Blood vessels from cardiac muscle, coloured scanning electron micrograph (SEM). Cardiac muscle is involuntary muscle that must be well supplied with blood for its continual contractions. Interrupted blood supply to cardiac muscle can lead to tissue death and a heart attack. This is a cast: the vessels were filled with a resin, which then set. The surrounding tissues were then eaten away chemically, leaving the cast of the vessels. Magnification: x570 when printed 10cm wide.