34 MENTAL EVOLUTION IN ANIMALS. 



CHAPTER III. 



The Physical Basis of Mind. 



We have already taken it for granted that Mind has a 

 physical basis in the functions of the nervous system, or that 

 every mental process has a corresponding equivalent in some 

 neural process. I shall next endeavour to show how precise 

 this equivalency is. 



We have seen that ganglionic action consists of waves of 

 nervous tremours originating in the cells, coursing along the 

 attached fibres to other cells, and there arousing fresh impulses 

 of the same kind. Moreover, we have seen that this coursing 

 of nervous impulses through nervous arcs is not, as it were, 

 promiscuous, but that, owing to the anatomical plan of a 

 ganglion, it takes place in certain determinate directions, so 

 that the result, when expressed in muscular movement, shows 

 the function of a ganglion to be that of centralizing nervous 

 action, or of directing nervous tremours into definite channels. 

 Lastly, we have seen that this directing or centralizing 

 function of ganglia has probably in all cases been due to the 

 principle of use combined with that of natural selection, 

 i^ow it is known from experiments on the lower animals, as 

 well as from the effects of cerebral disease in man, that the 

 part of the nervous system in all the Vertebrata which 

 appears to be exclusively concerned in all mental operations, 

 is the so-called " large brain," or cerebral hemispheres. This 

 is the convoluted part of the brain which appears imme- 

 diately below the skull, and is above all the series of ganglia 

 or nerve-centres which occupy the rest of the cerebro-spinal 

 tract. As some at least of the bewildering multitude of cells 

 and fibres which constitute the cerebral hemisjoheres are in 

 connection with these lower ganglia, there is no doubt that 

 the hemispheres are able to " play down " upon these ganglia 

 as upon so many mechanisms, whose function it is to throw 



