aracteristic costal type of respiration begins in
girls at the age of ten, for which reason some observers have assumed
that the wearing of the corset is the cause of its appearance; others,
however, among whom Hutchinson may be mentioned, deny this alleged
causal connexion, stating that they have observed costal respiration in
young girls who have never worn any constricting garments.
Unquestionably, sexual differences in the type of respiration become
apparent in the later years of childhood.

I have already pointed out that in girls the secondary sexual characters
begin to make their appearance at an earlier age than in boys. In the
onset of sexual differentiation, the boy thus plays a more passive part
than the girl, inasmuch as he retains longer the childish type. None the
less, in the boy also certain secondary sexual characters begin to
develop comparatively early. Thus, in the second period of childhood,
the boy's shoulders often become wider, his muscles stronger, than those
of the girl. Since at the same period there occurs in girls the greater
deposit of fat previously described, marked differences result in the
external contours of the respective bodies. The boy's body is therefore
much more angular and knobby, far less softly rounded, than that of the
girl. Towards the end of the second period of childhood, an additional
sexual character makes its appearance in the male sex, namely, the voice
breaks. The chief remaining differences, the growth of the beard and
the pubic hair, and the development of the characteristically masculine
larynx, usually manifest themselves after the close of the second period
of childhood--that is to say, during the period of youth.

As children become physically differentiated in respect of sex, so also
does a mental differentiation ensue. Authorities are not agreed as to
whether mental sexual differentiation exists in the very earliest years
of life. Many assume its existence, and profess to have observed sexual
differences even in the movements of quite small children. On the other
hand, it is urged that the alleged differences are made up out of
chance, auto-suggestion on the part of the observer, and the results of
education. There is, however, general agreement as to the fact that
during the second period of childhood mental differences become apparent
between the sexes. Such differences are observed in the matter of
occupation, of games, of movements, and numerous other det