NERVOUS SYSTEMS OF BACKBONED ANIMALS 



2 3 



of folding or convolution. These attain their maximum com- 

 plication in the human subject, where also the hemispheres are 

 of relatively enormous size (fig. 1027). The amount of convolu- 

 tion is related to the intelligence of the particular species, but 

 hasty deductions must be avoided, since they are also propor- 

 tionate to the bulk of the body. Some of the most brilliant 

 advances in modern surgery are due to the discovery that the 

 cerebral cortex is divided into 

 nerve-centres, some of which are 

 concerned with sight, hearing, 

 and other special sensations, while 

 others again control definite sec- 

 tions of the muscular system. 

 But so far it has not been found 

 possible to locate the higher 

 mental functions, such, for ex- 

 ample, as memory. The Cere- 

 bellum also possesses a very 

 complicated cortex. As might be 

 expected, the minute structure of the brain in a higher Mammal 

 is most remarkably complex. Details would be out of place 

 here, but fig. 1028, which represents one of the most specialized 

 neurons from the cortex, will suggest the elaboration which 

 exists, bearing in mind that the number of neurons in the brain 

 is enormous. And it is particularly interesting to know that, 

 as recent investigations have proved, these ultimate elements of 

 the nervous system maintain themselves during the entire life 

 of the animal. There are not successive crops of nerve-cells 

 as once supposed. Were this the case, indeed, such things as 

 memory would be almost unintelligible. 



Fig. 1028. Complex Neuron from Cortex of Human 

 Brain, greatly enlarged 



