a human physical type (somatotype) tending toward linearity, as determined by the physique classification system developed by the American psychologist W.H. Sheldon (Sheldon, William). Although classification by the Sheldon system is not absolute, a person is classed as an ectomorph if ectomorphy predominates over endomorphy and mesomorphy in his body build. The extreme ectomorph has a thin face with high forehead and receding chin; narrow chest and abdomen; a narrow heart; rather long, thin arms and legs; little body fat and little muscle; but a large skin surface and a large nervous system. If well fed he does not gain weight easily, and if he becomes fat he is still considered an ectomorph, only overweight. Compare endomorph; mesomorph.