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Heart muscle. Light micrograph of sectioned heart muscle (myocardium) fibres. The fibres are individual cells (blue) which contain contractile proteins. Mitochondria (purple) are seen at the cells' edges, which provide them with energy. Myocardium is not under voluntary control, and contracts to pump blood to the lungs and around the body. The heart beats some 2500 million times in an average human life. Magnification unknown.
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