The complexity of the musculoskeletal system is often a big issue for medical students. To learn about muscles effectively, a clear and logical grouping into systems with unique structures is needed. In this article, in addition to detailed descriptions of the origin, insertion, action and innervation, medical students will obtain an overview in tabular form for visualization of the muscle groups. Attention: Due to conflicting information in primary literature about the muscles' origins and insertions, it is advisable to always consult the latest copies for studying.
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pectoralis major left anterior lateral view

Image: “Pectoralis major (left anterior lateral view) – überarbeitet.” by Phil Schatz. Licence: CC BY 4.0


Muscles of the Thorax

In human anatomy, the chest wall muscles are divided into primary and secondary layers. While the primary chest wall muscles specifically include the intercostal region, the secondary, immigrated thoracic muscles correlate with the anterior shoulder and chest muscles.

Primary chest wall muscles

Thorax

Image: “The external intercostals are located laterally on the sides of the body. The internal intercostals are located medially near the sternum. The innermost intercostals are located deep to both the internal and external intercostals.” by Phil Schatz. License: CC BY 4.0

The primary chest wall muscles, in turn are divided into three sub-categories. A distinction is made between the intercostals, serratus posterior as well as an “other” group.

Intercostal Muscle Group

From outside to inside, the intercostal muscle group consists of external intercostal, internal intercostal and innermost intercostal muscles. 

Intercostales

Image: “The external intercostals are located laterally on the sides of the body. The internal intercostals are located medially near the sternum. The innermost intercostals are located deep to both the internal and external intercostals.” by Phil Schatz. License: CC BY 4.0

External Intercostal Muscles

The external intercostal muscles are eleven in number that originate from the rib tubercles to the costo-chondral border and insert into the upper border of the lower rib. They run dorso-cranial to ventro-caudal. They serve as muscles of inhalation, because they ensure the lifting (elevation) of the ribs through the innervation of the intercostal nerves (1-11).

Functional Note: In addition to their primary function, they serve as a tenser of the intercostal spaces, thus limiting the extent of movement of the ribs.

Internal Intercostal and Innermost Intercostal Muscles.

Starting from the sternum toward the costal angle, the internal and innermost intercostal muscles extend to the lower margin of the next higher rib. Their direction is dorso-caudal to ventro-cranial. They are innervated by the intercostal nerves 1-11 and act as muscles of exhalation through their rib-lowering function. The fibers of innermost intercostal muscle cross more than one intercostal space.

Functional Notes: In addition to their primary function, they serve as a tenser of the intercostal spaces, thus limiting the extent of movement of the ribs.

Furthermore, the part of internal intercostal muscle that is located between the costal cartilages (interchondral part) acts as an auto-antagonistical inhalation muscle i.e. it elevates the ribs. This part is considered a separate muscle in some medical reference books and is referred to as intercartilaginous muscle.

Serratus posterior-group

This group consists of only two muscles and so it bears their name. They are the serratus posterior superior and the serratus posterior inferior muscles.

Serratus posterior superior muscle

This muscle arises from the spinous processes of C6-Th2 and starts at the costal angle of the ribs 2-5. It is innervated by the intercostal nerves (Th1-Th4) and lifts the ribs. Thus it is a synergist with muscles of inhalation.

Serratus posterior inferior muscle

Beginning at the spinous processes of vertebrae Th 11-L2, the serratus posterior inferior muscle inserts at the lower margin of the 9th-12th rib. During active innervation by the intercostal nerves (T9-T12), it fixes the ribs posteriorly and thus acts as an antagonist to the costal diaphragm during inhalation.

“Other“ Group

Subcostalis and transversus thoracis are part of the “other” group of primary chest wall muscles.

Subcostalis muscle

The subcostalis muscle is originated from the inner surface of the lower ribs near their angles. From there, this muscle spread in the same direction as the internal intercostal muscle , extend over several ribs, to finally insert on the inside of the lower ribs. The intercostal nerves 4-11 provide the neural supply through which the ribs are elevated, and this consequently makes the subcostalis an exhalation muscle.

Special features: The subcostalis muscles are not inherent in every individual.

Functional Note: In addition to their primary function, they serve to tense the chest wall.

Transversus thoracis muscle

The transversus thoracis muscle starts at the posterior surface of lower sternum and xiphoid process, ascends in latero-cranial direction and inserts at the lower margin of the costal cartilages 2-6. During active innervation by the intercostal nerves 2-6, the muscle lowers the ribs, and thus, acts as an exhalation muscle.

