Blood-filled subcutaneous vein in the scalp. Coloured scanning electron micrograph (SEM) of a section through a vein (pink) in the scalp, showing the red blood cells (erythrocytes, red) inside. The scalp receives a rich arterial supply via the external carotid artery and the ophthalmic artery (a branch of the internal carotid). Underlying bone of the skull is in yellow. Magnification: x800 when printed at 10 centimetres wide.