Penis: The external male sex organ. The penis contains two chambers, the corpora cavernosa, which run the length of the organ. These chambers are filled with spongy tissue and surrounded by a membrane called the tunica albuginea. The spongy tissue contains smooth muscles, fibrous tissues, spaces, veins, and arteries. The urethra, which is the channel for urine and ejaculate, runs along the underside of the corpora cavernosa. The urethra emerges at the glans, the rounded tip of the penis.