Functional note: In addition to its primary function, it serves to tense the chest wall.

The secondary chest wall muscles

Pectoralis major minor Sternum

Image: “The external intercostals are located laterally on the sides of the body. The internal intercostals are located medially near the sternum. The innermost intercostals are located deep to both the internal and external intercostals.” by Phil Schatz. License: CC BY 4.0

The secondary chest wall muscles correspond with the ventral shoulder and chest muscles. This is important to mention, because it confuses many medical and physiotherapy students as muscle group assignation not play a decisive role while studying. Origin, insertion, innervation and function should be labeled. The group of secondary chest wall muscles includes: Pectoralis major, Pectoralis minor and Sternalis muscles.

Pectoralis Major

pectoralis major left anterior lateral view

Image: “Pectoralis major (left anterior lateral view) – revised.” by Phil Schatz. License: CC BY 4.0

This large breast muscle consists of two main parts: clavicular, and sternocostal. Their joined innervation is carried out by the lateral and medial pectoral nerves (C5-Th1) and their joined insertion is at the lateral lip of intertubercular groove of humerus.

Origin of the clavicular part is from the medial half of the clavicle, while the sternocostal has its origin from the sternum and upper six rib cartilage.

The pectoralis major muscle carries out adduction and internal rotation of the upper arm. Furthermore, the clavicular part, especially, ensures for anteversion of the arm. Additionally, the pectoralis major can move the shoulder girdle anteriorly, cranially and caudally.

Functional Note: In addition to its numerous primary functions, the pectoralis major is also an auxiliary respiratory muscle of inhalation.

Pectoralis Minor

The small pectoralis muscle arises from the 3rd-5th ribs, lateral to the bone-cartilage interface, and inserts at the coracoid process of scapula. During active innervation by the medial and lateral pectoral nerves (C6-Th1), it depresses the scapula.

Functional Note: In addition to its primary function, the pectoralis minor is also an auxiliary respiratory muscle of inhalation.

Sternalis

This is a very small muscle. Its points of origin and insertion vary greatly, but are always found along the sternum. Its function is not clearly described in the available literature. Functionally, the muscle is (generally) not verified to have a vital importance.

Special feature: Sternalis muscle is only found in 5-7% of the human population.

Muscles of the Abdomen

The muscles of the abdominal wall are divided into three different groups, according to their location: ventral, lateral and dorsal abdominal muscles.

Ventral abdominal muscles: Rectus Abdominis and External Oblique

From a muscular view, the ventral abdomen consists of the rectus abdominis, as well as the pyramidal muscles.

Rectus Abdominis

rectus abdominis

Image: “Superficial and deep abdominal muscles (anterior lateral view).” by Phil Schatz. License: CC BY 4.0

The rectus abdominis originates from the pubic crest and pubic symphysis, while it is inserted at the ventral surface of the thorax that includes the cartilage of the 5th-7th rib and xiphoid process of sternum. It receives its innervation through the intercostal nerves Th7-Th11, and subcostal nerve (Th12). This muscle acts as ventral flexor of the torso, dorsal tipper of the pelvis and lowers the ribs.

Special features: In some people, the muscle is also innervated by the segments T5-T6.

Functional Note: If unilateral innervation of the rectus abdominis muscle occurs, it is also capable of lateral flexion of the torso.

Pyramidal muscle

Pyramidal muscle originates from the superior margin of the pubic bone and the symphysis, located anterior to the rectus abdominis muscle. It is inserted on the linea alba and runs within the rectus sheath. During active innervation by the subcostal nerve (T12), it tenses the linea alba.

Special features: This muscle is not actually found in every individual. It can get additional innervation from the segments Th11 and L1-L2.

Lateral abdominal muscles

Internal and external obliquus transversus abdominis

Image: “Superficial and deep abdominal muscles (anterior lateral view).” by Phil Schatz. License: CC BY 4.0

External obliqueInternal oblique, Transverse abdominal as well as Cremaster muscles form the lateral abdominal muscle group.

External Oblique

This muscle arises from the outer surface of the 5th-12th rib and alternates here with the origin edges of the serratus anterior muscle. It inserts at the iliac crest, as well as the inguinal ligament, the ventral blade of the rectus sheath and the linea alba. The intercostal nerves (T5-T12) ensure the neural supply. The muscle’s function is the ventral flexion of the thoracic and lumbar spine, pelvic alignment and lowering of the ribs.

