212 THE NERVOUS SYSTEM 



9. The surface of the cerebrum is divided by a considerable 

 number of winding and irregular furrows, about an inch deep, 

 into " convolutions," as shown in Fig. 55. Into these furrows 

 the gray matter of the surface is extended, and, in this manner, 

 its quantity is vastly increased. The extent of the entire sur- 

 face of the brain, with the convolutions unfolded, is computed to 

 be equal to four square feet ; and yet it is easily enclosed within 

 the narrow limits of the skull. When it is stated that the 

 gray matter is the true source of nervous power, it becomes 

 evident that this arrangement has an important bearing on the 



PIG. 56. VERTICAL SECTION OP THE BBAIN 



A, Left Hemisphere of Cerebrum D, The Pons Varolii 



B, Corpus Callosum E, Upper extremity of the Spinal Cord 



C, Optic Thalainus F, The Arbor Vitse 



mental capacity of the individual. And it is noticed that in 

 children, before the mind is brought into vigorous use, these 

 markings or furrows on the surface are^comparatively shallow 

 and indistinct ; the same fact is true of the brain in the less 

 civilized races of mankind and in the lower animals. It is also 

 noticeable that among animals, those are the most capable 



9. The surface of the cerebrum, how marked ? The gray matter of the surface ? Ex- 

 tent of the entire brain surface ? Source of nervous power ? What further ? 



