The penis is made up of two tubes called the corpora cavernosa, which fill up with blood during an erection. The urethra is a tube running from the bladder to the end of the penis, which carries both urine and semen out of the body. The head of the penis (glans penis) is covered by the foreskin in uncircumcised men. In the s, the treatment for syphilis was mercury and urethral sounds, or large metal probes stuck up the pee hole. A few patients developed a fetish for this.