Those members of a man which may be useful to him in flight, and the unlawful deprivation of which by another amounts to a mayhem at common law. 1 Bl. Com. 130. If a man, se defendendo, commit homicide, he will be excused; and if he enter into an apparent contract, under a well-grounded apprehension of losing his life or limbs, he may afterwards avoid it. 1 Bl. 130.