Chest Wall, Diaphragm and Mechanisms of Ventilation

Question 1

[4-minute video]: the Diaphragm

Answer 1

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Question 2

[4-minute video]: anterior and posterior intercostal arteries

Answer 2

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Question 3

[9-minute video]: Cadaveric Demonstration of the Thoracic Wall Muscles and the Internal Thoracic Vessels

Answer 3

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Question 4

The thorax is the upper part of trunk, which extends from the root of neck to the abdomen. The cavity of trunk is divided by the _______________ into an upper thoracic cavity and a lower abdominal cavity.

Answer 4

diaphragm

Further notes:
✓ The thoracic cavity contains the principal organs of respiration– the lungs, which are separated from each other by bulky and movable median septum – the mediastinum. The principal structures in the mediastinum are heart and great vessels.
✓ The thorax is supported by a skeletal framework called thoracic cage. It provides attachment to muscles of thorax, upper extremities, back, and diaphragm. It is osteocartilaginous and elastic in nature. It is primarily designed for increasing or decreasing the intrathoracic pressure so that air is sucked into lungs during inspiration and expelled from lungs during expiration—an essential mechanism of respiration.

[Diagram]

Question 5

Outline the skeletal organisation of the thoracic cage.

Answer 5

It is formed by:
☛ sternum (breast bone) anteriorly
☛ 12 thoracic vertebrae and intervening intervertebral discs posteriorly
☛ 12 pairs of ribs and associated 12 pairs of costal cartilages laterally

Question 6

How do the ribs articulate with the other components of the rib cage?

Answer 6

Posteriorly:
➣ All the ribs articulate with the thoracic vertebrae
Anteriorly:
➣ The upper seven ribs articulate with the side of sternum through their costal cartilages (True ribs)
➣ The next three ribs articulate indirectly with the sternum by attaching onto the costal cartilage of the 7th rib through their costal cartilages (False ribs)
➣ The lower two ribs do not articulate and anterior ends of their costal cartilages are free (Floating ribs)

Question 7

The narrow upper end of the thoracic cage is continuous above with root of neck from which it is partly separated on either side by the ____(a)____. The broad lower end is completely separated from the abdominal cavity by the ____(b)____, but provides passage to structures like aorta, oesophagus,and inferior vena cava.

Answer 7

(a) suprapleural membranes/Sibson’s fascia
(b) diaphragm

Note:
The diaphragm is dome shaped with its convexity directed upwards. Thus, the upper abdominal viscera lies within the thoracic cage and are protected by it.

Question 8

Name the three openings of the diaphragm and state the level at which they occur as well as the structures traversing them.

Answer 8

Aortic hiatus [T12]t
descending aorta, thoracic duct, azygos vein

Esophageal hiatus [T10]
esophagus, vagus trunks, esophageal arteries

Caval opening [T8]
inferior vena cava, terminal branches of the right phrenic nerve (as it goes to supply the biliary structures)

Question 9

Describe the surface landmarks of the thorax.

Answer 9

Bony landmarks
✓ suprasternal notch — at the level of T2
✓ sternal angle — at the level of the 2nd rib
✓ xiphisternal joint — at the level of T9 vertebra
✓ costal margin — lowest point lies at the level of L3 vertebra
✓ subcostal angle
✓ thoracic vertebral spines

Soft tissue landmarks
nipple — at the level of the 4th intercostal space

[4-minute video]: bony landmarks of the thorax

Question 10

Describe the unique features of the first rib.

Answer 10

➣ It is wide and short, with a broad and flat shape.
➣ Unlike other ribs, it lacks a distinct angle.
➣ Its head articulates with the body of the first thoracic vertebra (T1) via a single articular facet.
➣ The first rib has two tubercles:
✓ Transverse Tubercle: Located posterior and lateral to the neck, it bears an articular facet for the transverse process of T1.
✓ Scalene Tubercle (Lisfranc Tubercle): Anteriorly, where the anterior scalene muscle inserts.

[Image: Typical Rib] [Image: First Rib] [8-minute video]: Anatomy of the First Rib

Question 11

State the neurovascular relations of the neck of the first rib.

