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Ishmael returns to the sperm whale’s head to draw the reader’s attention to something else that may be significant later: in the very front of its face, the head broad and flat, boneless, without any of its important organs.
The blubber is harder and more resilient, leading Ishmael to conclude that the front of the sperm whale’s head is designed to be an enormous battering ram, without too many nerve endings.
This "dead, impregnable, uninjurable wall" comes between the whale and anything else (76.4).