l6o THE GROWTH OF THE BRAIN. 



determined as follows : According to the table above 

 given, this system contains 1,005 cu. cm. of nerve sub- 

 stance. The average volume of a cell would be then 

 one 3000 millionth of this, which is '00033 cu. mm., and 

 taking the volume of the larger cells in the spinal cord 

 as -00246 cu. mm., then '00033 cu - mm - would be about 

 one-seventh of this figure. If, relying on histological 

 evidence, it is assumed that the production of cells 

 ceases at the end of the third month of fcetal life, and 

 that the volume of the nervous system at that time is 

 about 2*25 cu. cm., the volume of the neuroblasts would 

 be 750 cu. fj.. It was found previously that the germinal 

 cells had a volume of 687 cu. JJL, or nearly the same 

 figure ; the neuroblast soon becoming larger than the 

 cell from which it was derived. The importance of this 

 calculation is the following : Assuming that there are 

 3,000 millions of nerve elements in the central system of 

 1,005 cu - cm -> an d that these are condensed into a 

 nervous system whose volume is only 2^25 cu. cm., the 

 size of each of the elements is approximately the size of 

 the germinal cell, as determined by His from direct 

 observation, and the enlargement which each of these 

 3,000 millions of elements requires to undergo in order 

 to attain the average size found in the adult system 

 amounts to but one-seventh of the volume which is 

 possessed by the larger cells of the adult spinal cord. 

 In order that the nervous system, with a volume of 2*25 

 cu. cm., increase to 1,005 cu. cm., the number of elements 

 remaining the same, all the individual elements in the 

 central system must increase in average volume (1,005-^ 

 2*25) 447 times. The largest elements in the nervous 

 system do increase 10,000 times in volume, hence the 

 average increase here demanded is not large in com- 

 parison with that which may occur. Since many 

 neuroblasts increase far beyond the average volume 