Answer 11

1. first posterior intercostal vein
2. sympathetic chain
3. superior intercostal artery
4. T1 nerve root
5. [Diagram]

Question 12

State with reasons, the ribs that are most commonly fractured.

Answer 12

7th through 10th Ribs:
➣ These ribs are frequently involved in fractures due to their anatomical position.
➣ They are located in the middle of the rib cage, making them more susceptible to direct impacts from falls, car accidents, or contact sports.

Question 13

Define a flail chest and give its anatomical basis.

Answer 13

When a rib is fractured twice, it is called a floating rib due to the free fracture fragment.
If three or more contiguous floating ribs are present, it results in a condition known as a flail chest. [Diagram]

Question 14

What is a sternal puncture?

Answer 14

A sternal puncture is a medical procedure that involves sampling bone marrow from the anterior wall of the sternum (breastbone).

Question 15

Briefly describe the side determination and anatomical position of the ribs.

Answer 15

The side of the rib can be determined by holding it in such a way that its posterior end having head, neck, and tubercle is directed posteriorly, its concavity faces medially and its sharp border is directed inferiorly.
In an anatomical position, the posterior end is higher and nearer the median plane than the anterior end.

Question 16

List the imaginary lines of orientation on the chest wall.

Answer 16

(1) Midsternal line: It runs vertically downwards in the median plane on the anterior aspect of the sternum.
(2) Midclavicular line: It runs vertically downwards from
the midpoint of the clavicle to the midinguinal point. It
crosses the tip of the 9th costal cartilage.
(3) Anterior axillary line: It runs vertically downwards from
the anterior axillary fold.
(4) Midaxillary line: It runs vertically downwards from the point in the axilla located between the anterior and posterior axillary folds.
(5) Posterior axillary line: It runs vertically downwards from the posterior axillary fold.
(6) Scapular line: It runs vertically downwards on the posterior aspect of the chest passing through the inferior angle of the scapula with arms at the sides of the body.
[Diagram 1] [Diagram 2]

Question 17

Describe the sensory innervation of the thorax.

Answer 17

The skin above the horixontal line drawn at the level of the sternal angle is supplied by supraclavicular nerves (C3 and C4).
The skin below this horizontal line is supplied by anterior and lateral cutaneous branches of the 2nd - 6th intercostal nerves (T2 - T6).

Further notes:
➣ The anterior rami of C5–T1 innervate the skin of the upper limb.
➣ The cutaneous innervation on the back of thorax (on either side of midline for about 5 cm) is provided by posterior rami of thoracic spinal nerves.

Question 18

Why is the superficial fascia more dense on the posterior aspect of the chest?

Answer 18

To sustain the pressure of the body when lying in the supine position.

Further note:
The superficial fascia on the front of the chest contains breast (mammary gland), which is rudimentary in males and well-developed in adult females.

Question 19

Why is the deep fascia thin and ill-defined except in the pectoral region?

Answer 19

To allow free movement of the thoracic wall during breathing.

Question 20

The thoracic wall is liberally covered by extrinsic muscles. Name them.

Answer 20

1. Muscles of upper limb:
   (a) Pectoralis major and pectoralis minor muscles cover the front of thoracic wall.
   (b) Serratus anterior covers the side of thoracic wall.
2. Muscles of abdomen: Rectus abdominis and external oblique covers the lower part of the front of thoracic wall.
3. Muscles of back:
   (a) Trapezius and latissimus dorsi.
   (b) Levator scapulae, rhomboideus major and minor.
   (c) Serratus—posterior, superior, and inferior.
   (d) Erector spinae.

Further notes:
➣ The thoracic wall is more or less completely covered by extrinsic muscles except in the anterior and posterior median lines.
➣ On the back, the thoracic wall is thinly covered by musculature in the region of triangle of auscultation.

Question 21

Briefly discuss the triangle of auscultation.

