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 characteristic 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.