Instability is a striking characteristic of their bodies in general; for example, man's calcium metabolism is stable; women both retain less calcium salt and eliminate it during menstruation and pregnancy; the ovaries seem to have a catabolic action concerning calcium; this instability leads to disorders in the ovaries and in the thyroid, which is more developed in a woman than in a man: and the irregularity of endocrine secretions acts on the peripheral nervous system; muscles and nerves are not perfectly controlled.