Answer 21

Definition and boundaries:
It is a small triangular gap in the musculature on the back of the thorax near the inferior angle of scapula.
➣ Its boundaries are:
~ superior horizontal border of the latissimus dorsi
~ medial border of scapula
~ inferolateral border of the trapezius
➣ The floor of this triangle is formed by 6th and 7th ribs and the 6th intercostal space
[40-second video] [Image]

Clinical relevance:
➣ The upper part of the lower lobe of the lung lies deep to this triangle.
➣ When the scapulae are drawn anteriorly by folding the arm across the chest and the trunk is flexed, this triangle enlarges and becomes more subcutaneous. Now the lower lobe of the lung can be auscultated by putting the stethoscope in this region.
➣ The sounds are not muffled by the muscles of back in this area.

Question 22

The intrinsic muscles of the thoracic wall are arranged in three layers from superficial to deep. Name the muscles and the corresponding layers.

Answer 22

(1) External intercostal muscle (Superficial layer)
(2) Internal intercostal muscle (intermediate layer)
(3) Transversus thoracis muscle (deep layer)

Further notes:
➣ The muscle layer is lined by the endothoracic fascia, which in turn is lined by the parietal pleura.
➣ These three layers of muscles are comparable to the three layers of muscles in the abdominal wall.
➣ The intercostal nerve and vessels form neurovascular bundle lie between the intermediate and deep layer (neurovascular plane)
➣ In addition to the above-mentioned three intrinsic muscles there is another set of muscles called levatores costarum.
[Diagram]

Question 23

External intercostal muscle (11 pairs).
(1) Origin
(2) Insertion
(3) Extent
(4) Direction of fibers
(5) Nerve supply
(6) Action

Answer 23

(1) Origin: Lower border of rib above
(2) Insertion: Upper border (outer lip) of rib below
(3) Extent: From costochondral junction to tubercle of rib (anteriorly it continues as anterior intercostal membrane)
(4) Direction of fibers: Downwards, forwards, and medially (inferomedially)
(5) Nerve supply: Intercostal nerve of the same space
(6) Action: Elevates the rib during inspiration
(7) [44-second demonstration: External and internal intercostal muscles]
(8) [12-minute video: Muscles of Thoracic Wall]

Question 24

Internal intercostal muscle (11 pairs)
(1) Origin
(2) Insertion
(3) Extent
(4) Direction of fibers
(5) Nerve supply
(6) Action

Answer 24

(1) Origin: Floor of the costal groove of the rib above
(2) Insertion: Upper border (inner lip) of rib below
(3) Extent: From lateral border of sternum to the angle of rib (posteriorly it continues as posterior intercostal membrane)
(4) Direction of fibers: Upwards, forwards and medially (superomedially)
(5) Nerve supply: Intercostal nerve of same space
(6) Action: Depresses the rib during expiration
(7) [44-second demonstration: External and internal intercostal muscles]
(8) [12-minute video: Muscles of Thoracic Wall]

Question 25

The transverse thoracic muscle is divided into 3 parts; intercostalis intimus, subcostalis, sternocostalis.

Intercostalis intimus (innermost intercostal):
(1) Origin
(2) Insertion
(3) Extent
(4) Direction of fibers
(5) Nerve supply
(6) Action

Answer 25

(1) Origin: Inner surface of rib above
(2) Insertion: Inner surface of rib below
(3) Extent: Confined to the middle 2/4th of the intercostal space
(4) Direction of fibers: Upwards, forwards and medially
(5) Nerve supply: Intercostal nerve of same space
(6) Action: Depresses the rib during expiration
(7) [12-minute video: Muscles of Thoracic Wall]

Question 26

Subcostalis
(1) Origin
(2) Insertion
(3) Extent
(4) Direction of fibers
(5) Nerve supply
(6) Action

Answer 26

(1) Origin: Inner surface of rib near angle
(2) Insertion: Inner surface of 2nd or 3rd ribs below
(3) Extent: Confined to posterior parts of lower spaces only
(4) Direction of fibers: Upwards, forwards and medially
(5) Nerve supply: Intercostal nerves
(6) Action: Depressor of ribs
(7) [12-minute video: Muscles of Thoracic Wall]

Question 27

Sternocostalis
(1) Origin
(2) Insertion
(3) Extent
(4) Direction of fibers
(5) Nerve supply
(6) Action

Answer 27

(1) Origin:
– Lower 1/3rd of the posterior surface of the body of sternum
– Posterior surface of xiphoid process
– Posterior surface of costal cartilages of lower 3 or 4 ribs near sternum
(2) Insertion: Costal cartilages 2nd to 6th ribs
(3) Extent: Inner surface of front wall of chest
(4) Direction of fibers: Upwards and laterally
(5) Nerve supply: Intercostal nerves
(6) Action: Draws 2nd to 6th cartilages downwards
(7) [12-minute video: Muscles of Thoracic Wall]

Question 28

Levatores costarum (12 pairs)
(1) Origin
(2) Insertion
(3) Actions

Answer 28

These are a series of 12 small muscles placed on either side of the back of thorax, just lateral to the vertebral column.

