Men go bald on the front of the head, but turn grey
first on the temples; no one goes bald first on these or on the back
of the head. Some such affections occur in a corresponding manner also
in all animals which have not hair but something analogous to it, as
the feathers of birds and scales in the class of fish.
For what purpose Nature has made hair in general for animals has
been previously stated in the work dealing with the causes of the
parts of animals; it is the business of the present inquiry to show
under what circumstances and for what necessary causes each particular
kind of hair occurs. The principal cause then of thickness and
thinness is the skin, for this is thick in some animals and thin in
others, rare in some and dense in others. The different quality of the
included moisture is also a helping cause, for in some animals this is
greasy and in others watery. For generally speaking the substratum
of the skin is of an earthy nature; being on the surface of the body
it becomes solid and earthy as the moisture evaporates. Now the
hairs or their analogue are not formed out of the flesh but out of the
skin moisture evaporating and exhaling in them, and therefore thick
hairs arise from a thick skin and thin from thin. If then the skin
is rarer and thicker, the hairs are thick because of the quantity of
earthy matter and the size of the pores, but if it is denser they
are thin because of the narrowness of the pores.
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