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CHAPTER 1 The Thorax
11
Left Pulmonary Artery
• Connected at its origin with the arch of the aorta via the
ligamentum arteriosum.
• Runs in front of the le main bronchus and descending
aorta.
• Le recurrent laryngeal nerve loops below the aortic
arch in contact with the ligamentum arteriosum.
Bronchial Arteries
• Supply the air passages.
• Branches of the descending aorta.
PLEURA
• Each pleural cavity is composed of a thin serous membrane invaginated by the lung.
• e visceral pleura is intimately related to the lung surface and is continuous with the parietal layer over the
root of the lung.
• e parietal layer is applied to the inner aspect of the
chest wall, diaphragm and mediastinum.
• Below the root of the lung the pleura forms a loose fold
known as the pulmonary ligament, which allows for distension of the pulmonary vein.
• e lungs conform to the shape of the pleural cavities
but do not occupy the full cavity as this would not allow
expansion as in full inspiration.
• e two pleural cavities are totally separate from one
another.
Surface Anatomy of Pleura and Lungs
Pleura
• Cervical pleura extends above the sternal end of the
rst rib.
• It follows a curved line drawn from the sternoclavi cular
joint to the junction of the inner third and the outer
two-thirds of the clavicle, the apex arising 2.5 cm above
the clavicle.
• Line of pleural reection passes behind the sternoclavicular joint on each side to meet in the midline at the
angle of Louis (second costal cartilage level).
• e right pleural edge passes vertically down to the level
of the sixth costal cartilage and crosses:
• eighth rib in midclavicular line
• tenth rib in midaxillary line
• twelh rib at the lateral border of erector spinae.
• Le pleural edge arches laterally at the fourth costal cartilage and descends lateral to the border of the sternum
whence it follows a path similar to the right.
• Medial end of the fourth and h le intercostal spaces
are therefore not covered by pleura.
• e pleura descends below the twelh rib at its medial
extremity.
Lungs
• Apex of the lung follows the line of cervical pleura.
• Anterior border of the right lung corresponds to the
right mediastinal pleura.
• Anterior border of the le lung has a distinct notch (cardiac notch), which passes behind the h and sixth costal cartilages.
• e lower border of the lung (midway between inspiration and expiration) crosses:
• sixth rib in the midclavicular line
• eighth rib in the midaxillary line
• tenth rib at the lateral border of erector spinae.
• e oblique ssure is represented by the medial border
of the scapula with the arm fully elevated (abducted) or
by a line drawn from 2.5 cm lateral to the h thoracic
vertebrae to the sixth costal cartilage, about 4 cm from
the midline.
• e horizontal ssure of the right lung passes horizontally and medially from the oblique ssure at the level of
the fourth costal cartilage.
Clinical Points
• e pleura rises above the clavicle into the neck. It may
be injured by a stab wound, the surgeon’s knife or insertion of a subclavian or internal jugular line.
• A needle passing through the le fourth and h intercostal spaces immediately lateral to the sternal edge will
enter the pericardium without traversing the pleura.
• e pleura descends below the medial extremity of the
twelh rib and therefore may be inadvertently opened
in the loin approach to the kidney or adrenal gland.
Nerve Supply of Pleura
• Receives nerve supply from structures to which it is
attached.
• Visceral pleura obtains an autonomic supply from the
branches of the vagus nerve supplying the lung and is
sensitive only to stretching.
• Parietal pleura receives somatic innervation from the
intercostal nerves.
• Diaphragmatic pleura is supplied by the phrenic nerve.
• Parietal pleura and diaphragmatic pleura are therefore
sensitive to pain.
Clinical Points
• Pain from the parietal pleura of the chest wall may be
referred via the intercostal nerves to the abdomen, e.g.
right lower lobar pneumonia may irritate the parietal
pleura and refer pain to the right lower abdomen, mimicking acute appendicitis; irritation of the diaphragmatic
pleura may refer pain to the tip of the shoulder [irritating the phrenic nerve (C3, 4, 5) and referring pain to the
dermatomal distribution of C4 at the shoulder tip].
