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 fifty-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/z 3 (His* measurements give 697//), then this entire number 

 could easily be placed in 2.25 cu.cm. 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 



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). 



