134 C. M. Jackson. 



sexes being separated, and also the right and left in the case of the 

 paired organs. Data from the literature were utilized here as in 

 Table V. 



The Brain. 



Although subject to considerable individual variation (cf. Tables 

 IV and V) the relative size of the brain, when the average for lunar 

 months is taken (Table V), gives a fairly regular curve of growth, 

 as sho^vn in Fig. 3. ISTo data for the 1st month are available, but it 

 is very probable that, as in the case of the whole head, the maximum 

 relative size is not reached until the 2d month. Then it is seen to 

 form slightly more than 20 per cent of the entire body. From this time 

 onward, it decreases in relative size. The increase at the 9 th month 

 is probably accidental, due to the small number of observations. The 

 decrease is most rapid in the first half of the fetal period, the relative 

 size remaining fairly constant in the latter half, as noted by Legou, 

 (25). The brain reaches an average of about 12.8 per cent in the 

 still-born fetus (120 cases). In those born living, the average ap- 

 pears to be somewhat higher, being about 14.6 per cent (90 cases). 

 The reason for this increase in relative size in the live-born, which 

 is found in all of the organs (excepting pancreas and suprarenal 

 glands), is not clear. In the case of the lungs, it is evidently due 

 chiefly to a larger influx of blood, and this may perhaps in part ac- 

 count for the difference observed in the other organs. 



After birth, as is well known, the decrease in the relative size of 

 the brain continues, reaching about 2 per cent in the adult. Vierordt 

 (43) gives an estimate of 12.29 per cent of the total body weight for 

 the brain of the new-born, and 2.16 per cent for the adult. It may 

 be noted, however, that Vierordt's estimates (which are widely used) 

 are not calculated from individual data, and are therefore not free 

 from the possibility of error. 



Spinal Cord. 



The spinal cord attains its maximum relative size earlier than the 

 brain. In an embryo of the fifth week (11 mm.), it forms 4.85 per 

 cent of the entire body, and in models of earlier stages appears even 



