632 THE NERVOUS SYSTEM 



in the accompanying figures, 395 and 397. The gray matter of the brain 

 contains, however, not only these layers and cells, but an infinitely rich 

 mass of fibers, which can be shown to have a certain definite arrangement. 

 Some of the fibers are vertical, passing directly up to the most superficial 

 layers of cells; others have a horizontal direction, dividing the gray matter 

 into different layers. These layers of fibers have received different names. 

 A typical arrangement is shown in figure 398. The most conspicuous 

 fibers are those of certain large triangular or pyramidal cells. 



The efferent or axone fibers from the cerebral cortex may be divided into 

 three classes: i, the projection fibers, which descend through the corona 



FIG. 396. Scheme of Descending Conduction Pathways from the Cerebrum to Lower 



Nerve Centers. 



radiata and internal capsule, to end in lower centers; 2, the commissural 

 fibers, which cross to the opposite cerebral hemisphere, chiefly through the 

 corpus callosum; 3, the association fibers, which pass in bundles beneath 

 the cortex, to end in other regions of the same hemisphere. 



It is by means of projection fibers and collaterals that associations are 

 made with nerve cells in the thalamus, tegmentum, and pons, and through 

 the latter region with tracts going to the cerebellum. 



Weight of the Brain and Cord. The brain of an adult man weighs from 48 

 to 50 oz., about 1,550 grams, or about 2 per cent, of the body weight. It 

 exceeds in absolute weight that of all the lower animals except the elephant 

 and whale. Its weight, relatively to that of the body, is exceeded only by that 

 of a few small birds, and some of the smaller monkeys. 



In the new-born child the brain (weighing 10 to 14 oz.) is about 10 per 

 cent, of -the total body weight. At the age of 7 years the weight of the 

 brain already averages 40 oz., and about 14 years the brain not infrequently 



