THE CEREBRUM. 125 



these zones merge into one another, they are tolerably distinct. In 

 the middle (and widest) layer large multipolar cells are very nume- 

 rous, and the fibres may be seen to pass through the deeper layers 

 in bundles into the white matter. 



What chemical peculiarities does nervous tissue present? 



It contains some peculiar bodies allied to the fats, but contain- 

 ing nitrogen : of these cerebrin and lecethin are the more prominent. 

 Aside from this, the constituents are proteid and fatty substances, 

 with salts, chiefly potassium and magnesium phosphates, and 

 water. 



What is the weight of the adult brain ? 



About 3 pounds. In size it exceeds the brains of all the lower 

 animals except the elephant and whale. Its weight is about one- 

 fortieth of the total body-weight, and this ratio is greater than in 

 the lower animals, with a few exceptions among the smaller birds 

 and monkeys. In women the weight is about one-tenth less than 

 in men. 



Is the size of the brain a criterion of intellect ? 



In some degree it is. but this is not absolute. The depth of the 

 sulci, and consequent size and complexity of the convolutions, are 

 a more efficient measure of the brain-power. In the largest of the 

 apes the brain of an adult animal is about the same in weight as 

 that of a human infant at birth. Idiots, as a rule, have brains 

 much smaller than the normal, and in them the convolutions are 

 apt to be ill-marked and uncomplicated, as is the case in the lower 

 animals. 



What is known of the course of the fibres in the white substance 

 of the hemispheres ? 



The course of these fibres may be classified in three groups : 

 1, commissural fibres ; 2, fibres of association ; and 3, medullary 

 fibres. 



(1) The Commissural Fibres are those which connect one hemi- 

 sphere with the other, and it may be said that these fibres connect 

 each set of convolutions with the corresponding set of the opposite 

 side. The convolutions of the portion of the brain lying above the 

 fissure of Sylvius communicate by the corpus callosum, while those 

 at the base of the brain are joined by fibres passing through the 

 anterior commissure. 