12
the diaphragm
Right crus of the
SECTION I Anatomy
Inferior vena
cava
Right phrenic
nerve
Central
tendon
diaphragm
Psoas major
Fig. 1.8 The inferior aspect of the diaphragm.
THE DIAPHRAGM (Fig. 1.8)
• Dome-shaped septum separating the thorax from the
abdomen.
• Composed of a peripheral muscular part and a central
tendon.
• Muscular part arises from crura, arcuate ligaments, ribs
and sternum.
• Right crus arises from the front of the bodies of the rst
three lumbar vertebrae and the intervening intervertebral discs.
• Le crus arises from the rst and second lumbar vertebrae and the intervening disc.
• e lateral arcuate ligament is a condensation of the fascia over quadratus lumborum.
• e medial arcuate ligament is a condensation of the
fascia of psoas major.
• e medial borders of the medial arcuate ligament join
anteriorly over the aorta as the median arcuate ligament.
• e costal part is attached to the inner aspect of the
lower six ribs.
• e sternal portion arises as two small slips from the
back of the xiphoid process.
• e central tendon is trefoil in shape and receives
insertion of muscular bres. Above, it fuses with the
pericardium.
• ere are three main openings in the diaphragm:
Left phrenic
nerve
Oesophagus
Left crus of
Aorta
Quadratus lumborum
• aortic (strictly speaking the aortic ‘opening’ is not in
the diaphragm, but lies behind it): lies at the level of
T12; it transmits the abdominal aorta, the thoracic
duct and oen the azygos vein
• oesophageal: lies in the right crus of the diaphragm at
the level of T10. Transmits oesophagus, vagus nerves
and branches of the le gastric artery and vein
• IVC opening: lies at T8 level in the central tendon
of the diaphragm. Transmits IVC and right phrenic
nerve.
• Greater and lesser splanchnic nerves pierce the crura.
• Sympathetic chain passes behind the medial arcuate
ligament lying on psoas major.
Nerve Supply
• Phrenic nerve (C3, 4, 5): the phrenic nerve is the sole
motor nerve supply to the diaphragm. e sensory
innervation of the central tendon of the diaphragm
is via the phrenic nerve but the periphery of the diaphragm is supplied by the lower six intercostal nerves.
• Irritation of the diaphragm (e.g. in peritonitis or pleurisy) results in referred pain to the cutaneous area of
supply, i.e. the shoulder tip via C4 dermatome.
• Damage to the nerve (e.g. in the neck) leads to paralysis
of the diaphragm. Clinical examination reveals dullness
to percussion at the base on the aected side and absent
CHAPTER 1 The Thorax
Right coronary
and great cardiac vein
Left pulmonary artery
Arch of aorta
Left atrium
13
breath sounds. is is due to the diaphragm being
elevated as seen on chest X-ray. Paradoxical movement
of the diaphragm occurs on respiration.
ANATOMY OF RESPIRATION
• oracic breathing: movements of rib cage.
• Abdominal breathing: contraction of diaphragm.
Thoracic Breathing
• ‘Pump handle’ action of ribs. Anterior ends of ribs are
raised and, as these are below the posterior end, this
increases the anteroposterior diameter of the thorax.
• ‘Bucket handle’ action of ribs. Ribs 4–7 are raised. As the
centre of these ribs is normally below the anterior and
posterior ends, the transverse diameter of the chest is
increased when they move upwards.
Abdominal Breathing
• Muscular bres of the diaphragm contract and the central tendon descends, increasing the vertical diameter of
the thorax.
• As the central tendon descends it is arrested by the liver.
• e central tendon is now xed and acts as the origin for
muscle bres, which now elevate the lower six ribs.
• Combination of thoracic and abdominal breathing
increases all diameters of the thorax.
• e negative intrapleural pressure is increased and the
lung expands.
Inspiration
• Quiet inspiration is a combination of thoracic and
abdominal respiration.
• Forced inspiration (e.g. asthma) brings into action the
accessory muscles of respiration, i.e. sternocleidomastoid, scalenes, pectoralis major, pectoralis minor, serratus anterior.
