THE NKRVE ELEMENTS. 



161 



just indicated, there is, in order to account for the adult 

 system, not only no need of arrangement for the pro- 

 duction of new cells after the third month of foetal life, 

 but it must also be assumed that a large number of the 

 elements already formed undergo but a very slight 

 development, otherwise the final mass of the central 

 system would be far more voluminous than it is. The 

 relation of the mass of this system at the 1 2th week of 



i ^ 



FIG. 27. Cubes illustrating the relative volumes of the central 

 nervous system at the I2th week of foetal life and at maturity. 

 If the largest cube represent the volume of the central system 

 of an adult, the smallest will represent that of a foetus at the 

 i ith week, both reduced to one-tenth of their actual volume. 

 The cube indicated by the dotted line represents the esti- 

 mated volume of the central system in the foetus of the i2th 

 week. 



foetal life and at maturity is illustrated in Fig. 27, which 

 shows in solid lines two cubes having one-tenth their 

 true volume, and a third, indicated by the dotted lines, 

 which has the actual volume of the central system as 

 estimated for a foetus of the I2th week. 



