The back comprises the dorsal part of the neck and the torso (dorsal body cavity) from the occipital bone to the top of the tailbone.The muscles of the back can be divided in three main groups according to their anatomical position and function. The two spinotransversales muscles go upward and laterally via the spinous processes and ligamentum nuchae: The splenius capitis is a broad muscle attached to the occipital bone and mastoid process of the temporal bone.; The splenius cervicis is a narrow muscle attached to the transverse processes of the upper cervical vertebrae.; Independently, each muscle turns the head to one side that is. The interspinales and intertransversarii are small intrinsic muscles of the spine that are located between adjacent vertebrae, primarily in the lumbar and cervical regions. As their names indicate, the interspinales are located between the spinous processes, and the intertransversarii are located between the transverse processes (Figure 8-11).

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Muscles of the back can be divided into superficial, intermediate, and deep group.

Since the all the back muscles originate in embryo (fetus) form by locations other than the back, muscles in the superficial, as well as, intermediate groups, are extrinsic muscles. Anterior rami of spinal nerve innervate them.

Muscles associated and involved in movements of the upper limb belong to the superficial group. The intermediate group might perform a respiratory action and comprises of muscles connected to the ribs.

The deep group of muscles is called intrinsic muscles because they develop in the back. They are direct association with actions of the vertebral column and head and are supplied by posterior rami of spinal nerves.

Superficial Group of Back Muscles

Superficial Layer of Back Muscle

Superficial muscles of the back are located directly deep towards the skin along with superficial fascia. They are occasionally called the appendicular group as these muscles are mainly associated with activities of the appendicular skeleton. The superior part of the appendicular skeleton that includes clavicle, scapula, and humerus, is attached to the axial skeleton that consists of skull, ribs, and vertebral column by these muscles.

Trapezius

Origin: Spinous prominences of CVII to TXII, Superior nuchal line, external occipital protuberance, ligamentum nuchae.

Insertion: Lateral one third of clavicle, acromion, and spine of scapula.

Innervation: Motor stimulation is due to accessory nerve [XI], and sensory stimulation is due to sensory nerve endings C3 and C4.

Functions: During abduction of humerus above horizontal it assists in rotating the scapula; upper fibers help in elevation, middle fibers help in adduction, and lower fibers help in depression scapula.

Latissimus Dorsi

Origin: Spinous processes of TVII to LV and sacrum, iliac crest, ribs X to XII.

Insertion: Floor of intertubercular sulcus of humerus.

Innervation: Thoracodorsal nerve (C6 to C8).

Functions: Extends, adducts, and medially rotates humerus.

Levator Scapulae

Origin: Transverse processes of CI to CIV.

Insertion: Upper portion medial border of scapula.

Innervation: C3 toC4 and dorsal scapular nerve (C4, C5).

Functions: Elevates scapula.

Rhomboid Major

Origin: Spinous processes of TII to TV.

Insertion: Medial border of scapula between spine and inferior angle.

Innervation: Dorsal scapular nerve (C4, C5).

Functions: Retracts (adducts) and elevates scapula.

Rhomboid Minor

Origin: Lower portion of ligamentum nuchae, spinous processes of CVII and TI.

Insertion: Medial border of scapula at spine of scapula.

Innervation: Dorsal scapular nerve (C4, C5).

Functions: Retracts (adducts) and elevates scapula.

Intermediate Group of Back Muscles

Both Serratus posterior superior and Serratus posterior inferior move obliquely outwards via the vertebral column to connect to the ribs.

Serratus posterior superior is located deep to the rhomboid muscles, while serratus posterior inferior is located deep to the latissimus dorsi. Both serratus posterior muscles are attached to the vertebral column and associated structures medially and either descends like the fibers of the serratus posterior superior or else ascend the fibers of the serratus posterior inferior to connect to the ribs.

These two muscles elevate and depress the ribs. Segmental branches of anterior rami of intercostal nervesinnervate the serratus posterior muscles. Intercostal arteries give vascular supply to them.

Serratus Posterior Superior

Origin: Lower portion of ligamentum nuchae, spinous processes of CVII to TIII, and supraspinous ligaments.

Insertion: Upper border of ribs II to V just lateral to their angles.

Innervation: Anterior rami of upper thoracic nerves (T2 to T5).

Function: Elevates ribs II to V.

Serratus Posterior Inferior

Intermediate Layer of Back Muscles

Origin: Spinous processes of TXI to LIII and supraspinous ligaments.

Insertion: Lower border of ribs IX to XII just lateral to their angles.

Innervation: Anterior rami of lower thoracic nerves (T9 to T12).

Function: Depresses ribs IX to XII and may prevent lower ribs from being elevated when the diaphragm contracts.

Deep group of back muscles

The deep or intrinsic muscles of the back extend from the pelvis to the skull and are innervated by segmental branches of the posterior rami of spinal nerves. They consist of:

The extensors and rotators of the head and neck: the splenius capitis and cervicis (spinotransversales muscles).
The extensors and rotators of the vertebral column: the erector spinae and transversospinales. and
The short segmental muscles: the interspinales and intertransversarii.

The vascular supply to this deep group of muscles is through branches arteries like:

Vertebral
Deep cervical
Occipital
Transverse cervical
Posterior intercostal
Subcostal
Lumbar
Lateral sacral arteries

Spinotransversales Muscles

The two spinotransversales muscles go upward and laterally via the spinous processes and ligamentum nuchae:

The splenius capitis is a broad muscle attached to the occipital bone and mastoid process of the temporal bone.
The splenius cervicis is a narrow muscle attached to the transverse processes of the upper cervical vertebrae.

Independently, each muscle turns the head to one side that is the same side as the contracting muscle and collectively, the spinotransversales muscles move the head backward, elongating the neck.

Splenius Capitis

Origin: Lower half of ligamentum nuchae, spinous processes of CVII to TIV.

Insertion: Mastoid process, skull below lateral one third of superior nuchal line.

Innervation: Posterior rami of middle cervical nerves.

Function: Together, they draw head backward, extending the neck and individually, each one draws and rotates head to one side i.e. turns face to same side.

Levatores Costarum Interspinales And Intertransversarii