Clinical Anatomy: Superficial and Intermediate Layers of the Back

General Osteology and Terminology

• Definition of Osteology: Derived from the Greek word osteon, meaning bone, osteology is the scientific study of bones.
• Bone Formations and Markings: These physical features appear on bones in specific locations, including:     * Where tendons, ligaments, and fascias attach.     * Where arteries lie adjacent to the bone.     * Where arteries enter the bone.
• Bones of the Back: Muscles of the back attach to the following bones:     * Skull components: Occipital and Temporal bones.     * Axial skeleton: Spinal column and Ribs.     * Appendicular skeleton (Upper Limb): Scapula, Clavicle, and Humerus.     * Appendicular skeleton (Lower Limb): Ilium (which is a component of the os coxa or hip bone).

Osteology of the Back and Associated Structures

• Vertebral Features:     * Spinous process: The posterior projection of the vertebra.     * Transverse process: The lateral projections of the vertebra.
• The Scapula (Shoulder Blade):     * Spine of scapula: A prominent plate of bone crossing the posterior surface.     * Acromion: The lateral expansion of the spine of the scapula.     * Angles: Superior angle, Inferior angle, and Lateral angle.     * Borders: Medial (vertebral) border and Lateral (axillary) border.     * Glenoid fossa: Located on the lateral aspect; it articulates with the head of the humerus to form the glenohumeral joint.     * Base of spine: The medial starting point of the scapular spine.
• The Humerus (Bone of the Brachium):     * Head: Articulates with the glenoid fossa.     * Greater tubercle and Lesser tubercle: Sites for muscle attachment.     * Intertubercular sulcus (Bicipital groove): A furrow between the tubercles containing three specific parts:         1. Lateral lip         2. Floor         3. Medial lip

Osteology of the Ribs and Vertebral Relationship

• General Rib Anatomy:     * The human body typically possesses $12$ pairs of ribs.     * Head: Contains facets (e.g., superior articular facet) and a crest of the head.     * Neck: The flattened portion lateral to the head.     * Tubercle: Articulates with the transverse process of the corresponding vertebra.     * Body: The main part of the rib.     * Angle (Costal angle): The point where the rib curves most sharply.     * Cup for costal cartilage: Connection point for cartilaginous attachment to the sternum.
• Specific and Atypical Ribs:     * 1st Rib: Features include a tubercle, a scalene tubercle, and distinct grooves for the subclavian vein and subclavian artery.     * 2nd Rib: Notable for the tuberosity for the serratus anterior muscle.     * 8th, 11th, and 12th Ribs: Identified as part of the thoracic cage structure, with the $11^{th}$ and $12^{th}$ being atypical.

Osteology of the Pelvis and Skull

• Os Coxa (Hip Bone):     * Formed by the fusion of three primary bones: the Ilium, Ischium, and Pubis.     * Development: These bones are initially joined by hyaline cartilage to allow for growth and flexibility. Fusion begins between $15$ and $17$ years of age and is completed between $20$ and $25$ years of age.     * Landmarks: Iliac crest and the Posterior Superior Iliac Spine (PSIS).     * Sacrum: Articulates with the os coxa to form the pelvis.
• The Skull:     * Occipital Bone: Features include the External Occipital Protuberance (EOP), Superior nuchal line, Inferior nuchal line, and the Highest nuchal line. The external occipital crest runs perpendicular to these lines.     * Temporal Bone: Features the mastoid process.     * Parietal Bones: Located superior to the occipital bone.

Surface Anatomy and Soft Tissue Landmarks

• Soft Tissue Features:     * Nuchal groove: Vertical groove in the midline of the neck.     * Median longitudinal furrow: The vertical groove running down the center of the back.     * Muscular Landmarks: Trapezius, Latissimus dorsi, Teres major, and Erector spinae.
• Bony Landmarks (Palpable):     * Spinous process of $C7$: Known as the vertebra prominens.     * Spinous processes of various thoracic vertebrae.     * Scapular landmarks: Acromion, superior angle, spine of scapula, medial border, and inferior angle.     * Pelvic landmarks: Iliac crest and Posterior Superior Iliac Spine (PSIS).

Superficial Back Muscles

All five superficial back muscles act on the upper limb.

