Among the processes of growth that take place at puberty that of the heart is very marked. In the previous years this has remained rel atively small, increasing 6 per cent. to 7 per cent. each year, but at pu berty it grows rapidly and shows an increase as great as 20 per cent. The considerable growth of the lungs is evident from the figures that have been mentioned in regard to the circumference and capacity of the chest.' The growth of the liver and kidneys is also worthy of mention. The brain continues to grow even into the third decade.
The above-mentioned changes in the sexual organs that. serve to indicate puberty, take place inside a period of one to two years. The general bodily development as regards growth and increase in weight occupies a longer time. Upon these relations, important for an estima tion of the physiology, pathology and hygiene of later childhood, we possess a great number of facts which have been obtained through the systematic measurements of some 100,000 school children in the different countries of Europe and America. According to Axel Key the average growth could thus be decided with certainty. From this valuable ma terial the following facts are here quoted : the increase and growth in weight and length exhibit periodic changes and do not follow each other closely; growth in height is very apt to precede increase in weight. In later childhood, before puberty, a period of slow growth and increase in weight occurs; then from the tenth year on girls begin to grow more rapidly and this growth lasts about five years with its maximum in the twelfth year. An increased gain in weight which characterizes the more vigorous time of puberty lasts from the twelfth to the fifteenth year. While growth generally stops at the seventeenth year, marked increase in weight continues as late as the twentieth year. In the case of boys a decided growth lasting four years begins in the fourteenth year and the maximum of this is in the fifteenth year. The weight increases mostly in the sixteenth year. The sixteenth and seventeenth years are the years of greatest development for boys. Boys until the eleventh year exceed girls in length and weight., the latter are larger until the sixteenth year and from that time on drop behind the male sex.
The strength of build is generally dependent upon the develop ment of the chest, whose circumference stands in nearer relation to the weight than to the length. Children who have grown up in poor sur
roundings remain in weight and length constantly behind those of their own age in better circumstances. The period of slow development noticeable in all children before puberty is lengthened in the poor, in these puberty begins later but this stage lasts a shorter time and is ended in the same year as in children of the classes better situated.
Besides the above-mentioned effects of age and of outward circum stances upon the bodily development of the child, the influence of the time of year can in general also be noticed, in so far as during the winter months a slight increase takes place, in spring and summer a. growth in height, often with loss in weight, and finally in the fall decided in crease in weight results. Likewise changes in the outward temperature are operative, in so fat as an increase in temperature at any time of year, even though it lasts only a. few clays, causes an increase of growth and a fall in temperature causes a diminution of growth.
In general, puberty begins in girls of all peoples and climates earlier than in boys. The beginning of the formation of semen takes place in the latter, in the majority of cases, not until the fifteenth year, while complete maturity is generally considered to be reached at eighteen years. The age at which boys become sexually mature and girls men struate for the first time varies under the influence of climate, hered itary predisposition, race, social position, method of life, and individual peculiarity. The higher the mean temperature of the native climate the earlier puberty appears. Menstruation begins in Germany most commonly in the fourteenth or fifteenth year with less frequency be fore the thirteenth or after the eighteenth year. It is earlier in girls living in cities and in better circumstances than in those living in the country and in poorer circumstances. The previous use of alcohol can through the excitation of sexual desire, cause an abnormally early beginning of menstruation and premature sexual intercourse has the same effect. Girls of sanguine temperament, of nervous irritability, of large stature and of strong constitution menstruate earlier than phleg matic individuals or than small or weak girls.