This is a cross section through the human penis.
Can you identify the three cylindrical cavernous bodies (corpora) - a pair of corpora cavernosa, and the corpus spongiosum that surrounds the urethra, ending in the glans penis.
Penile skin moves freely over the underlying tissues due to the loose hypodermis - can you identify this? Unless circumcised, it extends over the glans as the prepuce (foreskin) a retractable protective fold of skin.
Can you identify the tough fibrous sheath, or tunica albuginea that surrounds each of the corpora?
The coprora contain irregular vascular spaces, lined by endothelium. Errection of the penis follows parasympathetic stimulation. Smooth muscle in the thick walled distributing arteries relaxes, and the vascular spaces fill with blood. As the corpora distend, they press against the inextensible tunica albuginea, compressing the veins, so blood is less able to drain away. The corpus spongiosum has a more extensible tunica albuginea, and so becomes less turgid, avoiding compression of the penile urethra.