378 DK. G. ELLIOT SMITH ON 



increase in number, so as to unite into a more exquisitely delicate machine the sensori- 

 projection cortex. The increase in number of these elements causes an insidious growth 

 of the sensory areas in which they are so intimately mingled with the sensory and 

 projection elements. But as these association-el emerits increase in number in the 

 formation of intricate groupings which determine increasingly complex and " skilful " 

 movements, they extend far beyond the limits of the "sensory " or " projection" areas, 

 and constitute regions wdiich .Flechsig calls " areas of association." 



In an interesting memoir upon the weight of the brain in mammals, Eugene Dubois * 

 calls attention to and discusses the influence of the size and richness of the sensory 

 surfaces of the body upon the cei'ebral cortex. Plechsig, who introduced the terms 

 "■ Projection-centre " and " Association-centre," has shown that the former develop veiy 

 late in the ontogeny of the mammalian brain ; and the sensory fibres, which are the first 

 to develop in the cortex, only receive their medullated sheaths after the lower executive 

 and jjhylogenetically ancient parts of the brain are fully developed. 



The facts which we liave discussed above to show the appearance of the pallium for the 

 first time in the mammal give a demonstration of the late phylogenetic appearance of the 

 more primitive pallium {I. e. the centres of projection), which is (|uitc in harmony with 

 their late appearance in ontogeny. The " centres of association " naturally make their 

 appearance after the " centres of i^rojection " are laid down. 



Elechsig says t that in E-odents (the House-Mouse and the Marmot) association-centres 

 are entirely lacking. In other words, the sensory centres are in close contiguity 

 (" Sinnesphiire stosst an Sinnesphiire "), so that the whole j^^Hium is one field of 

 projection. It is difficult to conceive of a large pallial area like that of the Rodents as 

 entirely devoid of association-elements, and we may with some reason interpret Flechsig's 

 statements as meaning that the elements of association are not yet sufficiently numerous 

 to form definite areas beyond the limits of the sensory. Such areas of association 

 begin to make their apjjearance in the Carnivora, according to Flechsig. The association- 

 elements which exist in the pallium of tlie Rodent are so intimately intermingled with 

 the sensori-projection elements that they are not recognizable as such, but go to swell 

 the dimensions of the sensory or j)roj action areas. 



The further discussion of these factors must be deferred until we can discuss the 

 specific cases afforded by the pallium in the Edentata. In the meantime it is convenient 

 to briefly discuss the brain-stem and cerebellum. 



In the evolution of the mammal the features of the brain-stem change very slightly 

 in comparison wdth the striking modifications which the cerebral hemisphere undergoes. 

 Most of the changes which affect the brain-stem are more or less directly associated 

 with the development of the pallium. Thus the fuller development of the optic thalamus 

 and the corj)ora geniculata are probably integral factors in the elaboration of the sensory 

 paths to the pallium. The changes which occur in the corpora quadrigemina are also in 

 all probal)ility expressions of the same series of changes. 



* Eugune Dubois, " Siir le Kappoit du Poids de TEncephale avec la Grandeur du Corps clicz Ics ilammiferes," 

 Biilk'tins de la Societe d'Anthropologie de Paris, t. viii. (4" serie) 1897. 

 t Paul iledisig, ' Gehiru und Socle,' i' Ausgabe : Leipzig, 1896, p. 84. 



