It shall devour the strength of his skin; literally, the bars of his skin, by which some understand "the muscles," some "the members," of his body. The general meaning is plain, that destruction shall always be close to him, and shall ultimately make him its own.
Even the firstborn of death shall devour his strength. By "the firstborn of death" is probably intended, either some wasting disease generally, or perhaps the special disease from which Job is suffering.