Final answer:
The externally visible ridge along the midline of the scrotum is called the raphe.
Step-by-step explanation:
The ridge that is visible externally along the midline of the scrotum is called the raphe. The scrotum is a pouch-like external structure of the male reproductive system that contains the testes, each of which is covered by protective layers, including the tunica albuginea. The dartos muscle, a subcutaneous muscle of the scrotum, forms the scrotal septum internally to divide the scrotum. The external ridge along the midline of the scrotum is known as the raphe, distinguishing it from other structures like the tunica albuginea, which covers the testis itself.