Scalenus Anterior

• Origin:
  • Anterior Tubercles of the four ‘typical’ Cervical Vertebrae (C3 – C6) by 4 slender tendons of origin which lie end to end with those of Longus Capitis
• Muscle passes forwards, laterally and downwards to end in a narrow tendon
• Insertion:
  • Scalene Tubercle and adjacent ridge on the inner border and upper surface of the 1st Rib
• Nerve Supply:
  • Separate branches from the Anterior Rami of C4-C6 nerves
• Actions:
  • Assists in Flexion and Rotation of the Neck
  • Helps to stabilize the 1st Rib
  • Shows some electromyographic activity even in quiet respiration

Anterior Relations

• Phrenic Nerve
  • Passes vertically down across the obliquity of the muscle
  • Plastered on the muscle by the Prevertebral Fascia and a pad of fat lies in front of the Prevertebral Fascia
  • Nerve leaves the medial border of the muscle low down
  • Crosses anterior to the Subclavian Artery and its Internal Thoracic Artery Branch
  • Behind the Subclavian Vein
    • Occasionally, the Phrenic Nerve may pass in front of the Subclavian Vein or posterior to the Internal Thoracic Artery
  • Lies on the Suprapleural membrane
  • Passes medial to the Apex of the Lung
  • Crosses in front of the Vagus Nerve as it enters the Superior Mediastinum
• Ascending Cervical Artery
  • Branch of the Inferior Thyroid Artery or the Thyrocervical Trunk
  • Runs up on the Prevertebral Fascia medial to the Phrenic Nerve
• Transverse Cervical Artery and Suprascapular Artery
  • Anterior to the Prevertebral Fascia
  • Lie between the Anterior Scalene and the Carotid Sheath (Internal Jugular Vein)
• Subclavian Vein
  • Lies in a groove on the 1st Rib
    • Due to the slope, lies at a lower level than the insertion of Scalenus Anterior
  • Runs medially
  • Joins the Internal Jugular Vein at the medial border of Scalenus Anterior to form the Brachiocephalic Vein
    • Thoracic Ducton the left and the Right Lymphatic Duct on the right enters the angle of confluence of the 2 veins
  • Used for central venous lines:
    • Infraclavicular approach – from a point 2cm below the midpoint of the Clavicle along a line that passes behind the Clavicle towards the Jugular Notch of the Sternum
    • Needle pierces the Clavipectoral Fascia and enters the vein just behind the Fascia

Medial Relations

• Medial edge of Scalenus Anterior makes a pyramidal shape with the lateral border of the lower part of Longus Colli
• Prevertebral Fascia in front of these muscles attached to bone at their opposing margins
• No fascial roof across the Pyramidal Space
• Base of space: Subclavian Artery (1st Part) (lying on Suprapleural Membrane)
• Apex: Carotid (Chassaignac’s) Tubercle on the Transverse Process of C6
• Common Carotid Artery:
  • Medial to the Internal Jugular Vein:
  • Lies deep to Sternocleidomastoid immediately anterior to the Pyramidal Space
• Inferior Cervical Sympathetic (or Stellate ganglion):
  • Posterior to the Common Carotid Artery (and Carotid Sheath)
  • Vertebral Artery and Vein anterior to the ganglion
• Inferior Thyroid Artery
  • Arches medially in a bold curve whose upper convexity lies in front of the Apex of the Pyramidal Space (C6 Level)
  • Sympathetic Chain (usually the Middle Ganglion) in front of the Artery
• Thoracic Duct (or Right Lymphatic Duct)
  • At a lower level and further anterior
  • Makes a similar convexity behind the Carotid Sheath
  • Arches over the Lung Apex and Subclavian Artery
  • Enters the confluence of the Subclavian Vein and Internal Jugular Vein
• Vagus Nerve (slightly medial - can also be considered anterior)
  • Within the Carotid Sheath
  • Passes down anterior to the Subclavian Artery
    • On the right side, gives off the Right Recurrent Laryngeal Nerve
      • Hooks around the artery and passes upwards
  • Inclines posteriorly and run on the medial surface of the Apex of the Lung to enter the Superior Mediastinum
  • Internal Jugular Vein has Deep Cervical Lymph Nodes closely adjacent to it

Posterior Relations

• Separated from the Scalenus Medius by:
  • Subclavian Artery
  • Anterior rami of the lower Cervical and 1st Thoracic Nerves, i.e. Roots of Brachial Plexus
• Subclavian Artery(2nd Part)

Lateral Relations

• Trunks of the Brachial Plexus:
  • Emerge from the lateral border of Anterior Scalene
• Subclavian Artery (3rd Part) – see below
• Both the above lie behind the Prevertebral Fascia on the floor of the Posterior Triangle