The Skeleton

Overview

The human skeleton provides the structural framework for the body, supports movement, and protects vital organs. It is divided into two main parts: the axial skeleton (skull, vertebral column, and thoracic cage) and the appendicular skeleton (limbs and girdles).

Vertebral Column

General Characteristics

• Function: The vertebral column transmits the weight of the trunk to the lower limbs, protects the spinal cord, and provides attachment points for ribs and muscles.

• Structure: It contains 26 irregular bones called vertebrae arranged in five major regions.

Regions of the Vertebral Column

• Cervical: 7 vertebrae (C1–C7)

• Thoracic: 12 vertebrae (T1–T12)

• Lumbar: 5 vertebrae (L1–L5)

• Sacrum: One bone formed from the fusion of several bones; articulates with the hip bones.

• Coccyx: Also fused bones that form the terminus of the column.

Mnemonic for remembering vertebral counts: 7 am (breakfast), 12 noon (lunch), 5 pm (dinner).

Spinal Curvatures

The vertebral column has four main curvatures that increase resilience and flexibility:

• Cervical and Lumbar Curvatures: Concave posteriorly

• Thoracic and Sacral Curvatures: Convex posteriorly

These curves help distribute mechanical stress during movement and standing.

Abnormal Spine Curvatures

• Scoliosis: Abnormal lateral rotation of the spine, most often in the thoracic region. Can be congenital or result from disease, poor posture, or unequal muscle pull.

• Kyphosis (hunchback): Exaggerated dorsal thoracic curvature, common in elderly individuals with osteoporosis, tuberculosis of the spine, rickets, or osteomalacia.

• Lordosis (swayback): Accentuated lumbar curvature, often seen in individuals with spinal tuberculosis, pot bellies, or pregnant women.

Intervertebral Discs

Structure and Function

• Location: Pads sandwiched between vertebrae that act as shock absorbers.

• Composition: Each disc has two parts:

  • Nucleus Pulposus: Inner gelatinous nucleus that gives the disc its elasticity and compressibility.

  • Anulus Fibrosus: Outer collar composed of collagen and fibrocartilage that limits the expansion of the nucleus pulposus when compressed.

Disc Herniation

• Definition: Trauma to the spine may cause herniated (prolapsed) discs, which is the rupture of the anulus fibrosus resulting in protrusion of the nucleus pulposus.

• Clinical Significance: The protruding nucleus can press on the spinal cord or nerves, causing pain and neurological symptoms.

• Treatment: Includes exercise, massage, heat, painkillers, or surgical intervention if conservative treatments fail.

Vertebral Characteristics by Region

Cervical Vertebrae (C1–C7)

• General: Smallest and lightest vertebrae.

• Atlas (C1): No body or spinous process; consists of anterior and posterior arches and two lateral masses. Articulates with the occipital condyles of the skull, allowing nodding motion ("yes").

• Axis (C2): Has a knoblike dens (odontoid process) projecting superiorly from the body, allowing rotation of the head ("no").

Thoracic Vertebrae (T1–T12)

• General: Larger than cervical vertebrae; articulate with ribs.

• Features: Heart-shaped body, circular vertebral foramen, long spinous process.

Lumbar Vertebrae (L1–L5)

• General: Largest vertebrae; receive the most stress.

• Features: Massive, kidney-shaped bodies; short, thick pedicles and laminae; flat, hatchet-shaped spinous processes.

Sacrum and Coccyx

• Sacrum: Triangular bone formed from five fused vertebrae; forms the posterior wall of the pelvis and articulates with the hip bones at the sacroiliac joints.

• Coccyx: Formed from three to five fused vertebrae; the "tailbone" that articulates superiorly with the sacrum.

Thoracic Cage

Structure and Function

• Components: Thoracic vertebrae posteriorly, sternum and costal cartilages anteriorly, and ribs laterally.

• Functions: Protects organs of the thoracic cavity, supports shoulder girdles and upper limbs, provides attachment sites for muscles of the neck, back, chest, and shoulders.

Sternum

• Manubrium: Superior portion; articulates with clavicles and ribs 1 and 2.

• Body: Midportion; articulates with costal cartilages of ribs 2–7.

• Xiphoid Process: Inferior end; site of muscle attachment.

Ribs

• True (vertebrosternal) ribs: Pairs 1–7; attach directly to sternum by individual costal cartilages.

• False (vertebrochondral) ribs: Pairs 8–10; attach indirectly to sternum by joining costal cartilage of rib above.

• Floating (vertebral) ribs: Pairs 11–12; do not attach to sternum.

Appendicular Skeleton

Overview

The appendicular skeleton consists of the bones of the limbs and their girdles, which attach the limbs to the axial skeleton.

Pectoral (Shoulder) Girdle

• Components: Clavicle and scapula.

• Function: Attaches upper limbs to the body trunk and provides attachment sites for muscles that move the upper limbs.

Pelvic (Hip) Girdle

• Components: Two hip bones (coxal bones) and the sacrum.

• Function: Attaches lower limbs to the axial skeleton, transmits weight of the upper body to the lower limbs, and supports pelvic organs.

• Structure: Each coxal bone is formed by the fusion of three bones: ilium, ischium, and pubis. The acetabulum is the socket that receives the head of the femur.

• Stability: The pelvic girdle is more stable but less mobile than the shoulder girdle.

Comparison of Male and Female Pelves

• Female pelvis: Wider, shallower, lighter, and rounder to accommodate childbirth.

• Male pelvis: Narrower and deeper.

Upper Limb

• Arm: Humerus (single bone of the upper arm).

• Forearm: Radius and ulna.

• Hand: 8 carpal bones (wrist), 5 metacarpal bones (palm), 14 phalanges (fingers).

Carpal Bones

Arranged in two rows (lateral to medial):

• Proximal row: Scaphoid, lunate, triquetrum, pisiform

• Distal row: Trapezium, trapezoid, capitate, hamate

Mnemonic: "Some Lovers Try Positions That They Can't Handle"

Lower Limb

• Thigh: Femur (largest and strongest bone in the body).

• Patella: Sesamoid bone in the quadriceps tendon that protects the knee joint.

• Leg: Tibia (weight-bearing) and fibula.

• Foot: 7 tarsal bones, 5 metatarsals, 14 phalanges.

Tarsal Bones

• Posterior half of foot: Talus and calcaneus (heel bone).

• Other tarsals: Cuboid, navicular, medial, intermediate, and lateral cuneiforms.

Developmental Aspects of the Skeleton

Infant and Child Skeleton

• Skull: More bones than adult skull; bones such as the mandible and frontal bones are unfused at birth.

• Fontanelles: Remnants of fibrous membranes between infant skull bones that allow for brain growth and ease of birth.

• Palate: Cleft palate is the most common congenital abnormality of the skull, resulting from incomplete fusion of the right and left halves of the palate.

Growth and Aging

• Proportions: At birth, the cranium is large relative to the face; the mandible and maxilla are foreshortened but grow faster than the cranium to reach adult proportions.

• Spinal Curvatures: Only thoracic and sacral curvatures are present at birth (C-shaped spine); cervical and lumbar curvatures develop later as the child lifts the head and begins to walk.

Aging Skeleton

• Intervertebral Discs: Become thinner, less hydrated, and less elastic with age, leading to height loss by age 55.

• Thorax: Costal cartilages ossify, causing shallow breathing and less efficient gas exchange.

• Bone Mass: Decreases with age, increasing fracture risk.

Summary Table: Comparison of Male and Female Pelves

Additional info: Some details, such as the mnemonic for carpal bones and the summary table, were inferred and expanded for academic completeness.