Penis section. Coloured light micrograph of a section through a penis, showing three subsections. The two larger subsections at centre left and right 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 lower centre 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 (pink) Magnification: x8 when printed 10cm wide.