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The sternalis is a muscle that is uncommonly but occasionally found in the human chest. It lies vertically alongside either side of the sternum, or breastbone, and covers the medial fibers of the pectoralis major muscle, those nearest the sternum. Present in less than ten percent of the population, it is more common in women than in men, and certain ethnic populations are more or less likely to demonstrate this anatomical variant as well. Anatomy experts debate its purpose, citing it as a portion of the pectoralis muscle that has been diverted somehow from the main body of muscle. Medically speaking, it is significant in that its presence can interfere with the interpretation of a mammography.
Shaped like a strap, the sternalis is a bilateral muscle, which means one is found on each side of the body. This particular muscle, when present, is situated parallel to the breastbone, which is the long, narrow flat bone that runs down the center of the chest between the ribs. Similar in length and width to the sternum, the sternalis sits immediately to either side of that bone superficial to or on top of the pectoralis major. It typically originates on the underside of the clavicle, or collarbone, as well as on the manubrium or head of the sternum.
The insertion point, or the surface to which the lower end of the sternalis attaches, can be less predictable. This muscle is said to insert anywhere from the fascia or sheath surrounding the pectoralis major muscle to the ribcage below the sternum to the fascia of the abdominal muscles. It can thus be several inches in length or almost nonexistent, only possessing a small number of fibers. In addition, it can be unilateral, with the muscle only present on one side of the body.
This anatomical variant is not well understood by medical experts, who debate its purpose. On the front aspect of the torso, there are several other longitudinal muscles running along the midline of the body, such as the rectus abdominus, the muscle commonly referred to as the “six-pack.” Such a muscle is designed to flex forward the part of the body it spans, as in performing an abdominal crunch.
The thoracic spine, however, does not allow much forward flexion. Instead, the shoulder blades move forward in a motion called protraction. As such, there is no need for a longitudinal muscle in the chest wall.
One concern that experts have expressed over the presence of the sternalis is that while anatomists are familiar with this muscle, medical professionals may not be. Therefore, they may misidentify it during routine procedures or mistake it for foreign tissue. During mammography, for instance, the practice of imaging the female breast to identify potentially cancerous lumps, the sternalis may be mistaken for an anomaly in the chest wall and therefore contribute to a misdiagnosis.