Anatomy of the Doctor: Part 1

      Although he looks like a human being (and once claimed human ancestry on his mother's side), the Doctor is an alien being and his Time Lord physiology has many differences from our own. Physically, the Doctor is stronger, has sharper senses, and greater powers of endurance than a human being, and can cope with heat, cold, radiation and powerful forces better than humans can. There are also some differences to his internal anatomy that have confused many Earthly doctors.

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     When the Doctor's right hand is severed by a Sycorax warrior, residual regeneration energy allows him to grow another. His friend Captain Jack acquires the original, and the liquid bubbles whenever the Doctor or a product of his DNA is near. Later, after a run-in with a Dalek triggers the Doctor's regeneration process, he blasts his regeneration energy into his severed hand, healing himself without having to change appearance - but effectively using up one whole regeneration.