(1) Origin: tip of transverse process from 7th to 11th thoracic vertebrae.
(2) Insertion: Each muscle passes obliquely downwards and laterally to be inserted on to the upper edge and outer surface of the rib immediately below in the interval between the tubercle and angle.
(3) Actions:
➣ Elevate and rotate the neck of rib in a forward direction.
➣ Are rotators and lateral flexors of the vertebral column
(4) [12-minute video]: muscles of Thoracic Wall, [levatores costarum]

Question 29

Why are the 3rd-6th intercostal spaces typical intercostal spaces?

Answer 29

This is because the blood and nerve supply of 3rd–6th intercostal spaces is confined only to thorax.

Further notes:
The spaces between the two adjacent ribs (and their costal cartilages) are known as intercostal spaces. Thus there are 11 intercostal spaces on either side.

Question 30

State the contents of a typical intercostal space.

Answer 30

1. Three intercostal muscles, viz.
   (a) External intercostal
   (b) Internal intercostal
   (c) Innermost intercostal (intercostalis intimus)
2. Intercostal nerves
3. Intercostal arteries
4. Intercostal veins
5. Intercostal lymph vessels and lymph nodes

Further notes:
➣ Plane of neurovascular bundle in the intercostal space: The neurovascular bundle consisting of intercostal nerve and vessels lies between the internal intercostal and innermost intercostal muscles, i.e., between the intermediate and deepest layers of muscles.
They are arranged in the following order from above downwards:
1. Intercostal Vein
2. Intercostal Artery
3. Intercostal Nerve
(Mnemonic: VAN)
➣ Strictly speaking, the intercostalis intimi is not present in the intercostal space as it lies on the deeper aspects of the ribs.

Question 31

The 12 pairs of thoracic spinal nerves supply the thoracic wall. As soon as they leave the intervertebral foramina they divide into anterior and posterior rami. The anterior primary rami of upper 11 thoracic spinal nerves (T1–T11) are called ____(a)____ as they course through the intercostal spaces. The anterior primary ramus of the 12th thoracic spinal nerve runs in the abdominal wall below the 12th rib, hence it is called ____(b)____.

Answer 31

(a) intercostal nerves
(b) subcostal nerve

Further notes:
Unique Features of Intercostal Nerves:
➣ The intercostal nerves are anterior primary rami of thoracic spinal nerves.
➣ They are segmental in character unlike the anterior primary rami from other regions of spinal cord which form nerve plexuses viz. cervical, brachial, lumbar and sacral.

Question 32

The intercostal nerves are classified into two groups. Name them, and state the basis of the classification.

Answer 32

1. Typical intercostal nerves (3rd, 4th, 5th, and 6th) - these are typical because they remain confined to their own intercostal spaces.
2. Atypical intercostal nerves (1st, 2nd, 7th, 8th, 9th, 10th, and 11th) - these are atypical because they extend beyond the thoracic wall and partly or entirely supply the other regions.

Question 33

State the branches of the typical intercostal nerve.

