THE GROWTH OF THK BRAIN. 



division occurring in this way, and accompanied by 

 changes in the size and form of the elements, the com- 

 plex individual is built up. 



What is thus accomplished in the case of man is 

 roughly indicated by the following calculations : Ac- 

 cording to the estimate of C. Francke, 1 there are, in the 

 entire body of a full-grown person, a number of fixed 

 cells represented by 4,000,000,000,000. The cells in the 

 blood are not fixed, and for these the best calculations 

 give 22,500,000,000,000, or, in the entire body, a total of 

 26,500,000,000,000 cells. This number may appear 

 surprisingly large, but it must be remembered that there 



are in a cubic millimeter 

 1,000,000,000 cubic //, (the 

 micron =/LL being O'OOi of a 

 millimeter, and the unit 

 adopted by histologists in 

 micrometry) ; so that if each 

 cell in the human body had 

 but the volume of one cubic 

 /ji, the whole number could 

 be condensed into 22,500 cu. 



This cube could contain very large mass, since it is 

 26 500,000,000 ooo cells pro- represented by a cube length 

 vided each cell hnd a volume r / *> 



of i cubic micron, the whose edge is 2'8 cm. (Fig. 3.) 

 micron being the unit of These facts can be expressed 

 measure employed by histo- . ... . r ... ... 



logists. in still another form which will 



indicate the average size of the 



cells. A man weighing 150 Ibs. the average specific 

 gravity of his body being taken as 1-030 would have 

 a volume of 66,200 cu. cms. As stated above, all the 

 cells of the body might be contained in 22'5 cu. cm., 

 provided each cell had a volume of only I cu. /m. By 

 1 Francke, Die menschliche Zelle, 1891. 



