Why do cats lick when their spines are scratched?

I have two cats that I live with (one is mine, the other my roommate's,
though they are siblings). When I scratch on either one's spine, they
seem to feel a desperate need to lick something, including the air, my
hand, themselves, or a random nearby object.

Does anyone know why they do this and if there's any evolutionary
reason for it?
--
--
alm

Weblog: alexloganmartin.wordpress.com