Answer 33

1. Rami communicantes: Each nerve communicates with the corresponding thoracic ganglion by white and grey rami communicantes.
2. Muscular branches: These are small tender branches from the nerve, which supply intercostal muscles and serratus posterior and superior.
3. Collateral branch: It arises in the posterior part of the intercostal space near the angle of the rib and runs in the lower part of the space along the upper border of the rib below in the same neurovascular plane. It supplies intercostal muscles, parietal pleura, and periosteum of the rib.
4. Lateral cutaneous branch: It arises in the posterior part of the intercostal space near the angle of the rib and accompanies the main nerve for some distance, then pierces the muscles of the lateral thoracic wall along the midaxillary line. It divides into anterior and posterior branches to supply the skin on the lateral thoracic wall.
5. Anterior cutaneous branch: It is the terminal branch of the nerve, which emerges on the side of the sternum. It divides into medial and lateral branches and supplies the skin on the front of the thoracic wall.
   [Diagram: Branches of typical intercostal nerve]
   [Diagram: Collateral nerves emerging from typical intercostal nerve]

Question 34

Atypical intercostal nerves
a) Which other area does the 1st intercostal nerve supply?
b) What other area does the 2nd intercostal nerve supply?
c) What other area does the 7th to 11th intercostal nerve supply?

Answer 34

a) The greater part of this nerve joins the ventral ramus C8 spinal nerve to form lower trunk of the brachial plexus.
b) Its lateral cutaneous branch is called intercostobrachial nerve. It courses across the axilla and joins the medial cutaneous branch of the arm. The intercostobrachial nerve supplies the skin of the floor of the axilla and upper part of the medial side of the arm.
c) Once they leave the intercostal spaces to enter the abdominal wall, they are called thoraco-abdominal nerves, and supply muscles of anterior abdominal wall, and skin and parietal peritoneum covering the outer and inner surfaces of the abdominal wall respectively.

Question 35

Considering the position of neurovascular bundle in the intercostal space, where is it safe to insert the needle?

Answer 35

A little above the upper border of the rib below.

Question 36

Each intercostal space contains one posterior and two anterior intercostal arteries (upper and lower). State the origin of the posterior intercostal arteries (there are 11 pairs of posterior intercostal arteries).

Answer 36

1. The 1st and 2nd posterior intercostal arteries are the branches of superior intercostal artery—a branch of the costocervical trunk (which part of subclavian artery?).
2. The 3rd–11th posterior intercostal arteries arise directly from the descending thoracic aorta.
3. [Diagram]

Question 37

State the terminal branches of the internal thoracic artery [aka. the internal mammary artery].

Answer 37

[The Internal Thoracic Artery terminates at the level of the sixth rib or the sixth intercostal space, dividing into two terminal branches:]
(a) Superior epigastric artery: this artery continues the course of the internal thoracic artery, travelling downward into the abdominal wall
(b) Musculophrenic artery: this artery roughly follows the costal margin.

Question 38

Describe the course and relations of the posterior intercostal arteries.

Answer 38

In front of the vertebral column:
✓ The right posterior intercostal arteries are longer than the left because the descending aorta lies on the left side of the front of the vertebral column. They pass behind the esophagus, thoracic duct, azygos vein, and sympathetic chain but in front of the anterior aspect of vertebral body.
✓ The left posterior intercostal arteries are smaller and pass behind the hemiazygos vein and sympathetic chain, but in front of the side of the vertebral body

In the intercostal space:
In the intercostal space, the posterior intercostal artery lies between the intercostal vein above and the intercostal nerve below. The neurovascular bundle in the intercostal space lies between the internal intercostal and intercostalis intimus muscles.

Question 39

Describe the termination of the posterior intercostal arteries.

Answer 39

Each posterior intercostal artery ends at the level of costochondral junction by anastomosing with the upper anterior intercostal artery of the space.
[Diagram]

Question 40

There are two anterior intercostal arteries in each intercostal space. They are present in the upper nine intercostal spaces only. State the origin and termination.

Answer 40

Origin:
1. In 1st–6th spaces they arise from the internal thoracic artery.
2. In 7th–9th spaces, they arise from musculophrenic artery.

Note: The 10th and 11th intercostal spaces do not extend forward enough to have anterior intercostal arteries.

Termination:
The anterior intercostal arteries are short and end at the level of costochondral junction as follows:
1. Upper anterior intercostal artery anastomoses with main trunk of corresponding posterior intercostal artery.
2. Lower anterior intercostal artery anastomoses with collateral branch of the corresponding posterior intercostal artery.

[Diagram]

Question 41

Intercostal veins just correspond to intercostal arteries. The only difference is the termination of posterior intercostal veins differ on the right and left sides. State the differences.

