Light micrograph of a longitudinal section through skeletal muscle. A muscle is composed of many fibres (several seen here, pink), containing contractile proteins arranged in smaller structures called myofibrils. The myofibrils lie in parallel along the fibres, and the regularly arranged proteins cause the fibres to have this cross-striated (banded) appearance. The light (I) bands and the dark (A) bands can be seen here as alternating light and dark vertical stripes. Skeletal muscle is responsible for voluntary muscle movement in the body. Across bottom is a capillary (red) embedded in collagen fibres (blue). Magnification: x200 when printed at 15cm wide.