ening of the
thigh and leg bones. In serial homology with the thigh and leg are the
bones of the arm and we find that these are undergoing an increase in
length commensurate with the increase of the legs. So the boy outgrows
his clothes; his coat sleeves are drawn up half way to his elbows and
his trousers half way to his knees. The muscles scarcely keep pace
with the bones in their growth, and tend to be flabby and to lack
usual tonicity. It is difficult for the youth to hold his back
straight and his shoulders back; he is awkward and ungainly in his
movements and becomes easily fatigued because of the condition of his
muscles. But the muscles follow immediately in their development and
rapidly gain volume and tonicity, filling out the arms, legs, back and
shoulders with large masses of firm muscular tissue. The growth of
these muscle masses changes the dimensions of the youth and he fills
out in his girths as rapidly as, in the previous period, he increased
in length measurements.

All of this increase in bulk can only be accomplished by increased
activity of all the nutritive processes. The appetite is practically
insatiable; the boy can eat three square meals in the day and lunches
between meals. If he wakes up in the night he is hungry. To accomplish
the digestion and absorption of this food material, the alimentary
tract throughout, and particularly the stomach is greatly increased in
size. To accomplish the distribution of the food (blood) the heart
also is increased in size and strength. With increased bulk of muscle
and increased quantity of food we have increased oxidation in the
tissues. This requires increased respiration, which demand is
satisfied by rapid development of the respiratory system. The thorax
increases in dimensions in all directions; it becomes deeper, broader
and longer. Not only does the thorax become more capacious but also
more mobile and more responsive to the varying requirements of the
system.

If we are interested in the biology of all these changes, we need not
go far to discover the natural causes at work to produce them. Nature
is preparing in the youth a home builder; it is preparing an
individual who can support and protect not only himself, but also a
family. This equipment in the case of primitive man must necessarily
be one of bone and brawn. While under the conditions of modern society
the necessity for bone and brawn is somewhat less marked, the plan of
nature is no less e