of all mammals        the human male
is among the few that has no
bone in the *****;       the coccyx
still functions as a vestigial         tail;

there are active parts of the brain
for which doctors  &    biologists
can find no known function;    when

women evolved the *******,    whose
sole purpose is to activate the libido;  
men lost the bone in their *****
thus ending their perpetual hard-ons