Serratus anterior muscle
Encyclopedia
The serratus anterior is a muscle that originates on the surface of the upper eight or nine ribs at the side of the chest and inserts along the entire anterior length of the medial border of the scapula
.
of the arm. As such, the muscle is an antagonist to the rhomboids. However, when the inferior and superior parts act together, they keep the scapula pressed against the thorax together with the rhomboids and therefore these parts also act as synergists to the rhomboids. The inferior part can pull the lower end of the scapula laterally and forward and thus rotates the scapula to make elevation of the arm possible. Additionally, all three parts can lift the ribs when the shoulder girdle is fixed, and thus assist in respiration.
The serratus anterior is occasionally called the "big swing muscle" or "boxer's muscle" because it is largely responsible for the protraction of the scapula — that is, the pulling of the scapula forward and around the rib cage that occurs when someone throws a punch.
The serratus anterior also plays an important role in the upward rotation of the scapula, such as when lifting a weight overhead. It performs this in sync with the upper and lower fibers of the trapezius.
(Nerve of Bell), a branch of the brachial plexus
. The long thoracic nerve travels inferiorly on the surface of the serratus. The nerve is especially vulnerable during certain types of surgery (for example, during lymph node
clearance from the axilla (e.g., in case of axillary dissection in a surgery for breast cancer
)). See winged scapula
for more details.
A 34 year-old man is examined in the prison infirmary after sustaining a superficial stab wound to the superolateral aspect of the thoracic wall at the level of the third rib. There is little bleeding and no difficulty breathing; however, the medial border of the scapula on the injured side pulls away from the body wall when the arm is raised. In addition, the arm cannot be abducted above the horizontal. Which of the following muscles is paralyzed? Answer: Serratus anterior. The long thoracic nerve runs very superficially on the superolateral thoracic wall, where it is prone to injury.
Functional demands has resulted in the evolution of individual muscles from the basal unit formed by the serratus anterior. In primitive life forms, the main function of the axioscapular group is to control the movements of the vertebral border of the scapula: fibers concerned with the dorsal movement of scapula evolved into the rhomboids, those with ventral motion into serratus anterior, and those with cranial movements into levator scapulae. The evolution of the serratus anterior itself has resulted in (1) grouping of its distal and proximal fibers, (2) size reduction of its intermediate fibers, and (3) the insertion of its dominant superior and inferior parts onto the superior and inferior angles of the scapula.
In primate
s, the thoracic cage is wide and the scapula is rotated onto its posterior side to have the glenoid fossa
face laterally. Additionally, the clavicle
take care of medial forces. In cursorial
mammals (for example the horse and other quadrupeds), the scapula is hanging vertically on the side of the thorax and the clavicle is absent. Therefore, in climbing animals, the serratus anterior supports the scapula against the reaction forces of the free limb and exerts high bending forces on the ribs. To sustain these forces, the ribs have a pronounced curvature and are supported by the clavicle. In cursorial animals, the thorax is hanging between the scapulae at the serratus anterior and pectoralis muscles.
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
.
Origin
Serratus anterior normally originates by nine or ten slips from either the first to ninth ribs or the first to eight ribs. Because two slips usually arise from the second rib, the number of slips is greater than the number of ribs from which they originate.Insertion
The muscle is inserted along the medial border of the scapula between the superior and inferior angles. The muscle is divided into three named parts depending on their points of insertions: the superior part is inserted near the superior angle; the intermediate part is inserted along the medial border; and the inferior part is inserted near the inferior angle.Function
All three parts described above pull the scapula forward around the thorax, which is essential for anteversionAnteversion
Anteversion and retroversion are complementary anatomical terms of location, describing the degree to which an anatomical structure is rotated forwards or backwards respectively, relative to some datum position.More specifically, neck anteversion is a neck tilted further forward than normal.A...
of the arm. As such, the muscle is an antagonist to the rhomboids. However, when the inferior and superior parts act together, they keep the scapula pressed against the thorax together with the rhomboids and therefore these parts also act as synergists to the rhomboids. The inferior part can pull the lower end of the scapula laterally and forward and thus rotates the scapula to make elevation of the arm possible. Additionally, all three parts can lift the ribs when the shoulder girdle is fixed, and thus assist in respiration.
The serratus anterior is occasionally called the "big swing muscle" or "boxer's muscle" because it is largely responsible for the protraction of the scapula — that is, the pulling of the scapula forward and around the rib cage that occurs when someone throws a punch.
The serratus anterior also plays an important role in the upward rotation of the scapula, such as when lifting a weight overhead. It performs this in sync with the upper and lower fibers of the trapezius.