Expiration
• Elastic recoil of lung tissue and chest wall.
• Forced expiration (e.g. coughing and trumpet playing)
requires use of muscles, i.e. rectus abdominis, external and
internal obliques, transversus abdominis, latissimus dorsi.
THE HEART (Fig. 1.9)
• Roughly conical in shape, lying obliquely in the middle
mediastinum.
• Attached at its base to the great vessels, otherwise lies
free in pericardial sac.
• Base directed upwards, backwards, to the right.
• Apex directed downwards, forwards, to the le.
• Consists of four chambers: right and le atria, right and
le ventricles.
Viewed from the front it has three surfaces and three
borders.
ree surfaces:
• anterior: right atrium, right ventricle and narrow
strip of le ventricle, auricle of le atrium
Pulmonary trunk
Auricle of left atrium
Anterior descending
(interventricular) branch
of left coronary artery
Pulmonary trunk
Left ventricle
Posterior
interventricular
artery
B
Superior vena cava
artery
Right atrium
Small cardiac
vein
A
Right ventricle
Fig. 1.9 The heart and great vessels in (A) anterior and (B) posterior view.
Right
pulmonary
veins
Coronary
sinus
Middle cardiac vein
14
SECTION I Anatomy
• posterior (base): le ventricle, le atrium with four
pulmonary veins entering it
• inferior (diaphragmatic surface): right atrium with
IVC entering it and lower part of ventricles.
ree borders:
• right: right atrium with IVC and SVC
• inferior: right ventricle and apex of le ventricle
• le: le ventricle, auricle of le atrium.
Chambers of Heart
Right Atrium
• Receives blood from IVC, SVC, coronary sinus, anterior
cardiac vein.
• Crista terminalis runs between cavae–muscular ridge,
separating smooth-walled posterior part of atrium
(derived from sinus venosus) from rougher area (due to
pectinate muscles) derived from true atrium.
• e fossa ovalis (the site of the fetal foramen ovale) is an
oval depression on the interatrial septum.
Right Ventricle
• icker-walled than atrium.
• Communicates with atrium via tricuspid valve.
• Connects with pulmonary artery via pulmonary valve.
• Tricuspid valve has three cusps: septal, anterior, posterior.
• Atrial surface of valve is smooth but ventricular surfaces
have brous cords, the chordae tendineae, which attach
them to papillary muscles on the ventricular wall. ey
prevent eversion of the cusps in the atrium during ventricular contraction.
• Moderator band is a muscle bundle crossing from the
interventricular septum to the anterior wall of the heart.
• Moderator band may prevent overdistension of ventricle. Conducts right branch of the AV bundle to anterior
wall of ventricle.
• Infundibulum is the outow tract of the ventricle.
Directed upwards and to the right towards the pulmonary trunk.
• Pulmonary orices guarded by the pulmonary valve
consisting of three semilunar cusps.
Left Atrium
• Smaller than the right.
• Consists of principal cavity and auricle.
• Auricle extends forwards and to the right, overlapping
the commencement of the pulmonary trunk.
• Four pulmonary veins open into the cavity (two from
each lung: superior and inferior).
• Shallow depression on septal surface corresponds to
fossa ovalis of right atrium.
• Largely smooth-walled, except for ridges in the auricle
owing to underlying pectinate muscles.
Left Ventricle
• Longer and more conical than right with thicker wall
(three times thicker).
• Communicates with atrium via mitral valve.
• Connects with aorta via aortic valve.
• Mitral valve has two cusps: anterior (larger) and
posterior.
• Chordae tendineae run from the ventricular surfaces of
cusps to papillary muscles.
• Aortic valve is stronger than pulmonary valve. Has three
cusps—anterior, right and le posterior—each having a
central nodule in its free edge and a sinus or dilatation
in the aortic wall alongside each cusp.
• e mouths of the right and le coronary arteries are
seen opening into the anterior and le posterior aortic
sinuses, respectively.
