Penis Anatomy

When a man ejaculates, semen spurts from the opening of the urethra out the tip of his penis. The root of the penis is attached to the pelvic bone. Inside are three cylindrical columns of tissue, abundantly supplied with blood. The lower cylinder, called the corpus spongiosum, is made of soft fibers that cushion the urethra. The two upper cylinders, which lie side by side, are made of erectile tissue. These are the corpora cavernosa. Through the veins of these cylinders, blood circulates continuously. When the penis is stimulated, the outflow veins that normally drain away blood become blocked, and the corpora cavernosa fill with blood. The penis becomes hard and warm, and lengthens and expands.