Why is there a line that rides up the middle of my scrotum?
The ridge up the middle of the scrotum is a seam that formed before you were born. It is called the raphe and actually extends from your anus and up the bottom side of your penis. It comes because two half of a sheet of cells came together and fused, inclosing your penis (leaving a slit at the end) and forming your scrotum covering your testicles.
Click here for a labeled image that shows the raphe.