In all animals the skin is one of
the parts that extends continuous and unbroken, and it comes to a stop
only where the natural ducts pour out their contents, and at the mouth
and nails.
All sanguineous animals, then, have skin; but not all such animals
have hair, save only under the circumstances described above. The hair
changes its colour as animals grow old, and in man it turns white or
grey. With animals, in general, the change takes place, but not very
obviously, or not so obviously as in the case of the horse. Hair turns
grey from the point backwards to the roots. But, in the majority of
cases, grey hairs are white from the beginning; and this is a proof
that greyness of hair does not, as some believe to be the case,
imply withering or decrepitude, for no part is brought into
existence in a withered or decrepit condition.
In the eruptive malady called the white-sickness all the hairs get
grey; and instances have been known where the hair became grey while
the patients were ill of the malady, whereas the grey hairs shed off
and black ones replaced them on their recovery. (Hair is more apt to
turn grey when it is kept covered than when exposed to the action of
the outer air.
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