NERVOUS SYSTEMS OF BACKBONED ANIMALS 23 
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 edie adi the 
cerebral cortex is divided into 
nerve-centres, some of which are NG a 
concerned with sight, hearing, ce ue “ 
and other special sensations, while NK ay i 
others again control definite sec- 
tions of the muscular system. We 
But so far it has not been found ay We 
possible to locate the higher Sy aS 2b ae 
Ne ST \ 
mental functions, such, for ex- . SS a 
Sts. 
ample, as memory. The Cere- 
bellum also possesses a very Fig. 1028.—Complex Neuron from Cortex of Human 
$ 5 Brain, greatly enlarged 
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. 
