Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Musculoskeletal System
Muscular System
Muscles and Fascia of the Trunk - Muscles of the Back

Located posteriorly, anteriorly, and laterally to the spine, the trunk Muscles ensure its mobility as well as the Movements of the Ribs, HEAD, and pectoral and pelvic girdles, and contribute to forming the walls of the Body Cavities (thoracic, abdominal, and pelvic). According to their Location, they are subdivided into the Muscles of the back, chest, abdomen, and Perineum. All skeletal muscles, except for the Diaphragm and the epicranius Muscle, are paired.

The back muscles are paired, located on the posterior surface of the trunk along its entire length, and are divided into superficial and deep groups (Fig. 35).

The superficial muscles include the thin, broad trapezius muscle and latissimus dorsi. Lying beneath these muscles are the rhomboid and posterior serratus muscles. The trapezius muscle originates from the Occipital bone, nuchal ligament, and the spinous processes of all thoracic vertebrae. It inserts into the acromial end of the clavicle, the acromion, and the spine of the scapula. When the entire muscle contracts, the scapula is retracted toward the spine. Its upper part pulls the scapula upward and medially, while the lower part pulls it downward and medially. The latissimus dorsi originates from the iliac crest, the spinous processes of all lumbar vertebrae, and the lower six thoracic vertebrae. Extending upward and laterally, the muscle continues into a narrow tendon that inserts into the crest of the lesser tubercle of the humerus.

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Fig. 35. Back muscles:

1 — trapezius muscle; 2 — splenius capitis muscle; 3 — Major and minor rhomboid muscles; 4 — posterior inferior serratus muscle; 5 — thoracolumbar fascia; 6 — latissimus dorsi

In the Second layer of superficial muscles, located directly beneath the trapezius and latissimus dorsi, lie the major and minor rhomboid muscles, as well as the posterior superior and inferior serratus muscles. The rhomboid muscles originate from the spinous processes of the lower cervical vertebrae (minor rhomboid) and the upper four thoracic vertebrae (major rhomboid) and insert into the medial border of the scapula. Both muscles retract the scapula toward the spine and elevate it. The posterior superior serratus muscle lies beneath the rhomboids. It originates from the spinous processes of the lower two cervical and upper two thoracic vertebrae, runs laterally and downward, and inserts into ribs II–V. This muscle elevates ribs II–V, thus aiding in inspiration. The posterior inferior serratus muscle lies beneath the latissimus dorsi. It originates from the spinous processes of the lower two thoracic and upper two lumbar vertebrae and inserts into the lower four ribs. The posterior inferior serratus muscle pulls the ribs downward, participating in expiration.

The deep back muscles are located on the posterior surface of the spine along its entire length, from the sacrum to the occipital bone, lateral to the spinous processes of the vertebrae. Within this group, the erector spinae muscle, along with the splenius capitis and cervicis muscles, lies more superficially. The deep back muscles proper are located directly on the spine. These include the short transversospinales, interspinales, and intertransversarii muscles, and, in the upper cervical region, the suboccipital muscles.

The erector spinae is a thick, powerful muscle originating from the posterior surface of the sacrum, the spinous processes of the lumbar and lower thoracic vertebrae, the posterior surface of the iliac crest, and the thoracolumbar fascia. The muscle extends upward, dividing into individual fascicles that insert into the spinous and transverse processes of the overlying lumbar, thoracic, and cervical vertebrae, the angles of the ribs, and the mastoid process of the Temporal bone. The erector spinae comprises three parts: the iliocostalis muscle, located laterally on the posterior part of the ribs; the longissimus muscle, lying more medially, closer to the spine; and the spinalis muscle, which directly adjoins the spinous processes of the vertebrae.

Beneath the erector spinae, directly on the posterior surface of THE Vertebral Column (in the longitudinal sulcus between the spinous and transverse processes), lies the transversospinalis muscle. It consists of short muscle fascicles that originate from the transverse processes of the underlying vertebrae, run obliquely upward and medially, and insert into the transverse processes of the overlying vertebrae. The more superficial fascicles span 5–6 vertebrae, while the deepest ones span a single vertebra. Bilateral contraction of the transversospinalis extends the spine, whereas unilateral contraction bends and rotates it toward the corresponding side.

The deepest back muscles, connecting the corresponding processes of adjacent vertebrae, are the short interspinales and intertransversarii muscles, which also assist in lateral flexion of the spine.

In the posterior neck region, beneath the trapezius muscle, lie the splenius capitis and cervicis muscles, which act as extensors of the Head and Neck. These muscles originate from the spinous processes of the lower cervical and upper thoracic vertebrae and insert into the transverse processes of the cervical vertebrae and the occipital bone. Bilateral contraction of these muscles extends the head and neck.

Deep within the suboccipital region, beneath the splenius muscles at the level of the atlanto-occipital joints, lie the paired suboccipital muscles, which include the rectus capitis posterior major and minor, and the superior and inferior oblique muscles of the head. These muscles participate in extending (tilting back) the head and rotating it to the sides.

The trapezius muscle and latissimus dorsi are covered by the superficial fascia. Deeper lies the thoracolumbar fascia, which separates the deep muscles from the superficial ones.