What does "sack" mean in "so that he'll fill the sacks along his backbone with air" in The Old Man and the Sea?
The full sentence is "Now that it is daylight let him jump so that he'll fill the sacks along his backbone with air and then he cannot go deep to die."
I looked up "sack" in the dictionary, but I cannot find any meaning of it that relates to backbones of a fish.
The quote is in reference to the swim bladder of a fish. Bony fish contain this important organ. A swim bladder is an organ that is usually a pair of gas-filled sacs, used to maintain buoyancy in a fish. It can allow the fish to float higher in the water or to sink lower in the water without having to expend a lot of energy due to swimming. It works by filling with gas causing the density of the fish to decrease and allowing it to float or to remove gas, making the fish more dense and causing it to sink. The swim bladder is located near the backbone of the fish. In the Old Man and the Sea, the fisherman is hoping that if the swim bladder fills with air, if it dies, it will die near the surface and not sink to the bottom of the sea.