Postnatal Growth of the Human Brain 



115 



growth of the major divisions of the structure, apparently no 

 attempt has been made to analyze these curves and to develop 

 formulae for the expression of the relation between brain weight 

 and age between birth and maturity. We have made a series 

 of calculations of this type and have comjputed empirical formulae 

 for the growth of the encephalon as a whole, the cerebrum, the 

 cerebellum, and the pons, medulla and mid brain, from birth to 

 20 years. These formulae have been determined from the 

 weighted average of male and female brain weights. While it 

 will no doubt be possible to develop slight varients of these 

 formulae for the weight of the entire brain and of the cerebrum 

 for males and females separately, our data indicate that it is 

 hardly practicable to establish separate curves for the sexes, on 

 the basis of the material now available, for the weight of the 

 cerebellum and the brain stem. Likewise no attempt has been 

 made to correct graphically or mathematically for the effect of 

 disease on the weight of the brain although all records of cases in- 

 volving any brain pathology were rigidly excluded. The curves 

 and formulae, therefore, represent the growth of the organ in a 

 hospital rather than in the general population. 



When plotted against age and tested graphically all the curves 

 of the postnatal growth of the brain approach hyperbolae and 



may be expressed approximately by the general formulae : 

 x + c x 



. Y == —— or ¥ = — -j- c. 



a -f- ox a -j- ox 



In these formulae, Y is the weight of the brain or brain-part, X 

 is the age in years and a, b and c are empirically determined 

 constants. 



A total of 2956 observations on the weight of the brain as 

 a whole in the postnatal developmental period were available 

 for study. Of these 317 were of newborn infants and 2639 

 were of individuals between birth and 20 years. The empirical 

 formula of the curve for these data as drawn by inspection is : 



Age (yrs.) + 0.315 



0.01 Encephalon weight (gin.) = — - — - — — — 



0.09 -f 0.0692 age (yrs.) 



The average deviation of the calculated values as determined by 

 this formula from the observed trimester averages for the first 

 year and the observed yearly averages thereafter to 20 years is 

 18.0 grams, and the average percentage deviation for the same 

 values is 1.75 per cent. 



