Is there any significance to Ahab having a leg of whalebone?
In Herman Melville's classic Moby Dick, Captain Ahab wears a prosthetic leg made out of whalebone ivory because the whale Moby Dick destroyed Ahab's leg in an earlier incident. Then, in Chapter 106, Ahab injures his ivory leg when being lowered into a boat. Melville writes of an earlier episode before the sailing of the Pequod: "his ivory limb having been so violently displaced, that it had stake-wise smitten, and all but pierced his groin." In this violent incident, Ahab's ivory leg nearly causes his own destruction, as it almost pierces his groin. After the incident in which he injures himself being lowered into the boat, he instructs the carpenter on the Pequod to make another leg made out of a whale's jaw. Ahab's whalebone leg is a constant reminder of his vendetta against Moby Dick.
Ahab's former injury and his memory of what Moby Dick did to him forever cause him to suffer. As Melville writes in Chapter 106, "Nor, at the time, had it failed to enter his monomaniac mind, that all the anguish of that then present suffering was but the direct issue of former woe." In other words, the monomaniacal Ahab, who always has Moby Dick on his mind, has a constant reminder of how much Moby Dick has caused him to suffer. Ahab's leg is a continual source of distress, and his pain makes him obsessed about killing Moby Dick.
Well, Moby Dick is a book that, among other things, is about vengeance, not just against a whale, but against God and the universe. The whalebone of Ahab's leg has many meanings, but ultimately becomes symbolic of that struggle.
For one thing, Ahab lost his leg to Moby Dick in a previous attack ("devoured, chewed up, crunched by the monstrousest parmacetty that ever chipped a boat!” says Capt. Peleg, ch 16) so on a basic level the missing leg is a reason for Ahab to seek vengeance. The fact that it is made of whalebone marks Ahab as a fearsome character ("So powerfully did the whole grim aspect of Ahab affect me, and the livid brand which streaked it, that for the first few moments I hardly noted that not a little of this overbearing grimness was owing to the barbaric white leg upon which he partly stood," Ishmael says in ch 28). Part of this "barbaric" quality no doubt has to do with the whalebone as a kind of trophy, evidence of Ahab's skill in killing whales. The whalebone leg evokes a skeleton leg, the whalebone replacing the human bone. The sound of Ahab's leg on the deck "sounded like a coffin-tap," (ch 51) a kind of prediction of death, either Moby Dick's, or Ahab's and his crew's.
There is also meaning in the fact that whalebone does not seem to be a very good material for a prosthetic limb -- Ahab breaks it twice, once before the voyage, when he was found in a Nantucket street, "his ivory limb having been so violently displaced, that it had stake-wise smitten, and all but pierced his groin," and a second time, when, returning to the Pequod from a meeting on a passing whaler, he "had lighted with such energy upon a thwart of his boat that his ivory leg had received a half-splintering shock." These breakages seem to suggest that the whalebone leg and Ahab are at odds: the leg effectively castrates Ahab when it breaks in Nantucket. Ahab realizes that "all the anguish of that then present suffering was but the direct issue of a former woe” -- in other words, his previous encounter with Moby Dick is the source for all his current trouble -- and that Ahab's hatred, and by extension all the sorrow in the world, -- not love -- is evidence of our connection to God ("The ineffaceable, sad birth-mark in the brow of man [e.g., man's mortal condition], is but the stamp of sorrow in the signers [e.g., the creators, or God].” (all ch 106).
This theme is brought into focus in ch 108, in which Ahab discusses his leg with the ship's carpenter, who is making a new one. Ahab asks the carpenter how it is that, although he will have a new whalebone leg, he will also feel his old flesh and blood leg in the same place? ("...when I come to mount this leg thou makest, I shall nevertheless feel another leg in the same identical place with it; that is, carpenter, my old lost leg; the flesh and blood one, I mean. Canst thou not drive that old Adam away?") The whalebone, representative of his hatred, is doubled by the phantom sensation of his old leg, a remnant of Ahab's humanity; this doubleness is itself representative of the root of Ahab's hatred -- the notion that reality itself is a lie. Ahab goes on to ask the carpenter "How dost thou know that some entire, living, thinking thing may not be invisibly and uninterpenetratingly standing precisely where thou now standest; aye, and standing there in thy spite?" The carpenter, of course, has no idea what Ahab is talking about, and Ahab laments that his cosmic quest for vengeance is dependent on mundane details like a sound false leg ("Here I am, proud as Greek god, and yet standing debtor to this blockhead for a bone to stand on!").
Ahab is a person who, to carry out his thirst for vengeance, must somehow transcend his human body. One gets the sense that if he had the chance, Ahab would have the carpenter remake him entirely in whalebone! But Ahab's problem is that his thirst for vengeance is directly linked to his humanity. His whalebone leg may represent Ahab's having "one foot" in the transcendent, but the rest of his body is firmly in the real.