GROWTH OF THE CEREBRAL CORTEX 505 



The increase of Ht. is somewhat more rapid than the increase 

 in width, but is rather slow as compared with the increase in 

 length, so that in relation to its length the brain flattens some- 

 what as the age advances. 



If the initial values of the diameters in the newborn brain 

 group (Group II) be taken as unity and the corresponding values 

 of the diameters in the other successive groups be compared 

 with these units, the series of ratios in columns D, E and F, table 

 4, are found. If the shape of the cerebrum remained the same 

 throughout growth, the product oi W. B X L. G X Ht. would 

 give the relative volume of the cerebrum at maturity as com- 

 pared with its volume at birth. 



As already stated, the weight of the cerebrum stands in an 

 almost fixed relation to the weight of the entire brain, so the ratio 

 of the total brain weight to its weight at birth would be the same 

 as the ratio for the cerebrum and the cube root of that ratio 

 would indicate the theoretical increase in one (mean) dimension, 

 if the cerebrum did not change in form. These calculated ratios 

 based on the data in table 3 are given in table 4. From among 

 the values given in table 4, the cube root of the brain weight 

 ratio and the ratios of W. B, L. B and Ht. are presented in chart 

 2 in smoothed graphs. 



On examining chart 2, we see that the diameter W. B increases 

 in an almost fixed relation to the theoretical curve denoted by 

 ■\/C (representing the cube root of the bran weight ratios), indi- 

 cating that W. B is growing almost in proportion to the increase 

 in total volume. The graph for L. G shows the rapid growth of 

 this dimension. The rate of increase is most rapid in brains 

 weighing 0.25 to 0.90 grams, then, in the brains weighing 0.90 

 to 1.25 grams, the curve runs nearly parallel to \/C, and after 

 that period the rate becomes more rapid again. Up to a brain 

 weight of 1.1 grams, the increase of Ht. is nearly equal to that of 

 \/C and after that becomes slower. Generally speaking, in the 

 period in which the brain weighs 0.9 to 1.2 grams, the three di- 

 mensions of the cerebrum increase in nearly the same proportions, 

 a fact to which we shall return later when considering the growth 

 in thickness of the cerebral cortex. 



