Sentence:  O momentary grace of mortal men, / Which we more hunt for than the grace of God! / Who builds his hopes in air of your good looks, / Lives like a drunken sailor on a mast, / Ready, with every nod, to tumble down / Into the fatal <WSD>bowels</WSD> of the deep. 
Meaning of the word marked with <WSD> tags:  The deepest or innermost part.