Innervation
The serratus anterior is innervated by the long thoracic nerveLong thoracic nerve
The long thoracic nerve supplies the Serratus anterior. This nerve characteristically arises by three roots from the fifth, sixth, and seventh cervical nerves but the root from C7 may be absent...
(Nerve of Bell), a branch of the brachial plexus
Brachial plexus
The brachial plexus is a network of nerve fibers, running from the spine, formed by the ventral rami of the lower four cervical and first thoracic nerve roots...
. The long thoracic nerve travels inferiorly on the surface of the serratus. The nerve is especially vulnerable during certain types of surgery (for example, during lymph node
Lymph node
A lymph node is a small ball or an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells. Lymph nodes are found all through the body, and act as...
clearance from the axilla (e.g., in case of axillary dissection in a surgery for breast cancer
Breast cancer
Breast cancer is cancer originating from breast tissue, most commonly from the inner lining of milk ducts or the lobules that supply the ducts with milk. Cancers originating from ducts are known as ductal carcinomas; those originating from lobules are known as lobular carcinomas...
)). See winged scapula
Winged scapula
A winged scapula is a condition in which the shoulder blade, or shoulder bone, protrudes from a person’s back in an abnormal position. It is a rare condition with the potential to lead to limited functional activity in the upper extremity with which it is adjacent. It can affect a person’s ability...
for more details.
A 34 year-old man is examined in the prison infirmary after sustaining a superficial stab wound to the superolateral aspect of the thoracic wall at the level of the third rib. There is little bleeding and no difficulty breathing; however, the medial border of the scapula on the injured side pulls away from the body wall when the arm is raised. In addition, the arm cannot be abducted above the horizontal. Which of the following muscles is paralyzed? Answer: Serratus anterior. The long thoracic nerve runs very superficially on the superolateral thoracic wall, where it is prone to injury.
Evolutionary variation
The muscles of the shoulder can be categorized into three topographic units: the scapulohumeral, axiohumeral, and axioscapular groups. Serratus anterior forms part of the latter group together with rhomboid major, rhomboid minor, levator scapulae, and trapezius. The trapezius evolved separately, but the other three muscles in this group evolved from the first eight or ten ribs and the transverse processes of the cervical vertebrae (homologous to the ribs).Functional demands has resulted in the evolution of individual muscles from the basal unit formed by the serratus anterior. In primitive life forms, the main function of the axioscapular group is to control the movements of the vertebral border of the scapula: fibers concerned with the dorsal movement of scapula evolved into the rhomboids, those with ventral motion into serratus anterior, and those with cranial movements into levator scapulae. The evolution of the serratus anterior itself has resulted in (1) grouping of its distal and proximal fibers, (2) size reduction of its intermediate fibers, and (3) the insertion of its dominant superior and inferior parts onto the superior and inferior angles of the scapula.
In primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
s, the thoracic cage is wide and the scapula is rotated onto its posterior side to have the glenoid fossa
Glenoid fossa
Glenoid fossa can refer to:* Glenoid cavity * Mandibular fossa...
face laterally. Additionally, the clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
take care of medial forces. In cursorial
Cursorial
Cursorial is a biological term that describes an organism as being adapted specifically to run. It is typically used in conjunction with an animal's feeding habits or another important adaptation. For example, a horse can be considered a "cursorial grazer", while a wolf may be considered a...
mammals (for example the horse and other quadrupeds), the scapula is hanging vertically on the side of the thorax and the clavicle is absent. Therefore, in climbing animals, the serratus anterior supports the scapula against the reaction forces of the free limb and exerts high bending forces on the ribs. To sustain these forces, the ribs have a pronounced curvature and are supported by the clavicle. In cursorial animals, the thorax is hanging between the scapulae at the serratus anterior and pectoralis muscles.
See also
- Pectoralis minor musclePectoralis minor muscleThe pectoralis minor is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major.-Origin and insertion:...
- Serratus posterior inferior muscleSerratus posterior inferior muscleThe Serratus posterior inferior muscle is a muscle of the human body.-Origin and insertion:The muscle lies at the junction of the thoracic and lumbar regions. The origin arises from the vertebrae T11 through L2...
- Serratus posterior superior muscleSerratus posterior superior muscleThe Serratus posterior superior is a thin, quadrilateral muscle, situated at the upper and back part of the thorax.It arises by a thin and broad aponeurosis from the lower part of the ligamentum nuchae, from the spinous processes of the seventh cervical and upper two or three thoracic vertebræ and...