Although Ishmael describes the sperm whale’s brain as being very small—no larger than a “nut,” and very far from the forehead—Ishmael attributes a “nobility” to the spinal column of the whale, which is quite thick, and whose vertebrae extend like “little skulls” all along its back. Ishmael says that the hump of the sperm whale, outlined by the spinal column, gives the whale an “air of indomitability,” and Ishmael then comments that this "air" is in fact true—the sperm whale, especially Moby Dick, is almost impossible to defeat or capture.
Note how Ishmael has over the past few chapters examined the whale from the outside in, from blubber to skull to brain. Ishmael also makes clear to the reader that Moby Dick, and other sperm whales, are not so terrifying because of their native intelligence—which cannot be so great, since their brains are so small. Instead, Moby Dick’s intelligence appears to manifest itself in the whale’s sheer physical strength and size—this is what makes Moby Dick “sublime” and fear-inducing, his whiteness, his strange appearance, and his taste for human blood.