358 DR. G. ELLIOT SMITH ON 



the cerebellum and the cerebral liemisjiberes. The posterior geniculate bodies still 

 retain their relativelyjlarge dimensions. 



Fig. 28. 



corp. qtraangem. post. 

 • Corp. quadrigem. ant. 

 thalam. opt. 



Corp. genicnlat. ant. 

 j i icorp. gemoulat. post. 



paraflocc. • crus cerebri 

 t 



pons Varol. 

 Riglit lateral aspect of brain-stem, and cerebellum of Dasypus vUlosus. Nat. size. 



In spite of all these variations in form and the relative proportions of different j)arts, the 

 mesencephalic region in all the Edentates conforms to the same type, which is pi-actically 

 constant in the Marsupialia, Insectivora, Rodentia, Ohiroptera, Ungulata, and Carnivora. 

 All the changes which take place are probably the direct expression of functional 

 adaptation to different modes of life. In animals which lead a life such as all the 

 Edentates follow, the sense of sight is of considerably less importance than the sense of 

 hearing and the all-important sense of smell. Eor most of the animals of tliis group are 

 nocturnal, and many of them are burrowing animals. They seek their food and pursue 

 the other objects which constitute the sum of their life's exertions at a time or in a manner 

 in which visual sensations must be of subsidiary importance. Even to the dog and cat, 

 who seek their visible prey with the directing aid of a highly-developed visual area of 

 the pallium, the sense of sight is quite subsidiary to that of smell and possibly also of 

 liearing. It is only natural, therefore, to expect in the Edentata, which are not so well 

 provided with a pallia! visual mechanism and which pursue tlieir ends under circum- 

 stances in which sight can avail them nothing, that the visual sense should be of even 

 more subsidiary importance. We have already seen that the olfactory parts of the brain 

 reach a very great development in this group. The auditory regions of the brain 

 ;i.re also exceptionally well-developed, as we shall see later ; 1)ut it is quite possible 

 that the enormous size of the posterior geniculate body and the large dimensions of 

 the posterior quadrigeminal bodies may also be witnesses to the high importance of the 

 auditory sense. 



The waning importance of the visual sense is markedly reflected in the anatomy of the 

 mesencephalon. The anterior quadrigeminal bodies, Avhich arc so well-developed in 

 Orycterojms and almost equally so in the 3Iyrmecopliagidce, itndergo a conspicuous 

 reduction in the DasypocUclce, and in Chlamydophorus, in which the visual activity has 

 become so slight as to have earned for it the name " Pichi-ciego " or " Blind Armadillo." 

 AVhile the anterior quadrigeminal bodies are undergoing this retrogression, the j)osterior 

 bodies remain, because their chief function is certainly not visual, and hence we get that 

 peculiar transverse ridge-like condition of the mesencephalon in the Armadillos. 



