Penis. Light micrograph (LM) of a section through a penis, showing the corpora cavernosa. 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.