222 CENTRAL NERVOUS SYSTEM 



Growth of Brain. At birth the brain weight is about one- 

 third of what it will be at maturity. The increase is very rapid 

 during the first year; quite/apid during the next seven or eight 

 years; after this it becomes very slow. The maximum weight 

 is attained in men between the fiftieth and sixtieth years; in 

 women, between the fortieth and fiftieth years. A premaximum 

 at thirteen to fifteen for males, and at about fourteen for 

 females, indicating a too vigorous growth, seems to be an im- 

 portant cause of death at this age. The encephalon reaches 

 maturity much sooner than the body as a whole. At the end of 

 the eighth year, when the brain has almost completed its growth, 

 the body has reached but a third of its mature weight. At birth 

 the brain forms 12 per cent, of the total weight of the body, 

 while in the adult it forms but 2 per cent., or even less. 



It has been estimated that the cortex alone contains 9,200,- 

 000,000 cell bodies, and that the entire nervous system must 

 contain at least 13,000,000,000 cells. It is generally agreed 

 that in the human being the number is not increased after the third 

 month of fetal life. All subsequent increase in the mass of the 

 brain is, therefore, due to an enlargement of individual cells. 

 There is very little direct evidence for this in man. Kaiser 

 has measured the diameters of the cells of the anterior horns 

 of the spinal cord, and in this manner has determined an increase 

 in size. In the frog there is a gradual increase in the number of 

 fibers of the ventral and dorsal spinal nerve roots. The rate of 

 increase is about 50 fibers for the ventral roots and 70 fibers 

 for the dorsal roots for each gram increase in the weight of the 

 frog. Moreover, the greatest number of medullated fibers is 

 to be found in the ventral roots near the cord, and in the dorsal 

 roots near the ganglion. This is explained by assuming the 

 presence of undeveloped cells which gradually become mature, 

 and in so doing push out their processes. 



The area of the cerebral cortex which is exposed has about one- 

 half the extent of that which forms the walls of the sulci. It has 

 been shown that the fibers of the cortex are greater in number 

 in middle age than in youth or old age. The association fibers, 

 moreover, form three parallel systems. The deepest of these 

 is first to become medullated, and the middle layer last. 



The average specific gravity of the brain for males is about 

 1036.3, and for females 1036. The gray matter has about 81 



