There are several muscles on the back which do not appear immediately under the skin. For ·instance, there is a pair of long muscles that run along each side of the spine called the erector-spine muscles. Their function is to straighten the back and control the movements of the spinal column. These muscles are round and thick, but for almost their full length they are overlaid by other muscles.
From the waist upwards to the middle of the back these muscles are covered by the thin fibers of the latissimus-dorsi muscle.