THE NERVE ELEMENTS. 153 



and conversely that of the micron in fractions of a milli- 

 meter ; also the weight of the milligramme in micro- 

 milligrammes. 



TABLE 32. SHOWING THE RELATIONS OF THE MICRON TO THE 

 MILLIMETER WHEN APPLIED TO THE MEASUREMENT OF 

 LENGTH, AREA, AND VOLUME. ALSO THE RELATION 

 BETWEEN THE MILLIGRAMME AND MlCROMILLIGRAMME. 



Length ... 

 Area 



Volume ... 

 Length ... 

 Area 



Volume ... 

 Weight ... 



mm. 1,000 fi 



sq. mm.=; 1,000,000 sq. /* 

 cu. mm.= i, 000,000,000 cu. \i. 

 \t -ooi mm. 



sq. \t = 'oooooi sq. mm. 



cu. n = 'ooooooooi cu. mm. 

 mgrm. = 1,000,000,000 //grm. 



This table will serve, among other purposes, as a 

 reminder of the difficulty of conceiving either very 

 small measures or very large numbers ; so unacquainted 

 are we with these extremes that most of us have no 

 feeling for the correctness of the facts expressed in 

 such terms, to say nothing of the difficulty in handling 

 them. 



The growth of the nerve cells may be briefly illus- 

 trated by a few measurements. His has determined 

 the diameter of the germinal cells in man to be 1 1 ^, 

 and the volume 697 //, (embryo R-length 5-5 mm., age 

 3-3*5 weeks). 1 The larger cells in the adult spinal cord 

 have a diameter of 50 //,, and a volume of 65,312 cu p. 

 The ratio of these two volumes is a trifle less than 

 i-ioo, but for convenience we may take the round 

 numbers. This liberty is justified by the fact that the 

 very largest cells attain a diameter of more than 100 /*, 

 and hence the figures for adult size given above are well 

 within the limits. The larger nerve fibres are 10 //, in 

 diameter, again a conservative figure, which gives 790 //, 2 

 as the area of the cross section, a relation which re- 

 1 His, Arch.f. Anat. und Physiol., 1889. 



