patch represents the forerunner of that highly 

 specialized and elaborately complex cortex which we know as the pallium, and whicli 

 in the mammal produces such profound and far-reaching modifications and attains to a 

 morjjhological importance which exceeds that of the rest of the nervous system. 



In this imperfect sketch of some of the salieiit features in the evolution of the cerebral 

 cortex we have wandered some distance from our main theme, in order to emphasize the 

 fact that the pallium is practically a new structure in the mammal : it is something that 

 has been added to the old Saurian brain, and immediately exercises so marked an influence 

 upon its possessor that it is perhaps not an exaggeration to say that the possession of a 

 pallium made mammals possible. Before its appearance the central nervous system 

 might be compared to a series of autonomous governments, of varying importance and 

 influence, united into a harmoniously working federation, witli no very decided centralized 

 control beyond the slightly dominant sway exercised by the insignificant cei'ebral hemi- 

 spheres. In the mammal this federation comes under the domination of the almost 

 absolute power of the dictatorial i:)allium. The pallium, in a sense, represents within 

 itself the whole body, because nerve-paths proceeding from all regions of the body 

 ultimately lead to the pallium, which is the Rome of the neural empire ; but it also 

 dominates and controls the whole of the nervous system, and through it the whole of 

 the body. 



It is of the utmost importance, therefore, to study the features of this organ, which 

 may truly be regarded as a reflection of the whole body. Keeping constantly in view 

 the fact of its recent appearance, its obvious influence on the rest of the organism, 

 and the multiplicity of factors which determine its size and configuration, we may 

 the better ajipreciate the meaning of its changing features which we are about to 

 consider. 



But before doing so we must briefly discuss the nature and constitution of the 

 pallium. 



The governing organ of the submammalian brain is probably the simple basal ganglion, 

 which is generally influenced in all probability to a predominant degree by the preco- 

 ciously developed olfactory areas of the hemisphere. In the mammal the development 



