CENTRAL NERVOUS SYSTEM. 



731 



weight of the nerve-tissues composing the encephalon, and of this one-half 

 would be medullary substance. 



Increase in the Mass of Nerve-cells. The amount of this increase under 

 various conditions has already been discussed, and been found to range between 

 zero and h'fty-thousaud-fold. 



Number of Cells. A conservative estimate of the number of cells in the 

 entire central system is 3,000,000,000. Giving each cell of this number a vol- 

 ume of at least 700// 3 (His' measurements give 697//), then this entire number 

 could easily be placed in 2.25 cu.cra. We assume that about three-quarters 

 of the total volume of the central system is nerve-tissue proper, while 

 the remaining quarter is composed of the supporting tissues and blood- 

 vessels. 



Volume of Central System. The volume of the entire system contain- 

 ing cells of the number and size chosen, as well as the supporting tissues, would 

 then, on the supposition made, be about 3 cu.cm., which is approximately that 

 found in the human fetus at the end of the twelfth week (see Fig. 207). The 

 enlargement occurring between this time and maturity is that between 3 cu.cm. 

 and 1340 cu.cm., the latter figure being the volume of the encephalon and cord, 



Maturity 



Birth 



'Fet-t,' 



FIG. 207. Cubes illustrating the relative volumes of the central nervous system at the twelfth week 

 of fetal life, at birth, and at maturity. The cubes as shown have exactly one-eighth of their true 

 volumes. 



weighing 1386 grams (encephalon 1360 grams, and spinal cord 26 grams), 

 and having together a specific gravity of 1036. This change demands an 

 average enlargement in the nerve-elements of four hundred and forty- 

 seven-fold, which, it is seen, is well within the limits of that found for 

 a cortical cell of medium size which had enlarged six hundred and sixty 

 times (pages 608, 609). 



