Muscles of the Thorax

The muscles of respiration / Muscles of the Thorax attached to the thoracic vertebrae, ribs and their cartilages and sternum constitute the muscles of the thorax or the costal region. They are

1. Serratus thoracis
2. Levatores costarum
3. External intercostals
4. Internal intercostals
5. Retractor costarum
6. Rectus thoracis
7. Transversus thoracis
8. Diaphragm

Serratus thoracis

• This muscle is thin and flat muscle covered by a strong glistening aponeurosis. It overlaps the serratus cervicis. Its ventral border is serrated and shows six digitations.
• Origin: External surfaces of ribs from 4 th to the 9 th .
• Insertion: The rough line at the dorso-posterior part of the ventral surface of the scapula. (See the picture at the top)
• Action: The two muscles act together as a sling to suspend the trunk between the two forelimbs. Acting singly it pulls the scapula upwards and backwards. When the limb is fixed, each side muscle acts as an extra-ordinary muscle of inspiration in respiratory exertions.
• Blood supply: Intercostal arteries.
• Nerve supply: Long thoracic nerve.

Levatores costarum

• These constitute a series of 10-12 small, flat, triangular musculo-tendinous fasciculi located at the upper part of the intercostal spaces. They are blended with the external intercostals at their insertion.
• Origin: Transverse process of the thoracic vertebrae.
• Insertion:Anterior borders of the succeeding.
• Action: To aid in inspiration.
• Blood supply: Intercostal arteries.
• Nerve supply:  Intercostal nerves.

External intercostals

• These muscles fill up the intercostal spaces from the levatores costarum to the level of the distal extremity of the ribs, but do not extend into the interchondral spaces.
• Origin: Posterior border of a rib.
• Insertion:Anterior border and external surface of the rib behind. The fibres of this muscle are directed downwards.
• Action: Inspiratory muscle.
• Blood supply: Intercostal arteries.
• Nerve supply:  Intercostal nerves.

Internal intercostals

• These muscles are situated internal to the external intercostals and also extend into the interchondral spaces.
• Origin: Anterior border of a rib and its cartilage, except the first.
• Insertion:Posterior border of the rib in front, and its cartilage. The fibres are directed downward and forward.
• Action: Assists in expiration, especially forced expiration.
• Blood supply: Intercostal and internal thoracic arteries.
• Nerve supply:  Intercostal nerves.

Retractor costarum (retractor of the last rib)

• It is a small triangular muscle situated behind the last rib under the cover of the last digitation of the serratus dorsalis posterior.
• Origin: Transverse processes of the first two or three lumbar vertebrae.
• Insertion:Posterior border of the last rib.
• Action: To retract the last rib and aid in forced expiration.
• Blood supply: 1 st –4 th lumbar arteries.
• Nerve supply:  1 st -4 th lumbar spinal nerves.

Rectus thoracis

• This is a thin quadrilateral muscle lying on the lateral face of the first few ribs and their cartilages under the cover of deep pectoral.
• Origin: Lower half of the lateral surface of the first rib.
• Insertion:Third or fourth costal cartilage.
• Action: Assist in inspiration.
• Blood supply: Internal and external thoracic arteries .
• Nerve supply:  First five intercostal nerves.

Transversus thoracis (triangularis sterni)

• This is a flat, triangular muscle situated on the dorsal face of the sternum.
• Origin: Sternal ligament.
• Insertion:Cartilages of ribs from the second to the eighth and the adjacent parts of ribs .
• Action: Assists in expiration.
• Blood supply: Internal thoracic artery.
• Nerve supply:  Intercostal nerves.

Diaphragm

• This is a broad and large dome shaped, unpaired muscle, which forms the partition between the thoracic and abdominal cavities.
• Its thoracic surface is convex and is covered by pleura and the abdominal surface is concave and is covered by peritoneum.
• Anteriorly the diaphragm is covered by the pleura and is related to the diaphragmatic lobes of the lungs and the last six ribs in part.
• Posteriorly, it is covered by the peritoneum and is related to the rumen, spleen, reticulum, liver, omasum, kidneys and adrenals.
• The muscle presents a tendinous centre, a muscular rim ( costal and sternal parts) and two crura (lumbar part).
• Attachments:  
  • The costal part is attached in a straight line from the last rib about a hand’s breadth above its middle to the junction of the eighth rib, and its cartilage. It consists of a series of digitations.
  • The sternal part is attached along the cartilage of the eighth rib and across the abdominal face of the xiphoid cartilage.
  • The lumbar part consists of the right and left crura. Each is fleshy and followed by a tendon attached to the lumbar vertebrae.
  • The right crus is attached to the body of the first lumbar vertebra through the ventral longitudinal ligament while the left is attached to the first two lumbar vertebrae through the ventral longitudinal ligament. The right crus is the larger of the two. It divides into two branches, which circumscribe the hiatus oesophagi, unite below and then spread out to the aponeurotic center. This hiatus is a little left to the median line. The aponeurotic centre is partially divided into right and left parts by the descent of the crura into it.
• The diaphragm is pierced by the three foramina
  • The hiatus aorticus that lies between the two crura below the 13 th thoracic vertebra and transmits the aorta, vena hemiazygos and the cisterna chyli.
  • Thehiatus oesophagi is formed between the two divisions of the right crus and transmits the oesophagus and the dorsal and ventral oesophageal continuations of the vagus nerve and
  • The foramen vena cavae is situated a little below and to the right of the hiatus oesophagus and transmits the posterior venacava.
• Action: Chief inspiratory muscle.
• Blood supply: Phrenic and musculo -phrenic arteries.
• Nerve supply:  Phrenic nerve.

Horse

• External intercostals are thinner.
• Internal intercostals are also thinner.
• The transverse thoracis is inserted to the seven sternal cartilages from the second to the eighth.
• Diaphragm is less wide but more sloping. The right crus does not divide into two branches. The hiatus oesophagi is found in the right crus a little above its junction with the aponeurotic centre. The foramen vena cavae is placed little higher than that in the ox.

Dog

• Diaphragm is strongly curved and its aponeurotic centre is small. The sternal part of the diaphragm has the pericardium attached to it by pericardio-phrenic ligament.
• The other muscles mostly resemble ox.