Muscle is a soft tissue found in most animals, and is one of the four basic animal tissues, along with nervous tissue, epithelium, and connective tissue. Muscle cells contain protein filaments called myofilaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion. They are primarily responsible for maintaining and changing posture, locomotion, as well as movement of internal organs, such as the contraction of the heart and the movement of food through the digestive system via peristalsis. <img src="https://upload.wikimedia.org/wikipedia/commons/c/c0/Skeletal_muscle.jpg" width="800"> From [Wikipedia](https://en.wikipedia.org/wiki/Muscle) under [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/) license.