Job 18:13

He calls the “soles of their feet” the products of their journeys, in order to say that the impious are not prosperous in their travels when they fall into “a violent ruin.” But also their fruits of season, that is, their children, are insatiately consumed by death; and from this entire condition, in which they find themselves, all healing withdraws. That is, their results will always be incorrigible and incurable. It is also possible to interpret the “soles of their feet” as their offspring or descendants.

Let it devour the beauty of his skin; and let the firstborn death consume his arms.

'The beauty of his skin' is temporal glory, which whereas it is coveted as an object without us, is retained as a beauty on the skin. But by the title of 'arms' works are not unfitly set forth, in that the work of the body is done by the arms. And what is death but sin, which kills the soul to the interior life? Whence it is written; Blessed and holy is he that hath part in the first resurrection, in that he shall hereafter rise again joyfully in the flesh, who whilst set in this life has risen again from the death of his soul. If then sin is death, 'the firstborn death' may not unsuitably be taken for pride; in that it is written, Pride is the beginning of all sin. And so 'the beauty of his skin and his arms the firstborn death devoureth,' in that the glory or the practice of the bad man is overthrown by Pride. For he might have been glorious even in this life without sin, if be had not been proud. He might in the judgment of His Creator have been commended for some works, if before His eyes pride had not overturned those very works. Thus we often see rich people, which might have had wealth and glory without guilt, if they would have had them with humility. But they are uplifted by possessions, they are flushed with honours, they disdain the rest of the world, and place their life's whole hope and trust in the mere abundance of good things alone. Hence a certain rich man said, Soul, thou hast much good laid up for many years; take thine ease, eat, drink, and be merry. Which thoughts of their hearts when the Judge above beholds, He plucks them away for this very confidence of theirs by itself.

The goods of the body which hunger begins to weaken are totally consumed by death. The principal goods of the body are beauty and strength, and so he then says, "may his skin lose its beauty," because beauty regards exterior appearance, and "may the arms of that man," in which strength is especially found, "be consumed by a premature," i.e. early, "death", before the end of the natural span of life.