"or a dead body or the bone of a man": Just as a dead body — flesh, sinews, and bones, so, a limb (cut off) from a living person, flesh, sinew, and bones, as in his natural state.
"or a grave": This refers to a closed grave (i.e., one in which there is less than a tefach between the body and the lid). — But perhaps it refers to an open grave? Would you say that? It follows (otherwise), viz.: If a tent, which is susceptible of tumah, does not confer tumah on all of its sides when it is open, how much more so, a grave, which is not susceptible of tumah! — But in that case, why do we not say that just as an open tent confers ("until) evening" tumah (when touched from the back), so, here, an open grave confers ("until) evening" tumah? Would you say that? Whence do we derive this for a tent? From an a fortiori argument (as above), and would you now come to derive something which is itself derived elsewhere? A derivation from a derivation?