Musannaf Ibn Abi Shayba 18:361

Abu Bakr narrated to us, he said: Muhammad ibn Bakr narrated to us, from Ibn Jurayj, from ‘Amr ibn Shu‘ayb, who said: For every limb there is a *ja’ifah* (a wound reaching the body cavity), and every bone is *ajyaf* (capable of being reached by a wound). So its *ja’ifah* is according to the reckoning of that bone.