From transverse processes of cervical vertebrae to the upper 2 ribs, the paravertebral muscles stretch out. They can, therefore, either bend the cervical part of the vertebral column laterally or elevate these ribs. These muscles create a thick mass behind the prevertebral fascia and supplied by twigs from ventral rami of the lower 5 or 6 cervical spinal nerves. The lateral vertebral group of muscles is composed of scalene muscles. The scalene muscles, generally 3 in number, are as follows:
- Scalenus posterior.
- Scalenus medius.
- Scalenus anterior.
The scalenus medius is the largest and scalenus poste-rior is the smallest.
It’s smallest and most deeply situated lateral vertebral muscle.
- From posterior tubercles of the transverse processes of C4, C5 and C6 vertebrae
- On to the outer surface of second rib, behind the tubercle for serratus anterior
- By ventral rami of the lower 3 cervical (C6, C7 and C8) spinal nerves
- When the 2nd rib is fixed, it bends the cervical vertebral column to precisely the same side
- When upper connection is fixed, it is helpful to elevate the 2nd rib and so functions as an accessory muscle of respiration.
It’s longest and largest of all the scalene muscles.
- From posterior tubercles of the transverse processes of C2 C6 cervical vertebrae.
- On to the upper surface of the very first rib between the tubercle of the rib and groove for subclavian artery.
- By ventral rami of C3 C8 spinal nerves.
- When upper end is fixed, it elevates the first rib and thereby functions as an accessory muscle of respiration.
It’s most superficial and is located deep to sternocleidomastoid muscle. It’s the key muscle in the root of neck due to its close connections to a lot of structures in this region. It gives a useful surgical landmark.
- From anterior tubercles of transverse processes of C3, C4, C5 and C6 vertebrae (i.e., all typical vertebrae)
- The fibres converge and descend just about vertically to be added by a narrow, flat tendon to the scalene tubercle on the inner border of the 1st rib and to the ridge on the upper surface of the rib anterior to the groove for the subclavian artery.
- By ventral rami of C4, C5 and C6 spinal nerves.
- Acting from below, it flexes the neck forwards and laterally and rotates it to the opposite side.
- Acting from above, it is helpful to elevate the 1st rib and so functions as an accessory muscle of respiration.
Relationships of the Scalenus Anterior Muscle
The scalenus anterior muscle creates an essential landmark in the root of the neck because
- phrenic nerve enters superficial to it
- subclavian artery deep to it and
- brachial plexus is located at its lateral border
For that reason the connections of scalenus anterior muscle are extremely significant. Anterior connections: Phrenic nerve runs downward across the anterior outermost layer of the muscle deep to prevertebral fascia.
- 2 arteries – transverse cervical and suprascapular- cross the anterior surface of muscle from medial to lateral side.
- 2 veins – Anterior jugular vein crosses the muscle from above downwards and Subclavian vein crosses the anterior surface near insertion from lateral to medial side.
- 2 muscles – Inferior belly of omohyoid crosses the anterior surface from medial to lateral side and Sternocleidomastoid (clavicular head) overlaps the muscle.
- Carotid sheath is located vertically in front of the muscle.
- Clavicle crosses from medial to lateral side.
- Roots of brachial plexus divide the scalenus anterior from scalenus medius.
- Subclavian artery enters deep to scalenus anterior near its insertion from medial to lateral side, dividing it from scalenus medius.
- Scalenus medius muscle (medial part).
- Cervical pleura.
- Suprapleural membrane.
- The medial border of scalenus anterior creates the lateral boundary of the scalenovertebral triangle or triangle of the vertebral artery. The contents of the triangle create the medial connections of the scalenus anterior.
- The lateral border of the scalenus anterior is related to trunks of brachial plexus, which appear underneath it.
- The upper part of scalenus anterior is split from longus capitis by the ascending cervical artery.
Scalene (Subclavian) Triangle
It’s a narrow triangular gap above the 1st rib between scalenus anterior and scalenus medius. The subclavian artery and brachial plexus go through this space.
It is composed of group of signs and symptoms created as a result of compaction of lower trunk of brachial plexus (C8 and T1) and subclavian artery in the scalene triangle either because of lifting of its base by the cervical rib, if present, or because of spasm of scalene muscles. Medically, this syndrome presents as:
- tingling sensation and numbness along the inner border of forearm and hand, i.e., along the supply of C8 and T1 spinal nerves,
- advancing paresis and wasting of intrinsic muscles of the hand (majority of them are supplied by C8 and T1 spinal nerves) and
- Ischemic pain and absence of radial pulse because of compaction of subclavian artery.
Scalenovertebral Triangle (Triangle of Vertebral Artery)
It’s a deeply placed triangular space in the root of the neck between scalenus anterior laterally and longus colli (cervicis) medially. The cervical pleura and apex of lung project upward into this space.
- Medial: Lower oblique part of the longus colli.
- Lateral: Medial border of scalenus anterior.
- Apex: Transverse process of C6 vertebra.
- Base: First part of the subclavian artery.
- Floor (posterior wall): From above downwards, it’s created by: transverse process of C7 vertebra, Ventral ramus of C8 spinal nerve, which enters laterally above the neck of first rib, neck of first rib and cupola of cervical pleura.
- Roof: It’s created by carotid sheath.
- First part of vertebral artery and accompanying veins.
- The artery stretches upwards vertically from the base to goes into the foramen transversarium of C6 at the apex.
- Thyrocervical trunk and inferior thyroid artery; thelater arches medially across the apical part of the triangle.
- Sympathetic chain with stellate ganglion is located posterolat-eral to vertebral artery.
- Thoracic duct on the left side arches laterally at the level of transverse process of C7 vertebra.
- Ansa subclavia.
- Scalenus minimus (pleuralis): The scalenus minimus is the fourth vestigial scalene muscle.
- It appears from the anterior border of the transverse process of C7 vertebra and fit to the inner border of the 1st rib supporting the groove for the subclavian artery and to the dome of the cervical pleura.
- The suprapleural membrane is regarded as the flattened tendon of the muscle.
- The contraction of scalenus minimus pulls up the dome of pleura to ensure it is anxious.