Fibrous Skeleton of the Heart
• e AV orice is bound together by a gure-of-eight
conjoined brous ring.
• Acts as a brous skeleton for attachment of valves and
muscles of atria and ventricles.
• Helps to maintain shape and position of heart.
Conducting System
• Sinoatrial (SA) node situated in right atrial wall at upper
end of crista terminalis (SA node = pacemaker of heart).
• From SA node, cardiac impulse spreads to reach AV
node.
• AV node lies in interatrial septum immediately above
opening of coronary sinus.
• Cardiac impulse is conducted to ventricles via AV
bundle (of His).
• AV bundle passes through brous skeleton of heart to
membranous part of interventricular septum, where it
divides into right and le branch.
• Le AV bundle is larger and both run under endocardium to activate all parts of the ventricular muscle.
• Papillary muscles contract rst and then wall and
septum in a rapid sequence from apex towards outow
tract, both ventricles contracting together.
• AV bundle is normally the only pathway through which
impulse can reach ventricles.
Blood Supply of Heart (see Fig. 1.9)
Right Coronary Artery
• Arises from anterior aortic sinus.
• Passes to the right of the pulmonary trunk between it
and the auricle.
• Runs along the AV groove around the inferior border of
the heart and anastomoses with the le coronary artery
at the posterior interventricular groove.
CHAPTER 1 The Thorax
15
• Branches include:
• marginal branch along the lower border of the heart
• posterior interventricular (posterior descending)
branch, which runs forward in the inferior interventricular groove to anastomose near the apex with the
corresponding branch of the le coronary artery.
Left Coronary Artery
• Arises from the le posterior aortic sinus.
• Larger than the right coronary artery.
• Main stem varies in length (4–10 mm).
• Passes behind and then to the le of the pulmonary trunk.
• Reaches the le part of the AV groove.
• Initially lies under cover of the le auricle where it
divides into two equally sized branches.
• Branches:
• anterior interventricular (le anterior descending):
runs down to the apex in the anterior interventricular
groove supplying the wall of the ventricles, to anastomose with the posterior interventricular artery
• circumex: continues round the le side of the heart
in the AV groove to anastomose with the terminal
branches of the right coronary artery.
• Occlusion of the le coronary artery will lead to rapid
demise.
Variations
• Le coronary and circumex arteries may be larger and
longer than usual and give o the posterior intraventricular artery before anastomosing with the right coronary artery, which is smaller than usual (known as ‘le
dominance’; occurs in 10% of population).
• Right and le coronary arteries may have equal contribution to posterior interventricular artery (known as
codominance; occurs in 10% of population).
• Le main stem may divide into three branches. e
third lies between the anterior interventricular and circumex arteries and may be large, supplying the lateral
wall of the le ventricle.
• In just under 60% of the population the SA node is supplied by the right coronary artery, while in just under
40% it is supplied by the circumex artery. In 3% it has
a dual supply.
• e AV node is supplied by the right coronary artery in
90% and the circumex in 10%.
Venous Drainage (see Fig. 1.9)
• Venae cordis minimae: tiny veins draining directly into
the chambers of the heart.
• Anterior cardiac veins: small, open directly into the
right atrium.
• Coronary sinus:
• main venous drainage
• lies in posterior AV groove
• opens into the right atrium just to the le of the
mouth of the IVC.
• Tributaries of coronary sinus:
• great cardiac vein: ascends in anterior interventricular groove next to anterior interventricular artery
• middle cardiac vein: drains posterior and inferior
surfaces of heart and lies next to the posterior interventricular artery
• small cardiac vein: accompanies marginal artery and
drains into termination of coronary sinus.
Nerve Supply of Heart
• Sympathetic (cardioaccelerator).
• Vagus (cardioinhibitor).
