| Age | Weight of body in grams | Increase |
|---|---|---|
| At birth | 3000 | — |
| 1 month | 3750 | 750 |
| 2 months | 4450 | 700 |
| 3 months | 5100 | 650 |
| 4 months | 5700 | 600 |
| 5 months | 6250 | 550 |
| 6 months | 6750 | 500 |
| 7 months | 7200 | 450 |
| 8 months | 7600 | 400 |
| 9 months | 8000 | 400 |
| 10 months | 8350 | 350 |
| 11 months | 8700 | 350 |
| 12 months | 9000 | 300 |
But these figures give no idea of the laws of growth that govern each separate organ, and that have been studied by Vierordt. According to this authority, the total weight of the body increases nineteen-fold from birth to complete development. Certain ductless glands, on the contrary, diminish in weight in the course of growth; the thymus, for instance, is reduced to half what it weighed originally.
Furthermore, the various organs all differ in such varying degrees, as compared with their respective weights at birth, that it facilitates comparison to reduce the weight of each separate organ to a scale of 1. On this basis we find that when complete development is attained, the eyes weigh 1.7; the brain 3.7; the medulla oblongata (spinal marrow) 7; the liver 13; the heart 15; the spleen 18; the intestines, stomach and lungs 20; the skeleton 26; the system of striped muscles 48.
And these widely different augmentations are not uniform in their progress, nor is the complete development of each organ attained at the same epoch. As a matter of fact, the brain acquires one-half its final weight at the end of the first year of age; the organs of vegetative life attain half their weight at the beginning of the period preceding puberty (eleventh year). To offset the lack of indications regarding such increases in weight, we have a guide in the morphology of growth, which reveals how differently the various parts of the body develop.
However empirical it may be from an analytical point of view, the datum of weight is a valuable index, and represents, taken by itself, a synthetic anthropological measure of prime importance.
It obeys certain laws of growth which are themselves of great interest; namely, there exist two periods of rapid growth: at birth and during puberty; while at various periods in childhood, between the ages of three and nine, there are alternations of greater and lesser growth analogous to those already noted in relation to stature.
Accordingly, the weight confirms the fact that the organism does not proceed uniformly in its evolution, but passes through crises of development during which the forces of the organism are all devoted to its rapid transformation; such periods represent epochs at which the organism is more predisposed to maladies, more subject to mortality and less capable of performing work (compare the observations already made in relation to stature).
Index of Weight.—Accordingly, weight and stature stand in a certain mutual relationship, but the correspondence between them is not perfect. In the study of individual physiological development it is necessary to know the anthropological relation between weight and stature; in other words, the ponderal index. Without this, we cannot get a true idea of the weight of an individual. For instance, if two persons have the same weight, 65 kilograms for example, and one of them has a stature of 1.85 metres and the other of 1.55 m.; it is evident that the first of these two will be very thin, because his weight is insufficient, while the second, on the contrary, will have an excessive weight.