Special features: It can also be innervated by segment L1.

Functional Note: In addition to its numerous primary functions, it is an accessory respiratory muscle of exhalation. During unilateral innervation, it rotates contralaterally and performs an ipsilateral lateral flexion of the torso.

Internal Oblique

The internal oblique abdominal muscle has numerous points of origin: the deep blade of the thoracolumbar fascia, anterior superior iliac spine and the lateral two-thirds of the inguinal ligament. Its insertions are equally various: the caudal edges of the 9th-12th rib, linea alba and the front and rear blade of the rectus sheeth, cranial of the linea arcuata. During active innervation by the intercostal (Th7-Th11), subcostal (Th12), iliohypogastric, and ilioinguinal nerves, it performs a ventral flexion of the thoracic and lumbar spine, pelvic alignment and lowers the ribs.

Functional Note: During unilateral innervation, it acts as an ipsilateral rotator and lateral flexor of the torso.

Transversus Abdominis

Similar to previously mentioned muscles, the transverse abdominal muscle has multiple origins, which includes lower six costal cartilages, anterior superior iliac spine, the lateral one-third of the inguinal ligament and thoracolumbar fascia. It is inserted at the rear blade of the rectus sheath, the linea alba and the cranial side of the linea arcuata. Its innervation corresponds to the internal oblique muscle i.e. intercostal (Th7-Th11), subcostal (Th12), iliohypogastric, and ilioinguinal nerves. It sucks in the stomach and can cause a minimum flexion of the torso.

Functional Note: In addition to its primary function, it is an auxiliary muscle of exhalation and rotates the torso ipsilaterally during unilateral innervation.

Cremaster

The cremaster muscle is originated from the inguinal ligament and the lower border of internal oblique muscle, while it is inserted at the pubic crest and public tubercle. It is wrapped around the spermatic cord and serves as its protective sheath but can also lift the testicles during active innervation by the genitofemoral nerve (L1-L3). The nerve function can be examined via the so-called cremasteric reflex (in some textbooks this is spelled with a “K”).

Dorsal abdominal muscles

quadratus lumborum

Image: “Superficial and deep abdominal muscles (anterior lateral view).” by Phil Schatz. License: CC BY 4.0

Only one muscle forms the group of posterior abdominal muscles. It is the quadratus lumborum muscle.

Quadratus lumborum

This muscle mainly originates from the iliolumbar ligament and iliac crest, while it is inserted at the transverse processes of L3-L5. It receives its neural supply from the subcostal nerve (Th12) and via direct branches of the lumbar plexus (Th12-L3). This muscle is responsible for lordosis of the lumbar spine. It also fixes the 12th rib and serves as a countertraction to the diaphragm during breathing functions.

Functional Note: During unilateral innervation, the quadratus lumborum muscle acts as a lateral flexor of the lumbar spine.

Thoracic and Abdominal Muscles: Summary Table

Muscle group Muscles
Primary chest wall: intercostals group
  • External intercostal
  • Internal intercostal
  • Innermost intercostal
Primary chest wall: serratus posterior group
  • Serratus posterior superior
  • Serratus posterior inferior
Primary chest wall: “Others” group
  • Subcostalis
  • Transversus thoracis
Secondary thoracic wall
  • Pectoralis major
  • Pectoralis minor
  • Sternalis
Ventral abdominal muscles
  • Rectus abdominis
  • Pyramidal
Lateral abdominal muscles
  • External oblique
  • Internal oblique
  • Transverse abdominal
  • Cremaster
Dorsal abdominals
  • Quadratus lumborum

Popular Exam Questions about Thorax and Abdominal Muscles

The answers are located below the references.

1. Which of the following muscles is not a part of the primary chest wall muscles?

  1. External Intercostal
  2. Internal intercostal
  3. Subcostalis
  4. Serratus anterior
  5. Transversus thoracis

2. Which of the following nerves does not innervate the internal oblique muscle?

  1. Intercostal
  2. Pudendal
  3. Iliohypogastric
  4. Ilioinguinal
  5. Genitofemoral

3. Which of the following structures is not a point of origin of the transverse abdominal muscle?

  1. Iliac crest
  2. Anterior inferior iliac spine
  3. Inguinal ligament
  4. Thoracolumbar fascia
  5. Costal cartilages
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