The sternalis muscle is an

anatomical variation An anatomical variation, anatomical variant, or anatomical variability is a presentation of body structure with morphological features different from those that are typically described in the majority of individuals. Anatomical variations are categ ...

that lies in front of the sternal end of the

pectoralis major The pectoralis major () is a thick, fan-shaped or triangular convergent muscle, situated at the chest of the human body. It makes up the bulk of the chest muscles and lies under the breast. Beneath the pectoralis major is the pectoralis minor, a ...

parallel to the margin of the

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sh ...

. The sternalis muscle may be a variation of the pectoralis major or of the

rectus abdominis The rectus abdominis muscle, ( la, straight abdominal) also known as the "abdominal muscle" or simply the "abs", is a paired straight muscle. It is a paired muscle, separated by a midline band of connective tissue called the linea alba. It exte ...

Structure

The sternalis is a

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

that runs along the anterior aspect of the body of the sternum. It lies superficially and parallel to the sternum. Its origin and insertion are variable. The sternalis muscle often originates from the upper part of the sternum and can display varying insertions such as the

pectoral fascia The pectoral fascia is a thin lamina, covering the surface of the pectoralis major, and sending numerous prolongations between its fasciculi: it is attached, in the middle line, to the front of the sternum; above, to the clavicle; laterally and bel ...

, lower ribs,

costal cartilage The costal cartilages are bars of hyaline cartilage that serve to prolong the ribs forward and contribute to the elasticity of the walls of the thorax. Costal cartilage is only found at the anterior ends of the ribs, providing medial extension. ...

rectus sheath The rectus sheath, also called the rectus fascia,. is formed by the aponeuroses of the transverse abdominal and the internal and external oblique muscles. It contains the rectus abdominis and pyramidalis muscles. Structure The rectus sheath ca ...

, aponeurosis of the abdominal external oblique muscle. There is still a great deal of disagreement about its

innervation A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

and its embryonic origin. In a review, it was reported that the muscle was innervated by the external or internal thoracic nerves in 55% of the cases, by the

intercostal nerves The intercostal nerves are part of the somatic nervous system, and arise from the anterior rami of the thoracic spinal nerves from T1 to T11. The intercostal nerves are distributed chiefly to the thoracic pleura and abdominal peritoneum, and diffe ...

in 43% of the cases, while the remaining cases were supplied by both nerves. However, innervation by the pectoral nerves has also been reported. This appears to indicate that the sternalis is not always derived from the same embryonic origin.

Prevalence

Cadaveric studies showed that the sternalis muscle has a mean prevalence of around 7.8% in the population with the range from 0.5% to 23.5%. It has a slightly higher incidence in females. Though, It was proposed that a possible reason for the high prevalence may result from the existence of small, ill-defined or tendinous fibres, which could be misidentified for a sternalis muscle.

Variations

A 2014 study classified the sternalis into three types depending on morphology. * Type I (single head and single belly) * Type II (double-headed/multi-headed) * Type III (double-bellied/multi-bellied) Type I, the single head and single belly was seen in the majority of reported cases (58.5%), type II in 18.1%, and type III in 23.4%. In addition to the above classification, triple-bellied/double-headed sternalis has also been reported.

Function

There is no apparent physiological function of the sternalis muscle. However, there are many theories for a function. It may function as a

proprioceptive Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...

sensor for thoracic wall movements. It may also take part in the movement of the

shoulder joint The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint. ...

or have an additional role in elevation of the chest wall.

Clinical significance

Musculus sternalis rechts - CT sagittal und Volume rendering - 001

The presence of the sternalis is asymptomatic but aesthetic complaints have been reported as it was reported to cause chest

asymmetry Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...

or deviation of the nipple-areola complex. The presence of the sternalis may cause alterations in the electrocardiogram or confusion in mammography. However, there is a potential benefit of the muscle as it can be used as a flap in a

reconstructive surgery Reconstructive surgery is surgery performed to restore normal appearance and function to body parts malformed by a disease or medical condition. Description Reconstructive surgery is a term with training, clinical, and reimbursement implica ...

of the head and neck and the anterior chest wall.

History

The sternalis was first reported by Carbolius in 1604 and the name was first given by Turner in 1867.Turner W (1867) On the musculus sternalis. J Anat Physiol 1(2):246–378 Different terminologies have been given to the sternalis due to its highly varied morphology and the disagreement on its embryonic origin. The sternalis was referred to as the ''rectus sternalis,'' ''sternalis brutorum, musculus sternalis, episternalis, parasternalis, presternalis, rectus sterni, rectus thoracis, rectus thoracicus superficialis, superficial rectus abdominis, japonicas, and thoracicus'' depending on studies''.''

Additional images

Image:PSM_V25_D748_Rectus_sternalis_and_the_great_pectoral_chest_muscle.jpg, The sternalis muscle as it lies on top the pectoralis major. Image:Braus 1921 137.png, Sternalis muscle visible in an individual who also has an accessory abdominal

, and an axillary arch of the latissimus dorsi. Image:Musculus sternalis rechts - CT axial und Volume rendering - 001.jpg, Musculus sternalis seen at axial CT and volume rendering.

References

{{Authority control Accessory muscle