The axes indicate the direction of the volumes of the body. The body volumes themselves are created for the most part by the muscles and bones, and the separate body volumes define the various regions of the body (head, chest, forearm). Individual muscles often group together with muscles of similar function to create masses that attach to, surround, and are supported by the skeleton. As an animal changes position, the body volumes are redirected, and the shapes of the individual volumes are subject to change—a muscle or group of muscles becomes thinner as it is stretched and elongated, and thicker and more massive when it shortens.
The volumes of the head and chest are basically determined by the skeleton, and are covered by relatively thin to medium-thickness muscles.
An important body volume not created by bone or muscle, and therefore one that is highly variable, is the abdominal volume. This region contains the intestines and other soft abdominal organs held in place by a sling created by the thin abdominal muscles and their wide tendinous sheets. When the spine is flexed and the rib cage and pelvis approach one another, the abdomen shortens, becomes compressed, and bulges. When the spine is extended, straightening the body, the rib cage and pelvis move apart and the abdomen is stretched and narrowed.
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