What is the deformity down the middle of my penis?
Answers: It's a scar
your urethra? It is your "middle line"
There is a medical permanent status for it which I have forgotten, but essentially to be exact a scar tissue that formed when you be a fetus and your sex determination was surrounded by development.
If you turned within to a girl, the tissue that is presently your penis would have be used to form a vagina.
In fact women also hold a middle line, but we seldom can see it.
It is inside the vagina and may be noticeable above her Venus mound when she is pregnant because it darkens.
the industrial term for it is 'raphe', pronounced rafe.