Thoracic Inlet and Thoracic wall

Learning Objectives:

After reading this post you will know:

  • Boundaries and structures passing through the thoracic inlet.
  • Contents of typical  intercostal space.
  • Intrinsic muscles of thoracic wall.
  • Arterial supply and venous drainage of thoracic wall.
  • Branches of a typical intercostal nerve.
  • Mechanism of respiration and muscles involve in inspiration and expiration.

Q. Write vertebral level of the following:

  • Jugular notch 
  • Sternal angle l
  • Xiphisternal joint
  • Tip of ninth costal cartilage
  • Lowest extent of costal margin

A. Verterbral levels:

Structure Vertebral level
Jugular notchLower border of T2 
Sternal angleLower border of T4
Xiphisternal jointLower border of T8
Tip of ninth costal cartilage L1
Lowest extent of costal margin L3

Q. Enumerate the:

  1. Structures forming boundaries of superior thoracic aperture (thoracic inlet)
  2. Structures passing through the thoracic inlet
  3. Contents of atypical intercostals space
  4. Intrinsic muscles of the thoracic wall.
  5. Branches of a typical intercostal nerve
  6. Arteries supplying thoracic wall
  7. Tributaries of azygous vein
  8. Structures at the level of sternal angle

 A. 1. Boundaries of superior thoracic aperture(thoracic inlet):

  • Posteriorly: first thoracic vertebra
  • Laterally : first pair ribs  and their costal cartilage .
  • Anteriorly: superior border of  manubrium sterni.

   2. Structures that pass through the superior thoracic aperture include:

  1. Trachea
  2. Oesophagus
  3. Thoracic duct
  4. Nerves
    • Vagus
    • Phrenic
    • Left recurrent laryngeal
    • Sympathetic trunks
  5. Vessels
    • Arteries
      • Left and right common carotid arteries
      • Left and right subclavian arteries
    • Veins
      • Internal jugular veins
      • Brachiocephalic veins
      • Subclavian veins
    • Lymph nodes and  lymphatic vessels

3.Contents of a typical intercostals space:

  1. Intercostal muscles ( external, internal and innermost intercostal)
  2. Intercostal nerve and its branches
  3. Anterior and posterior intercostal arteries
  4. Anterior and posterior intercostal veins.

 4. Intrinsic muscles of thoracic wall:

  1. External intercostals (outer layer)
  2. Internal intercostals ((intermediate layer)
  3. Innermost intercostals
  4. Sternocostalis
  5. Subcostalis

 5. Branches of a typical intercostals nerve (3rd – 6th intercostal nerves ):

  1. Two communicating branches (white and gray ramus) which connect it to the corresponding sympathetic ganglion.
  2. Muscular branches: Number of muscular branches that suppy intercostal muscles, subcostalis and sternocostalis.
  3. Collateral branch that supplies parietal pleura and periosteum of rib besides supplying intercostal muscles.
  4. Cutaneous branches:
    • Lateral cutaneous branch (reaches skin by piecing muscles at midaxillary line and divides into anterior and posterior branches)
    • Anterior cutaneous branch(reaches skin by piecing muscles about 1cm. lateral to the sternum and divides into medial and lateral branches).

   6. Arteries supplying thoracic wall:

  1. Superior intercostal artery ( a branch of costocervical trunk from the second part of subclavian trunk) gives posterior intercostals arteries first two intercostal spaces.
  2. Two anterior intercostal arteries for each intercostal space (in upper six intercostal spaces they are direct branches of internal thoracic artery(ITA) and in 7th , 8th & 9th  they aris from musculphrenic artery ,a branch of ITA, 10th and 11th intercostals spaces don’t have anterior intercostal arteries ).
  3. Posterior intercostal arteries: for 3rd to 11th intercostal space they arise from descending thoracic aorta.

7. Structures at the level of sternal angle:

  1. Ascending aorta ends
  2. Arch of aorta begins and ends
  3. Descending aorta begins
  4. Trachea bifurcates into right and left principal bronchii.
  5. Pulmonary trunk divides into right and left pulmonary arteries.
  6. Azygous veins drains into superior vena cava
  7. Upper border of heart lies at this level

Sternal angle(angle of Louis) which can be easily palpated is used for counting ribs. The second costal cartilage articulates with sternum at this level.

Q. Draw labeled diagram of the following:

  1. Structures passing through the thoracic inlet (transverse section)
  2. Typical intercostals space
  3. TS showing intercostal muscles and course & branches of intercostal nerve.
  4.  Tributaries of azygous vein.

Ans. 1.  Structures passing through the thoracic inlet (transverse section)

2. Typical intercostal space

3.TS  of intercostal space showing intercostal muscles and course & branches of intercostal nerve.

typical intercostal nerve

 

4. Tributaries of azygous vein

Q. Enumerate the factors responsible for the increase in various diameters of the thoracic cavity during  inspiration.

A.  During inspiration the volume of thoracic cavity increases to allow expansion of lungs. The following factors are responsible for increase in the diameters of thoracic cavity.

  • Vertical diameter is increased by contraction of diaphragm.
  • Anteroposterior diameter is increased by anterior movement of sternum due to elevation of vertebrosternal (1st-7th) ribs (Pump handle movement).
  • Transverse diameter is increased due to elevation of vertebrochondral (8th, 9th & 10th ) ribs ( bucket handle movement).

Q. Name the muscles that act during inspiration and expiration.

 InspirationExpiration
Quite respirationDiaphragmPassive (Diaphragm and intercostal muscles relax)
External intercostal muscles
Forced respitationDiaphragm and  intercostal musclesAnterior abdominal wall muscles
SternocleidomastoidInternal intercostal muscle
Scalene muscles (raise 1st and 2nd ribs)Serratus posterior muscles.
Levator scapulae, trapezius and rhomboid major & minor(fix and elevate scapula)
Quadratus lumborum (fixes last rib)
Pectoralis major (when the arm is fixed)

 

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