Light micrograph of transverse section of a penis. Lightly visible are three subsections. The two larger subsections on center left are the corpora cavernosa, which each surround an artery on each side of the penis. These arteries dilate during sexual arousal, enabling the corpora cavernosa to fill with blood, causing the penis to swell and lengthen. The section at right is the corpus spongiosum, which surrounds the urethra (white). These sections are separated by the septum pectiniforme and surrounded by a fibrous envelope of connective tissue and skin.