Whales are not fish, but mammals, and the inside of a whale looks similar to that of a land mammal. They have skeletons with long backbones, circulatory systems similar to warm-blooded animals, large hearts and lungs, large teeth and inner ears that are adapted to hear underwater. To provide buoyancy, store energy and insulate them against cold temperatures, whales have a layer of blubber under their skin.
On the spinal columns of whales, the neck vertebrae are usually fused to provide stability while they are swimming. They have flexible rib cages to prevent their ribs from breaking under the pressure of diving deep. Whales' fore-flippers have five fingers with segmented finger bones. Whales also have vestigial pelvic bones, and some have vestigial back leg and foot bones. To remain buoyant in water, their bones are light and soft. When whales accidentally become beached, their bones struggle to withstand the weight and their internal organs may become crushed.
Whales' lungs and hearts are immense to accommodate the oxygen needed to spend long periods of time underwater. Unlike most mammals, sound passes through a whale's throat into its inner ear. Whales have a skull with special air-filled sinus chambers that amplify their ability to hear underwater. Sperm whales have some of the largest teeth of any animals, whereas baleen whales have plates of material, called keratin, in place of teeth. Though sperm whale heads are immense, their skulls are flattened, and most of their box-like foreheads are taken up by a large organ with a cavity full of a waxy substance called spermaceti.