Skeleton and giant's leg, 18th-century illustration. Gigantism (excessive growth) is usually caused by pituitary gland defects causing an excess secretion of growth hormones. This illustration is from 'Osteographia, or the Anatomy of the bones' (London, 1733) by English surgeon and anatomist William Cheselden (1688-1752). It was the first full and accurate description of the anatomy of the human skeletal system.