half of the body begins to resemble that of the woman in type.
During the second period of childhood, this peculiarity becomes more
marked; the pelvis and the hips widen, the thighs and the buttocks
become more and more rounded; the enduring feminine characteristics in
these respects are acquired. More gradually, the feminine development of
the upper half of the body succeeds that of the lower; the transition
from the lower jaw to the neck become less abrupt, and the face becomes
fuller. The sexual difference in the growth of the hair also manifests
itself in childhood. Whether cut or uncut, the girl's hair tends to grow
longer than the boy's. Later, the typical development of the breasts
occurs. As early as the beginning of the second period of childhood, the
surface of the areola mammae may become slightly raised; but the typical
deposit of fat, leading to the hemispherical prominence of the breast,
does not begin until towards the close of the second period of
childhood. Even later than this is the growth of the axillary and pubic
hair. Various answers are given to the question as to the relation in
time between the appearance of menstruation and the development of the
sexual characters just described. Unquestionably there are great
differences in this respect. Whereas Axel Key declared that the
secondary sexual characters appeared before the first menstruation,
according to C. H. Stratz this is true only of girls belonging to the
lower classes; whilst according to his own observations on girls
belonging to the upper classes of society, the first menstruation
precedes the development of the breasts and the growth of the pubic and
axillary hair.

Concerning a number of sexual differences, during childhood, authors are
not agreed. As regards the type of breathing, for instance, in the adult
man, the abdominal type prevails; that is, the respiratory exchange of
gases is effected chiefly by movements of the diaphragm and the
abdominal muscles: whereas in the adult woman the respiration is costal,
the respiratory exchange being effected chiefly by movements of the
thorax. How unsettled our views are in respect of the types of
respiration in children is well displayed by the collection of opinions
given by Havelock Ellis.[14] According to Boerhaave, sexual differences
in the type of respiration were manifest even in very small children;
but his observations have not been confirmed by others. Thus, Sibson
states that the ch