There are probably no conditions which contribute more to physical inferiority in man than defects in skeletal development. Most important among these is the result of a syndrome designated as rickets. This disease varies in severity, and individual cases present, therefore, pictures which differ in some details, but all cases have certain characteristics in common. Rickets leads to deformity, due to abnormal enlargement of the ends of the bones, and to distortion due to bending owing to lack of resistance of the bones to the body weight, to muscular tension and to atmospheric pressure. The latter factor is especially important in changing the form of the thorax. Bow legs, knock knees, enlarged joints, flat or deformed chest and abnormal conformation of the skull, are the result of the failure of the bones to develop in a normal manner.

Rickets is essentially a disease of infancy and early childhood, although it may under exceptional conditions appear in children of five or six years or even later (rachitis tarda). Since the war this "late rickets" has become very common in the children of central Europe. The disease may be manifest by the second month of life. It is most frequent from the seventh month to the end of the second year. It is usually accepted as true that the disease is never or only very rarely present at birth, but since clinically recognizable symptoms may occur very early in life, and the development of the disease is slow, it must in many children have its beginning in the earliest days of extrauterine life.