LOCALISATION OF FUNCTION. 255 



those forms in which cephalisation is but little advanced 

 the primary centres of the cranial nerves when alone 

 present may assume a guiding control over the re- 

 mainder of the system. It thus happens that a frog 

 after loss of the cerebral hemispheres can still direct its 

 jumping movements so as to avoid a visible obstacle in 

 its path ; in other words, impressions reach the central 

 system of such a frog through its eyes, and these impres- 

 sions influence the reactions of the muscles of hind legs 

 despite the absence of the hemispheres. 'In man, on the 

 other hand, the parts of the brain corresponding to the 

 optic lobes of the frog do not represent a locality in 

 which such connections are established, so that in him 

 the hemispheres alone do the work which in the less 

 specialised form may be performed by the lower 

 centres. In this connection we naturally inquire how 

 the cerebral hemispheres may have acquired in the higher 

 vertebrates capabilities which belong to them in a less 

 and less degree as we descend from man through the 

 zoological scale. In the higher forms it appears that 

 the incoming impulses, instead of passing over in the 

 primary centres to cells which discharge downwards, 

 pass to a group of afferent central cells which carry 

 impulses to the cortex, that with the organisation of 

 this second pathway the first becomes less passable, and 

 thus the function is transferred, though the causes 

 determining the growth of the central cells on which 

 the change depends are still obscure. 



One difficulty felt by all who have touched this 

 problem of localisation is due to the very large portion 

 of the encephalon, especially in the cerebrum, to which 

 our observations and conclusions have not yet been 

 applied. At first explanation is incommoded by the 

 existence of these enigmatic parts, but only too readily 

 do we neglect what cannot be easily explained, and by 



