less active from its beginning in the ovum until the adult type of the
species is attained. As determined by the volume, the embryo increases
more than ten thousand times in size during the first month of
intra-uterine life. At birth the average weight is six and a half
pounds; at the end of the first year eighteen and a half pounds, a
gain of twelve pounds; at the end of the second year twenty-three
pounds, a gain of four and a half pounds. The growth is coördinated,
the size of the single organs bearing a definite ratio, which varies
within slight limits, to the size of the body, a large individual
having organs of corresponding size. Knowing that the capacity of
growth is one of the inherent properties of living matter, it is much
easier to understand the continuance of growth than its cessation. It
is impossible to avoid the conclusion that there is some internal
mechanism of the body which controls and regulates growth. In the
first chapter reference was made to organs producing substances which
pass directly into the circulation; these substances act by control of
the activities of other parts, stimulating or depressing or altering
their function. Two of these glands, the thymus, lying in front, where
the neck joins the body and which attains its greatest size at
puberty, and the pituitary body, placed beneath the brain but forming
no part of it, have been shown by recent investigations to have a very
definite relation to growth, especially the growth of the skeleton.
The growth energy chiefly resides in the skeleton, and if the growing
animal has a diet sufficient only to maintain the body weight, the
skeleton will continue to grow at the expense of the other tissues,
literally living upon the rest of the body. Disease of the glands
mentioned leading to an increase or diminution or alteration of their
function may not only inhibit or unduly increase the growth of the
skeleton, but may also interfere with the sexual development which
accompanies the skeleton growth.