Clinical Point
• Cardiac pain is experienced not only in the chest but is
referred down the inner side of the le arm and up to
the neck and jaw. Cardiac pain is referred to areas of the
body surface which send sensory impulses to the same
level of the spinal cord that receives cardiac sensation.
e sensory bres from the heart travel through the
cardiac plexus, sympathetic chain and up to the dorsal
root ganglia of T1–4. Excitation of spinothalamic tract
cells in the upper thoracic segments contribute to the
anginal pain experienced in the chest and inner aspect
of the arm via dermatomes T1–4. Cardiac vagal aerent
bres synapse in the nucleus of the tractus solitarius of
the medulla and then descend to excite upper cervical
spinothalamic tract cells. is innervation contributes
to the angina pain experienced in the area of the neck
an d jaw.
PERICARDIUM
Fibrous
Heart and roots of the great vessels are contained within
the conical brous pericardium.
• Apex: fuses with adventitia of great vessels about 5 cm
from the heart.
• Base: fuses with central tendon of the diaphragm.
Relations
• Anterior sternum: third to sixth costal cartilages,
thymus, anterior edges of lungs and pleura.
• Posterior: oesophagus, descending aorta, T5–8 vertebrae.
• Lateral: roots of lung, phrenic nerves, mediastinal
pleura.
16
pulmonary
Superior vena cava
Aorta
Left pulmonary
mediastinum
mediastinum
SECTION I Anatomy
Serous
• e brous pericardium is lined by a parietal layer of
serous pericardium.
• e parietal layer is reected to cover the heart and roots
of great vessels to become continuous with visceral layer
of serous pericardium.
Oblique and Transverse Sinuses (Fig. 1.10)
At the pericardial reections, veins are surrounded by one
sleeve of pericardium and arteries by another.
Transverse Sinus
• Lies between the aorta and pulmonary trunk in front,
and the SVC and le atrium behind.
Oblique Sinus
• Bounded by the pulmonary veins.
• Forms a recess between pericardium and le atrium.
Clinical Points
• Fibrous pericardium can stretch gradually if there is
gradual enlargement of the heart.
• Sudden increase in pericardial contents as in sudden
bleeds: stretching does not occur and cardiac function
is embarrassed (cardiac tamponade).
Surface Anatomy of Heart
• Superior: line from second le costal cartilage 1.2 cm
from sternal edge to third right costal cartilage, 1.2 cm
from sternal edge.
• Inferior: line from the sixth right costal cartilage 1.2 cm
from sternal edge to h le intercostal space, 9 cm
from midline (i.e. position of apex beat).
• Le border: curved line joining second le costal cartilage 1.2 cm from sternal edge to h le intercostal
space, 9 cm from midline.
• Right border: curved line joining third right costal
cartilage 1.2 cm from sternal edge to sixth right costal
cartilage, 1.2 cm from sternal edge.
MEDIASTINUM (Fig. 1.11)
e space between the two pleural cavities is called the
mediastinum. It is divided into:
• superior mediastinum
• anterior mediastinum
• middle mediastinum
• posterior mediastinum.
Superior Mediastinum
Boundaries are:
• anterior: manubrium sterni
• posterior: rst four thoracic vertebrae
• above: continues up to root of neck
• below: continues with inferior mediastinum at level of
horizontal line drawn through angle of Louis.
Pulmonary trunk
Tranverse sinus
Right
veins
Inferior vena cava
Fig. 1.10 The posterior surface of the pericardial
veins
Oblique sinus
cavity after removal of the heart. The reflection of
the pericardium around the great vessels is shown.
(From Rogers AW. Textbook of Anatomy. Churchill
Livingstone, Edinburgh, 1992, with permission.)
1
2
3
Angle of
Louis
Anterior
mediastinum
Middle
Diaphragm
4
5
6
7
8
9
10
11
12
Fig. 1.11 The divisions of the mediastinum.
Superior
mediastinum
Posterior
CHAPTER 1 The Thorax
Body of sternum
Descending aorta
TracheaBody of T5 vertebraOesophagus
Superior vena cava
17
Contents:
• lower end of trachea
• oesophagus
• thoracic duct
• aortic arch
• innominate artery
• part of carotid and subclavian arteries
• innominate veins
• upper part of SVC
• phrenic and vagus nerves
• le recurrent laryngeal nerves
• cardiac nerves
• lymph nodes
• remnants of thymus gland.
