Countless animals, including us, bear external structures without blood supply or nerves -- for example, our hair and fingernails, or the scales, claws, and horns of other animals.

• 包括人在内的无数动物都有一些没有血液供应和神经的体外构造,例如,我们的头发和指甲,或者鳞片、爪子和其它一些动物的角。