FORM AND GROWTH 133 



cal distance (ordinate) the percentage of the final weight (pre- 

 natal or postnatal) reached at the corresponding age. Thus in 

 prenatal growth the suprarenal glands and brain appear some- 

 what precocious j while the uterus and thymus lag below the 

 body weight curve, rising by more rapid growth in the latter 

 part of the prenatal period. In general, however, the relative 

 growth of the various organs appears more uniform in the 

 prenatal (fetal) period than later. 



In the postnatal period, we may note first the curve for the 

 body weight and splanchnic group (most of the viscera). This 

 curve shows in general an initial steep ascent, corresponding to 

 the rapid growth in infancy. This is followed by a long phase 

 of steady but slower increase during childhood. At the age of 

 puberty, there is a second increase, tapering off more slowly to 

 nearly adult size at twenty years of age. 



Several organs and systems, however, depart more or less 

 widely from this prevailing type of growth, and four extreme 

 cases are represented on the chart. Two of these organs, the 

 brain and the thymus, are precocious in their growth, increasing 

 more rapidly in the earlier part of the postnatal period, and 

 slowing up later. The brain has nearly reached its adult size 

 at six years, and is full-grown at fifteen. The thymus continues 

 to increase steadily until the age of puberty. After this it de- 

 creases, not only relatively but absolutely, so that its adult 

 weight is less than half the maximum at puberty. 



On the other hand, the suprarenal glands undergo after birth 

 an involution with actual loss of half their absolute birth 

 weight. This loss is made good by more rapid growth in late 

 childhood and adolescence. There is also a slighter postnatal 

 loss in weight of the uterus, which remains relatively small 

 during childhood, but grows with extreme rapidity at the age 

 of puberty. 



While there are many variations in the character of postnatal 

 growth among the individual organs, Scammon 43 has grouped 

 them (with a few exceptions) under four types, as shown in 



