Deep Muscles of the Thorax

• Purpose of the Deep Muscles of the Thorax


  the deep muscles of the thorax promote movements necessary for breathing

  breathing consists of 2 phases - inspiration + expiration - produced by cycic changes in the vol/pressure of the thoracic cavity
• Thoracic Muscles


  v short muscles

  most run only from one rib to the next

  they form 3 layers in the wall - superficial, intermediate + inferior
• Superficial Muscular Layer of the Thorax - External Intercostals


  the external incostal muscles form the more superficial layer

  they lift the rib cage, which increases the anterior to posterior + side-to-side dimensions

  for this reason, the external intercostals function during inspiration
• External Intecostals - Location & Structure

  11 pairs lie between ribs

  fibrs run obliquely (down + forward) from each rib to rib below

  in lower intercostal spaces, fibres r continous w external oblique muscle, forming part of abdominal wall

  origin - inferior border of rib above

  insertion - superior border of rib below

  
• External Intercostals - Action


  w first ribs fixed by scalene muscles, pull ribs towards one another to elevate rib cage

  aid in inspiration

  synergists of diaphragm
• Intermediate Layer of the Thoracic Muscular Layers - Internal Intercostals


  internal intecostal muscles form the intermediate muscle layer of the thorax

  they may aid forced expiration by depressing the rib cage
• Internal Intercostals - Location & Structure


  11 pais lie between ribs

  fibres run deep to and at right angles to those of external intercostals (run downward + posteriorly)

  lower intercostal muscles r continous w fibres of internal oblique muscle of abdominal wall

  
• Internal Intecostals - Action, Origins & Insertions

  origin - superior border of rib below

  insertion - inferior border of rib above (costal groove)

  w 12th ribs fixed by quadratus lumborum, muscles of posterior abdominal wall + oblique muscles of the abdominal wall, they pull ribs together + depress rib cage

  aid forced expiration

  antagonistic to external intercostals
• Deepest Muscle Layer of the Thoracic Wall


  the deepest muscle layer of the thoracic wall attaches to the internal surfaces of the ribs

  it has 3 discontinous parts

  they r small + their function is unclear
• Autonomic Contraction of the Diaphragm


  most important muscle of inspiration

  forms a muscular partition between the thoracic + abdominopelvic cavities

  in the relaxed state, the diaphragm is dome-shaped

  when it contracts, it moves inferiorly + flattens, increasing the vol of the thoracic cavity, which pulls air into the respiratory passageways

  the alternating rhythmic contraction + relaxation of the diaphragm also causes pressure changes in the abdominopelvic cavity below that facilitate the return of blood to the heart
• Forced Expiration


  forced breathing (as during exercise) calls into a play a no of other muscles that insert into the ribs

  e.g. during forced inspiration the scalene + sternocleidomastoid muscles of the neck help lift the ribs

  forced expiration is aided by muscles that pull the ribs inferiorly + those that push the diaphragm superiorly by compressing the abdominal wall muscles
• Diaphragm - Location & Structure


  broad muscle pierced by the aorta, inferior vena cava + oesophagus

  forms floor of thoracic cavity

  dome shaped in relaxed state

  fibres converge from margins of thoracic cage toward a boomerang-shaped central tendon

  
• Diaphragm - Origin & Insertion & Action


  origin - inferior, internal surface of rib cage + sternum, costal cartilages of last 6 ribs + lumbar vertabrae

  insertion - central tendon

  prime mover of inspiration

  flattens on contraction, increasing vertical dimensions of thorax