Anterior Mediastinum
Boundaries are:
• anterior: sternum
• posterior: pericardium.
Contents:
• part of the thymus gland in children
• anterior mediastinal lymph nodes.
Middle Mediastinum
Boundaries are:
• anterior: anterior mediastinum
• posterior: posterior mediastinum.
Contents:
• heart
• great vessels
• phrenic nerves
• pericardiophrenic vessels.
Posterior Mediastinum
Boundaries are:
• anterior: pericardium, roots of lungs, diaphragm
below
• posterior: vertebral column from lower border of fourth
to twelh vertebrae
• above: horizontal plane drawn through the angle of
Louis
• below: diaphragm.
Contents:
• descending thoracic aorta
• oesophagus
• vagus and splanchnic nerves
• azygos vein
• hemiazygos vein
• thoracic duct
• mediastinal lymph nodes.
Fig. 1.12 shows some of the structures in the anterior,
middle and posterior mediastinum.
Thymic residue
in anterior
mediastinal fat
Ascending aorta
Azygos vein
Subscapularis
Fig. 1.12 Contrast CT at the level of the fifth thoracic vertebra showing some of the structures in the anterior,
middle and posterior mediastinum.
Pulmonary trunk
Left pulmonary
artery
Scapula
Infraspinatus
18
Left common carotid artery
Recurrent laryngeal
Right vagus nerve
Azygos vein
SECTION I Anatomy
Oesophagus
Left subclavian artery
Left vagus nerve
Arch of aorta
Oesophagus
Trachea
Sympathetic chain
Superior
vena cava
nerve
Left phrenic nerve
Descending
thoracic aorta
Fig. 1.13 The mediastinum seen from the left side.
The Mediastinal Surfaces (Figs. 1.13 and 1.14)
Because of the arrangements of structures in the mediastinum, it appears dierently when viewed from le and right
sides.
The Angle of Louis
e angle of Louis (manubriosternal junction) is an important anatomical landmark. It corresponds to the plane of
OSCE SCENARIOS
Right
phrenic
nerve
Fig. 1.14 The mediastinum seen from the right side.
T4, which is an important landmark. e following occur
at T4:
• commencement and termination of aortic arch
• bifurcation of trachea
• junction of superior and inferior mediastinum
• second costosternal joint
• conuence of azygos vein with superior vena cava
• thoracic duct runs from right to le
• ligamentum arteriosum lies on this plane.
OSCE Scenario 1.1
A 19-year-old male is admitted with a right-sided spontaneous pneumothorax. He has a past history of a treated
coarctation of the aorta. He requires a chest drain.
1. Describe the anatomy of a typical intercostal space.
2. Why is this knowledge important in your technique of
insertion of an intercostal drain?
3. What is the ‘triangle of safety’ when inserting a chest
drain?
4. Explain the anatomical basis for notching of the lower
border of a rib seen on a chest X-ray of a patient with
coarctation of the aorta.
OSCE Scenario 1.2
A 35-year-old male sustains a crushing upper abdominal
injury in a road trac accident. On admission to A&E he
has a tachycardia of 120 and a systolic blood pressure of
90 mmHg. He is complaining of abdominal and bilateral
shoulder tip pain. Urgent CT scan reveals liver and splenic
trauma as well as a ruptured le hemidiaphragm.
1. Describe the three origins of the muscular part of the
diaphragm.
2. At what vertebral levels do the oesophagus and the IVC
pass through the diaphragm?
3. What is the nerve supply of the diaphragm?
CHAPTER 1 The Thorax
19
4. Explain why in some cases irritation of the diaphragm
may result in referred pain to the shoulder while in others it may result in referred pain to the abdomen.
1. Describe the surface anatomy of the heart.
2. Why does cardiac tamponade result in drop in the blood
pressure and clinical shock?
3. Describe how you would treat a cardiac tamponade.
OSCE Scenario 1.3
A 60-year-old female undergoes a right open nephrectomy via a loin approach through the bed of the twelh
rib. A postoperative chest X-ray shows a small right
pneumothorax.