• Trapezius:     * Origins (O): Medial $1/3$ of the superior nuchal line; External Occipital Protuberance (EOP); nuchal ligament; spinous processes (SPs) of $C7-T12$.     * Insertions (I): Lateral $1/3$ of the clavicle; acromion; spine of the scapula.     * Innervation (n): Motor supply via the Spinal Accessory Nerve (CN XI). Pain and proprioception via $C3$ and $C4$ spinal nerves.     * Actions (A):         * Descending (superior) part: Elevates the scapula.         * Middle (transverse) part: Retracts the scapula.         * Ascending (inferior) part: Depresses the scapula.     * Arterial Supply (a): Branches of the occipital artery, superficial branch of the transverse cervical artery, dorsal scapular artery, and posterior intercostal arteries.
• Latissimus Dorsi:     * Origins (O): SPs of $T7-T12$; thoracolumbar fascia; iliac crest; inferior $3$ or $4$ ribs.     * Insertion (I): Floor of the intertubercular groove of the humerus.     * Innervation (n): Thoracodorsal nerve ($C6, C7, C8$).     * Actions (A): Extends, adducts, and medially rotates the humerus.     * Arterial Supply (a): Thoracodorsal artery.
• Levator Scapulae:     * Origins (O): Posterior tubercles of transverse processes of $C1-C4$.     * Insertion (I): Medial border of the scapula (superior to the root of the scapular spine).     * Innervation (n): Dorsal scapular nerve ($C5$).     * Actions (A): Elevates the scapula and rotates the glenoid cavity inferiorly.     * Arterial Supply (a): Dorsal scapular artery.
• Rhomboid Minor:     * Origins (O): Nuchal ligament; SPs of $C7-T1$.     * Insertion (I): Medial border of the scapula at the level of the medial end of the scapular spine.     * Innervation (n): Dorsal scapular nerve ($C4, C5$).     * Actions (A): Retracts the scapula, rotates the glenoid cavity inferiorly, and fixes the scapula to the thoracic wall.     * Arterial Supply (a): Dorsal scapular artery.
• Rhomboid Major:     * Origins (O): SPs of $T2-T5$.     * Insertion (I): Medial border of the scapula from the spine to the inferior angle.     * Innervation (n): Dorsal scapular nerve ($C4, C5$).     * Actions (A): Retracts the scapula, rotates the glenoid cavity inferiorly, and fixes the scapula to the thoracic wall.     * Arterial Supply (a): Dorsal scapular artery.

Intermediate Back Muscles

• Serratus Posterior Superior:     * Origins (O): Nuchal ligament; SPs of $C7-T2$ or $T3$.     * Insertion (I): Superior border of external surfaces of ribs $2-5$ (just lateral to their angles).     * Innervation (n): $2^{nd}-5^{th}$ intercostal nerves.     * Actions (A): Elevates the ribs; potentially involved in proprioception.     * Arterial Supply (a): Posterior intercostal arteries.
• Serratus Posterior Inferior:     * Origins (O): SPs of $T11-L2$ or $L3$.     * Insertion (I): Inferior border of external surfaces of ribs $9-12$ (lateral to their angles).     * Innervation (n): $9^{th}-11^{th}$ intercostal nerves and the subcostal nerve ($T12$).     * Actions (A): Depresses lower ribs; potentially involved in proprioception.     * Arterial Supply (a): Posterior intercostal arteries.

Anatomical Triangles and Spaces

• Triangle of Auscultation:     * Clinical Significance: Located at the level of the $6^{th}$ intercostal space; provides a thinning of muscle for clearer respiratory organ sounds.     * Boundaries:         1. Latissimus dorsi         2. Trapezius         3. Rhomboid major
• Lumbar Triangle:     * Boundaries:         1. Iliac crest         2. Latissimus dorsi         3. External abdominal oblique

Neurovascular Structures and Dissection Observations

• Nerves of the Superficial Back:     * Greater occipital nerve ($C2$).     * Spinal accessory nerve ($CN XI$).     * Cutaneous branches of posterior rami.     * Posterior branch of the lateral branch of intercostal nerves.
• Arteries:     * Occipital artery.     * Thoracodorsal artery.     * Dorsal scapular artery.     * Posterior intercostal arteries.
• Fascial Layers: The interface between superficial fascia and deep fascia is a critical landmark during dissection.
• Dissection Techniques: Skin reflection involves making incisions along specific dashed lines and reflecting the skin layer laterally to expose underlying superficial and deep fascia.
• Transverse Section of Thorax: Highlights the relationship between the spinal cord, spinal nerve (dividing into anterior ramus and posterior ramus), the erector spinae muscle, rhomboid major, latissimus dorsi, and trapezius.