The Intercostal Area
Advertisements help pay for this website. Thank you for
your support.
Joints
of the sternum | Main Anatomy
Index | Mammary glands
Last updated 30 March 2006
The
Intercostal Muscles
The
External Intercostal Muscles (p. 50)
- Each of the 11 pairs of
muscles occupy the intercostal spaces from the tubercles
of the ribs posteriorly to the costochondral junctions
anteriorly.
- Anteriorly, the external intercostal membranes replace
the muscle fibres.
- The muscles run inferoanteriorly from the rib above to
the rib below.
- Each muscle is attached superiorly to the inferior border
of the rib and inferiorly to the superior border of the
rib below.
- The external intercostal muscles are continuous
inferiorly with the external
oblique muscles of the anterolateral abdominal wall.
The Internal Intercostal Muscles (pp.
50-1)
- The 11 pairs of muscles
run deep to and at right angles to the external
intercostal muscles.
- Their fibres run inferoposteriorly from the floors of the
costal grooves to the superior borders of the ribs
inferior to them.
- The internal intercostal muscles attach to the shafts of
the ribs and their costal cartilages as far anteriorly as
the sternum and as far posteriorly as the angles of the
ribs.
- Between the ribs posteriorly, the internal intercostal
membrane replaces the internal intercostal muscles.
- The inferior internal intercostal muscles are continuous with the internal
oblique muscles of the anterolateral abdominal wall.
The
Innermost Intercostal Muscles (p. 51)
- Theses are the deepest intercostal muscles and are
similar to the internal intercostal muscles, and are
really deep portions of them.
- The innermost intercostal muscles are separated from the
internal intercostal muscles by the intercostal nerves
and vessels.
- These muscles pass between the internal surfaces of
adjacent ribs and occupy the middle part of the
intercostal spaces.
The
Subcostal Muscles (p. 51)
- These are variable in size and shape, these muscles are
thin muscular slips that extend from the internal surface
of the angle of the rib to the internal surface of the
rib inferior to it.
- It crosses one or two intercostal spaces.
- They run in the same direction as the internal
intercostal muscles and lie internal to them.
The
Transversus Thoracic Muscle (p. 51)
- These are thin muscular consisting of four or five slips
that are attached posteriorly to the xiphoid process, the
inferior part of the body of the sternum, and the
adjacent costal cartilages.
- They pass superolaterally and are attached to the second
to sixth costal cartilages.
- The transversus thoracis muscles are continuous
inferiorly with the transversus
abdominis muscle. The internal thoracic vessels run
anteriorly to these muscles, between them and the costal
cartilages and internal intercostal muscles.
Actions of the Intercostal Muscles (p. 51)
- All these inspiratory muscles elevate
the ribs.
- This movement expands the thoracic cavity in the
transverse and anteroposterior diameters.
- All three layers of intercostal muscles keep the intercostal spaces rigid,
thereby preventing them from bulging out during
expiration and from being drawn in during inspiration.
The
Intercostal Spaces
- These spaces between the ribs are deeper anteriorly than
posteriorly and deeper between the superior than the
inferior ribs.
- The intercostal spaces widen on
inspiration.
- Each space contains three muscles and a neurovascular bundle (vein,
artery and nerve; often referred to as VAN).
Intercostal
Nerves (Moore p. 52, Snell 52-3)
- There are 12 pairs of costal nerves.
- The intercostal nerves are the anterior
rami of the first 11 thoracic spinal nerves.
The anterior ramus of the twelfth thoracic nerve lies in
the abdomen and runs forward in the abdominal wall as the
subcostal nerve.
- Each intercostal nerve enters the intercostal space
between the parietal pleura and the posterior intercostal
membrane.
- It then runs forward, inferior to the intercostal vessels
in the costal groove of the corresponding rib, between
transversus thoracis and internal intercostal muscle.
- The first 6 nerves are distributed within their
intercostal spaces.
- The seventh to ninth intercostal nerves leave the
anterior ends of their intercostal spaces by passing deep
to the costal cartilages, to enter the anterior abdominal
wall.
- In the case of the tenth and eleventh nerves, since the
corresponding ribs are floating, these nerves pass
directly into the abdominal wall.
- The lower intercostal nerves supply the skin of the
abdomen. Also, these nerves also supply the muscles of
the abdomen-leading to the tickle response.
Branches of the intercostal nerves (Moore p. 56,
Snell p. 53)
- Rami communicantes
connect the intercostal nerve to a ganglion of the
sympathetic trunk. The grey ramus joins the nerve medial
to the point at which the white ramus leaves it.
- A collateral branch,
which runs forward inferiorly to the main nerve on the
upper border of the rib below.