1. Describe the surface anatomy of the pleura.
2. Why has this patient developed a right pneumothorax?
3. At which other site, other than surgery on the thorax,
may surgery or trauma result in a pneumothorax?
OSCE Scenario 1.5
An 18-month-old girl developed sudden-onset bouts of
cough and wheezes. A bowl of peanuts was found nearby
while she was playing unwitnessed. She was rushed to A&E
and found to be conscious but distressed, tachypnoeic and
wheezy. A chest X-ray revealed a collapsed lung.
1. In which main bronchus a foreign body is more likely to
be dislodged and why?
2. In relation to the surface anatomy, where does the tra-
OSCE Scenario 1.4
A 22-year-old male is brought to A&E with a penetrating
chea commence and terminate?
3. Describe briey how you would treat the patient.
injury in the le third intercostal space, anterior to the midaxillary line. His blood pressure is 80/40, pulse rate 140
Answers in Appendix pages 431–433
beats/min and has mued hear sounds and distended neck
veins. A diagnosis of cardiac tamponade is established.
Please check your eBook at https://studentconsult.inkling.com/ for more self-assessment questions. See inside cover for
registration details.
2
Данная книга находится в списке для перевода на русский язык сайта https://meduniver.com/
The Abdomen, Pelvis and Perineum
DEVELOPMENT
Development of the Gut
e gut develops from a primitive endodermal tube. It is
divided into three parts:
• foregut: extends to the entry of the bile duct into the
duodenum (supplied by the coeliac axis)
• midgut: extends to distal transverse colon (supplied by
superior mesenteric artery)
• hindgut: extends to ectodermal part of anal canal (supplied by inferior mesenteric artery).
Foregut
• Starts to divide into the oesophagus and the laryngotracheal tube during the 4th week.
• If it fails to do so correctly, there may be pure oesophageal atresia (8% of cases), or atresia associated with
tracheo-oesophageal stula (the commonest, 80% of
cases), the stula being between the lower end of the
trachea and the distal oesophagus (Fig. 2.1).
• Distal to the oesophagus, the foregut dilates to form the
stomach.
• Rotates so that the right wall of the stomach now
becomes its posterior surface, forming the lesser sac
behind.
• Vagus nerves rotate with the stomach so that the right
vagus nerve becomes posterior and the le anterior.
• As the stomach rotates to the le, so the duodenum
swings to the right, its mesentery fusing with the peritoneum of the posterior abdominal wall, leaving all but
the rst inch retroperitoneal.
Midgut (Fig. 2.2)
• Enlarges rapidly in early fetal life, becoming too big for
the developing abdominal cavity, and herniates into the
umbilical cord.
• e apex of the herniated bowel is continuous with the
vitellointestinal duct into the yolk sac.
• While the midgut is within the cord it rotates 90° counterclockwise around the axis of the superior mesenteric
A
Fig. 2.1 Types of oesophageal atresia. (A) Oeso-
pha geal atresia with distal tracheo-oesophageal
fistula—most common type, with an incidence of
80%. (B) Isolated oesophageal atresia—second commonest, with an incidence of about 8%.
artery, bringing the third and fourth parts of the duodenum across to the le of the midline behind the superior mesenteric artery; this part of the duodenum is now
xed retroperitoneally.
• e midgut returns to the abdomen at the 10th week
and during this time it continues to rotate counterclockwise through a further 180°, bringing the ascending
colon to the right side of the abdomen with the caecum
lying immediately below the liver.
• e caecum descends into its denitive position in the
right iliac fossa, pulling the colon with it.
• e mesenteries of the ascending and descending colon
blend with the posterior abdominal wall, except for the
sigmoid colon, which retains a mesentery.
Clinical Points
• In early fetal life, growth obliterates the lumen of the
developing gut. It then recanalizes. If recanalization is
incomplete, areas of atresia or stenosis may result.
• e communication between the primitive midgut and
yolk sac may persist as a Meckel’s diverticulum. is
B
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