Answer 41

Both 1st (highest) posterior intercostal vein of both sides drain into their corresponding brachiocephalic veins.
➣ 2nd, 3rd, and 4th right posterior intercostal veins join to form right superior intercostal vein, which in turn drains into the azygos vein, while 2nd, 3rd, and 4th left posterior intercostal veins join to form left superior intercostal vein, which in turn drains into left brachiocephalic vein.
➣ 5th to 11th right posterior intercostal veins drain into the azygos vein while 5th to 8th left posterior intercostal veins drain into accessory hemiazygos vein, and 9th to 11th left posterior intercostal veins drain into hemiazygos vein.
➣ Right subcostal vein drains into the azygos vein while left subcostal vein drains into the hemiazygos vein

Question 42

The respiration consists of two alternate phases of ____(a)____ and ____(b)____, which are associated with alternate increase and decrease in the volume of thoracic cavity, respectively.

Answer 42

(a) inspiration
(b) expiration

NB: The average rate of respiration is 18 per minute in normal resting state of an adult. It is faster in children and slower in the elderly.

Question 43

During inspiration the volume of thoracic cavity increases, creating a negative intrathoracic pressure, consequently air is sucked into the lungs. An increase in capacity of thoracic cavity occurs vertically, anteroposteriorly, and transversely. State the factor responsible for increase in each of these diameters.

Answer 43

1. Vertical diameter increase; descent (contraction) of the diaphragm.
2. Anteroposterior diameter increase; pump-handle movement of the sternum [sternum moving forward and upward] (brought about by the elevation of vertebrosternal ribs).
3. Transverse diameter increase; bucket-handle movement of the vertebrochondral ribs (ribs move outwards like the bucket handle)

Note: Vertebrosternal ribs/true ribs, vertebrochondral ribs/false ribs

Question 44

What brings about expiration?

Answer 44

(a) elastic recoil of the alveoli of the lungs,
(b) relaxation of the intercostal muscles and the diaphragm,
and
(c) increase in the tone of the muscles of anterior abdominal wall.

Question 45

Explain the three types of respiration.

Answer 45

1. Quiet respiration: movements are normal.
2. Deep respiration: movements described for quiet respiration are increased. The 1st rib is elevated by scalene and sternocleidomastoid muscles.
3. Forced respiration: all movements are exaggerated. The scapula is fixed and elevated by trapezius, levator scapulae, rhomboideus major, and rhomboideus minor muscles, so that pectoral muscles and serratus anterior can raise the ribs.

Question 46

What muscles act during quiet respiration?

Answer 46

Inspiration
1. External intercostal muscles
2. Diaphragm

Expiration
Passive, no muscles

Question 47

What muscles act during deep respiration?

Answer 47

Inspiration
1. External intercostal muscles
2. Scalene muscles
3. Sternocleidomastoid
4. Levatores costarum
5. Serratus posterior superior
6. Diaphragm

Expiration
Passive, no muscles

Question 48

What muscles act during forced respiration?

Answer 48

Inspiration
1. All the muscles involved in deep inspiration (vide supra)
2. Levator scapulae
3. Trapezius
4. Rhomboids
5. Pectoral muscles
6. Serratus anterior

Expiration
1. Quadratus lumborum
2. Internal intercostal muscles
3. Transverse thoracis
4. Serratus posterior inferior

Question 49

Describe the boundaries and clinical significance of the triangle of safety.

Answer 49

Location: The triangle is situated in the axilla (armpit) with the arm abducted.
Apex: The apex of the triangle is the axilla itself.
Borders:
Anterior Border: Formed by the lateral border of the pectoralis major muscle.
Posterior Border: Formed by the lateral border of the latissimus dorsi muscle.
Inferior Border: Defined by a horizontal line from the nipple, commonly corresponding to the 5th intercostal space.
[Diagram]

Clinical Significance:
➣ The Triangle of Safety provides a safe zone for intercostal catheter (ICC) placement. Needles inserted within this area have the lowest rate of complications.
➣ Complications can occur if ICCs are placed outside this triangle, including lung perforation, damage to the pericardium, heart, and liver.
➣ Ultrasound guidance is recommended to enhance accuracy and safety during ICC placement.