What is the penis made of?

The penis is basically three cylinders of spongy erectile tissue full of blood vessels. The urethra, the body's outlet tube for both sperm and urine (although only one at a time), passes through the middle of the smallest of these - the corpus spongiosum - which is found on the underside of the penis. The corpus spongiosum expands at the tip to form the head of the penis called the glans. The glans is protected by the foreskin.