Cardiac muscle. Coloured scanning electron micrograph (SEM) of a freeze-fractured section through healthy heart (cardiac) muscle fibres. The muscle fibres run from left to right, and are made up of numerous myofibrils (not clearly seen). The fibres are crossed by transverse tubules (raised ridges running vertically). These tubules mark the division of the myofibrils (within the fibres) into contractile units known as sarcomeres. Cardiac muscle, mostly under subconscious control, continuously contracts and pumps blood around the body without tiring. Magnification: x4000 at 6x7cm size.