Sentence:  And it is a hard and cruel thing thus in early youth to taste beforehand the pangs which should be reserved for the stout time of manhood, when the <WSD>gristle</WSD> has become bone, and we stand up and fight out our lives, as a thing tried before and foreseen; for then we are veterans used to sieges and battles, and not green recruits, recoiling at the first shock of the encounter. 
Meaning of the word marked with <WSD> tags:  Bone not yet hardened by age and hard work.