At the end of the third month of life the infants are about 9 cm longer than at birth; in the second quarter the length increases S cm and in the third and fourth quarters 3 to 4 cm. each. Thus the total gain in length is about 24 cm. during the first year of life. With regard to the growth in length during the first year (see Fig. 6S, red curve). During the second year boys and girls grow about 10 cm., during the third year 7 to 9 cm., during the fourth year about 5 cm., and at the end of the fourth year the total length reaches about 95 to 100 cm. (see Fig. 6S, blue curve). From this time on the yearly increase in the length of boys is rather constantly 5 cm. until about the thirteenth year; the next three years it rises to 6 to 7 cm., and then drops rapidly. Girls gain about 4 to 5 cm. yearly from the fifth to tho twelfth year; in the thirteenth and fourteenth years the yearly gain increases to 6 to 7 cm., and decreases then rapidly. The growth in length is chiefly finished after the fif teenth year in girls and after the seventeenth year in boys. The latter can be seen particularly well on the side curve of Fig. 71. This curve is from continuous observations on twenty cadets from the fourteenth to the nineteenth year, and shows that after the seventeenth year the length did not increase more than 3 cm. The course of the growth in length is laid down in the following table: At first sight it seems surprising that the growth in height stops at the given times, since usually the litnit is reached in the twenty-third to year. And actually a small increase in length is nearly always observed after the fifteenth to eighteenth year of life, and a ber of boys and girls will even gain considerably in height after this time. But such growth must be regarded as delayed by previous dis turbances. German recruits twenty years of age (Fig. 69) measure on an average 169 cm., while Germans thirty-one years of age measure 169.5 cm. Since a number of recruits (namely, those whose growth was delayed) increase considerably in height after the twentieth year (Fig. 70), there must be many men who do not grow any more after the twentieth year, because the average difference in length between the twentieth and the thirty-first year is only 0.5 em. In other nations, the differences for the given ages are surprisingly small; in England and France, for instance, the difference between recruits and men thirty-five years of age is 1 cm. The investigations of Ammons furnish exact figures for the population of Baden. His statistics include over 30,000 persons fit for service in the army. Among other data he found that the average height of the recruits in Baden is 165 cm.; it is therefore considerably
lower than the average of a large part of the other recruits in Germany. Racial and social conditions are of influence on the growth in height and this is shown in Figs. 69 and 70. Fig. 69 shows that considerable differences in growth exist in the different regions of Germany, and still more pronounced are these differences in comparing different coun tries. In Fig. 70 we see that pupils belonging to well-to-do classes present a better growth in height than do pupils coming from socially lower classes, although it is probable that this difference may change in favor of the latter after the 15th year.
The growth in height like the growth in weight, presents two periods of greater intensity, the one during the first year of life, the sec ond after the twelfth year in girls and after the thirteenth year in boys. The reasons are undoubtedly the same as for the growth in weight. It is of interest that the curve of growth from the second to about the twelfth year is somewhat like a parable From the fact that during the first half year of life the gain in length is as great as that of three to four years together later on, we may obtain sonic information about the strength of the impulse of growth in the first months Of life. The curve of growth in girls crosses that of the boys at about the twelfth to the fourteenth year. This is occasioned by the fact that in girls the second period of growth occurs considerably earlier in boys. After a short time the curve sinks again below that of the boys. It is therefore only from about the twelfth to the fourteenth year that the girls surpass the boys in length for a short time. Figs. 71 and 72 show these conditions graphically.
The length of human beings depends chiefly on the size of theit skeletons, and in close connection to this stands the size of the muscles.
The size of the skeleton and of the muscles must exercise a considera ble influence on the gain in weight of the body, since the combined weight of skeleton and muscles form 40 per cent. of the total weight of the newborn and GO per cent. of that of the adult. The size of the skel eton and muscles has a dominating influence on the growth in length. It was to be expected, therefore, that the curves of the growth in weight and length would follow a similar course. This is actually the case, but a remarkable difference is to be noted, in that the second period of greater intensity of growth occurs somewhat earlier with regard to the length than to the weight, and that the growth in length is finished somewhat earlier than is the growth in weight.