By Elizabeth A. Murray, Mount St. Joseph University
Several muscles on the anterior and lateral chest are also upper limb muscles—specifically, the pectoralis major and minor, the serratus anterior, and the subclavius. These aren’t designed to move the bones of the thorax; rather, they move the arm or the pectoral girdle—either the scapula or clavicle.
Pectoralis Muscles
The pectoralis major muscles—often called the pecs—take origin from the sternum and clavicle and insert on the anterior humerus. Because the pectoralis major is located anteriorly, it flexes the humerus; also, because it crosses the axilla, pectoralis major adducts the upper limb, pulling it toward the body.
And, lastly, since it attaches to the anterior side of the proximal humerus, the pectoralis major acts in medial or inward rotation, rolling the head of the humerus toward the body.
The pectoralis minor, on the other hand, originates entirely on the axial skeleton—typically on ribs 3, 4, and 5 and inserts on the coracoid process of the scapula. The pectoralis minor mainly acts to stabilize and depress the scapula. So, both pectoralis muscles act on the upper limb; their actions are not on the chest itself.
Serratus Anterior
The serratus anterior is a muscle of the lateral chest wall, and as its name implies, it has a serrated, or jagged, origin—mostly off of the true ribs. It must be remembered that those are the first seven ribs. It inserts on the deep and medial border of the scapula and is supplied by the long thoracic nerve off the brachial plexus.
As an antagonist to the rhomboids—which retract the scapulae, drawing them together—the serratus anterior pulls the scapulae apart—often called protraction, or occasionally termed abduction. It also holds the scapula to the rib cage and can rotate the scapula to allow the upper limb to reach past the horizontal plane.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Subclavius
The subclavius is a small muscle that helps hold the clavicle to the first rib and gets its own tiny branch of the brachial plexus as its nerve supply. The ventral rami have two possible fates: they can assume a simple pattern in the thorax, as intercostal nerves traveling between ribs, or they can braid into a plexus, such as the cervical or brachial plexus.
Because the pectoralis major, pectoralis minor, serratus anterior, and subclavius are all upper limb muscles, they are supplied by branches of the brachial plexus.
Thoracic Muscles
The thoracic muscles are the innermost complex of chest muscles. This deepest layer really consists of three subcomponents.
On the deep side of the anterior rib cage is the transversus thoracis muscle. It’s an interesting, almost star-shaped muscle that attaches the deep side of some anterior ribs to the posterior side of the sternum. The transversus thoracis is said to aid deep expiration.
Then, laterally are the innermost intercostals, which have roughly the same fiber direction and function as the internal intercostals.
The third member of the innermost complex is found on the deep side of the ribs posteriorly; these are called the subcostales or subcostal muscles. These posterior muscles are still inside the rib cage, but unlike the innermost intercostal muscles, the subcostales span two ribs, not just one.
There’s also a group of muscles called the levator costarum, which are on the posterior side of the ribs—deep to the erector spinae, but lateral to the transverse processes. They are supplied by dorsal rami, yet as their name implies, can also elevate the ribs to aid deep inspiration.
Common Questions about Muscles of the Chest and the Thorax
The subclavius is a small muscle that helps hold the clavicle to the first rib and gets its own tiny branch of the brachial plexus as its nerve supply. The ventral rami have two possible fates: they can assume a simple pattern in the thorax, as intercostal nerves traveling between ribs, or they can braid into a plexus, such as the cervical or brachial plexus.
The pectoralis major muscles—often called the pecs—take origin from the sternum and clavicle and insert on the anterior humerus. Because the pectoralis major is located anteriorly, it flexes the humerus; also, because it crosses the axilla, pectoralis major adducts the upper limb, pulling it toward the body. Since it attaches to the anterior side of the proximal humerus, the pectoralis major acts in medial or inward rotation, rolling the head of the humerus toward the body. The pectoralis minor, on the other hand, originates entirely on the axial skeleton—typically on ribs 3, 4, and 5 and inserts on the coracoid process of the scapula. The pectoralis minor mainly acts to stabilize and depress the scapula. So, both pectoralis muscles act on the upper limb; their actions are not on the chest itself.
There are three subcomponents of the thoracic muscles. On the deep side of the anterior rib cage is the transversus thoracis muscle. It’s an interesting, almost star-shaped muscle that attaches the deep side of some anterior ribs to the posterior side of the sternum. The transversus thoracis is said to aid deep expiration. Then, laterally are the innermost intercostals, which have roughly the same fiber direction and function as the internal intercostals. The third member of the innermost complex is found on the deep side of the ribs posteriorly; these are called the subcostales or subcostal muscles.