- A lateral cutaneous branch,
which reaches the skin near the midaxillary line. It
divides into an anterior and a posterior branch.
- An anterior cutaneous branch,
which is the terminal portion of the main trunk, reaches
the skin near the midline. It divides into a medial and a
lateral branch.
- The main nerve and its collateral branch give off
numerous muscular branches.
- The first intercostal nerve has no
anterior cutaneous branch and usually has no
lateral cutaneous branch.
- It divides into a large superior part and a small
inferior part.
- The superior part joins
the brachial plexus. The
inferior part becomes
the first intercostal nerve
- The second intercostal nerve
may also contribute a small branch to the brachial
plexus.
- The lateral cutaneous branch of the second intercostal
nerve is called the intercostobrachial
nerve because it supplies the floor of the
axilla and then communicates with the medial brachial
cutaneous nerve to supply the medial side of the upper
limb as far as the elbow.
The
Intercostal Arteries
- Three arteries, a large posterior intercostal artery and
a small pair of anterior intercostal arteries supply each
intercostal space.
The Posterior Intercostal Arteries (p. 59)
- The first two posterior intercostal arteries arise from
the superior intercostal artery,
a branch of the costocervical trunk of the subclavian
artery.
- Nine pairs of posterior intercostal arteries and one pair
of subcostal arteries arise posteriorly from the thoracic
aorta.
- Each posterior intercostal artery gives off a posterior branch, which
accompanies the dorsal ramus of the spinal nerve to
supply the spinal cord, vertebral column, back muscles,
and skin.
- Each artery also gives off a small collateral
branch that crosses the intercostal space and
runs along the superior border of the rib inferior to the
space.
- The terminal branches of the posterior intercostal artery
anastomose anteriorly with the anterior intercostal
artery.
- The posterior intercostal artery accompanies the
intercostal nerve through the intercostal space.
- Close to the angle of the rib, it enters the costal
groove, where it lies between the intercostal vein and
nerve. At first, the artery runs between the pleura and
the internal intercostal membrane and then it runs
between the innermost intercostal
and internal intercostal muscles.
The
Anterior Intercostal Arteries (p. 59)
- The anterior intercostal arteries supplying the superior six intercostal spaces
are derived from the internal
thoracic arteries.
- There are two anterior intercostal arteries for each
intercostal space. The arteries supplying the seventh to ninth intercostal spaces
are derived from the musculophrenic
arteries, branches of the internal thoracic
arteries.
- These arteries pass laterally, one near the inferior
margin of the superior rib and the other near the
superior margin or the inferior rib.
- At their origins, the first two
arteries lie between the
pleura and the internal intercostal arteries.
- The next four arteries are separated from the pleura by
the transversus thoracis muscle.
- The anterior intercostal arteries supply the intercostal
muscles, and send branches through them to the pectoral
muscles, breast and skin.
- There are no anterior intercostal
arteries in the inferior two intercostal spaces.
The posterior intercostal arteries and their collateral
branches supply these spaces.
The
Intercostal Veins (p. 59)
- The intercostal veins accompany the intercostal arteries
and nerves; they lie deepest in the costal grooves.
- There are eleven posterior
intercostal veins and one subcostal
vein on each side.
- They receive lateral cutaneous, muscular, intervertebral,
and posterior tributaries.
- The intercostal veins contain
valves, which direct the blood posteriorly.
- The posterior intercostal veins anastomose with the anterior intercostal veins,
which are tributaries of the internal thoracic veins.
- Most intercostal veins end in the azygos
vein which conveys blood to the superior vena cava.
- The superior intercostal veins drain directly into the
superior vena cava.
The Internal Thoracic Vessels
The
Internal Thoracic Artery (p. 59)
- This vessel arises in the root of the neck from the
inferior surface of the first part of the subclavian artery at the
medial border of the scalenus anterior muscle.
- The internal thoracic artery descends into the thorax
posterior to the clavicle and the first costal cartilage.
It runs on the internal surface of the thorax, a little
lateral to the sternum.
- It lies on the pleura posteriorly and is crossed by the phrenic nerve.
- The internal thoracic artery runs inferiorly in the
thorax posterior to the superior six costal cartilages
and intervening intercostal muscles.
- At the level of the third costal cartilage, it continues
inferiorly, anterior to the
transverse thoracis muscle, to end in the
sixth intercostal space where it divides
into the superior epigastric and musculophrenic arteries.
The Internal Thoracic Veins (pp.
59-60)
- These are the venae comitantes
of the internal thoracic artery.
- In the region of the first to third intercostal space,
the usually unite to form a trunk (single vein) that
empties into the corresponding brachiocephalic
vein; however, the right internal thoracic
trunk may empty into the superior vena cava.