The Skeletal System: Structure, Function, and Anatomy

The Skeletal System

Overview of the Skeleton

The human skeleton is a complex framework of approximately 206 bones and associated cartilages. It provides structural support, protection for internal organs, and facilitates movement through articulations (joints). The skeleton is divided into two main divisions: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Forms the longitudinal axis of the body and is primarily structured for protection. It includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Structured for motion, it includes the bones of the girdles and the upper and lower limbs.

Major Components of the Skeleton

• Skull: The most complex structure, consisting of 22 bones (8 cranial, 14 facial).

• Vertebral Column: Composed of 33 vertebrae, including the sacrum and coccyx (fused vertebrae).

• Thoracic Cage: Includes 12 pairs of ribs, the sternum, and part of the vertebral column, protecting thoracic organs.

• Pectoral Girdle: Clavicle and scapula, supporting the upper limb.

• Pelvic Girdle: Two pelvic bones and the sacrum, supporting the lower limb.

• Upper and Lower Limbs: Arms, forearms, wrists, hands, thighs, legs, ankles, and feet, each with specific bones for movement and support.

Bone Markings

Types of Bone Markings

Bone markings are surface features that serve as sites for muscle, ligament, and tendon attachment, or as passages for nerves and blood vessels. They are classified as depressions, openings, or projections.

• Depressions: Allow passage of blood vessels and nerves or articulation between bones.

• Openings: Enclose delicate structures and allow passage through bones.

• Projections: Sites for ligament and tendon attachment or articulation with other bones.

Skull Structure

Overview of Skull Bones

The skull is composed of cranial and facial bones, united by immovable joints called sutures (except the mandible). The cranial bones protect the brain, while the facial bones form the structure of the face.

• Cranial Bones: Frontal, occipital, ethmoid, sphenoid (single); temporal, parietal (paired).

• Facial Bones: Mandible, vomer (single); maxilla, zygomatic, nasal, lacrimal, palatine, inferior nasal concha (paired).

Major Cavities of the Skull

• Cranial Cavity: Houses the brain, with the cranial vault (calvaria) and cranial base. The base contains anterior, middle, and posterior cranial fossae for brain support.

• Orbits: Contain the eyeballs and associated structures.

• Nasal Cavity: Houses olfactory receptors and forms the first part of the respiratory tract.

• Oral Cavity: Contains the teeth and tongue, forming the entry to the digestive tract.

• Paranasal Sinuses: Air-filled spaces that lighten the skull and enhance voice resonance.

Selected Skull Bones and Features

• Frontal Bone: Forms the forehead and part of the cranial vault.

• Parietal Bones: Form the superior and lateral aspects of the skull.

• Occipital Bone: Forms the posterior part of the skull and contains the foramen magnum.

• Temporal Bones: House the structures of the ears.

• Sphenoid and Ethmoid Bones: Contribute to the cranial floor and orbits.

Facial Bones

• Maxillae: Form the upper jaw and part of the orbits.

• Zygomatic Bones: Form the cheekbones.

• Mandible: Lower jaw bone, only movable skull bone.

• Nasal, Lacrimal, Palatine, Inferior Nasal Concha, Vomer: Contribute to the structure of the nasal cavity and orbits.

Special Features of the Skull

Fetal Skull and Fontanels

In infants, the skull contains membranous areas called fontanels, which allow for flexibility during birth and brain growth. These close by 18–24 months of age.

Forensic Skull Anatomy

Skull features can help determine sex, age, and ethnic heritage. For example, males typically have a more prominent supraorbital ridge and a more acute mandibular angle.

Hyoid Bone

The hyoid bone is a small, C-shaped bone in the neck that does not articulate with other bones. It serves as an attachment for muscles involved in swallowing and speech.

The Vertebral Column

Structure and Regions

The vertebral column consists of 33 vertebrae, divided into cervical, thoracic, lumbar, sacral, and coccygeal regions. It supports the head, protects the spinal cord, and provides attachment points for ribs and muscles.

• Cervical (7): Neck region

• Thoracic (12): Articulate with ribs

• Lumbar (5): Lower back

• Sacral (5 fused): Articulate with pelvic bones

• Coccygeal (3–5 fused): Tailbone

Spinal Curvatures

• Primary Curvatures: Thoracic and sacral, present during fetal development.

• Secondary Curvatures: Cervical and lumbar, develop after birth.

Abnormal Spinal Curvatures

• Scoliosis: Lateral curvature, C or S shaped.

• Lordosis: Exaggerated cervical and lumbar curvatures ("swayback").

• Kyphosis: Exaggerated thoracic curvature ("humpback").

Structure of a Typical Vertebra

• Body (Centrum): Weight-bearing region.

• Vertebral Foramen: Passage for the spinal cord.

• Pedicles and Laminae: Form the vertebral arch.

• Processes: Spinous, transverse, and articular processes for muscle and ligament attachment.

Specialized Vertebrae

• Atlas (C1): Supports the skull, allows nodding motion.

• Axis (C2): Contains the dens (odontoid process), allows rotation of the head.

The Thoracic Cage

Structure and Function

The thoracic cage consists of the sternum, 12 pairs of ribs, and thoracic vertebrae. It protects the heart, lungs, and major blood vessels, and supports the shoulder girdles and upper limbs.

• Sternum: Manubrium, body, and xiphoid process.

• Ribs: True ribs (1–7), false ribs (8–12), and floating ribs (11–12).

The Pectoral Girdle and Upper Limb

Pectoral Girdle

The pectoral girdle consists of the clavicle and scapula, supporting the upper limb and providing attachment points for muscles.

Humerus

The humerus is the only bone of the arm, articulating proximally with the scapula and distally with the radius and ulna.

Radius and Ulna

The radius (lateral) and ulna (medial) are the bones of the forearm, articulating with each other and with the humerus and carpals.

Carpals, Metacarpals, and Phalanges

The wrist contains eight carpal bones, the hand contains five metacarpals, and the fingers contain 14 phalanges.

The Pelvic Girdle and Lower Limb

Pelvic Girdle

The pelvic girdle consists of two pelvic bones and the sacrum, forming the pelvis. Each pelvic bone is composed of the ilium, ischium, and pubis, which fuse during childhood.

Femur and Patella

The femur is the only bone of the thigh and the strongest bone in the body. The patella is a sesamoid bone within the quadriceps tendon.

Tibia and Fibula

The tibia (medial) and fibula (lateral) are the bones of the leg, connected by an interosseous membrane and forming the ankle joint with the tarsals.

Tarsals, Metatarsals, and Phalanges

The ankle contains seven tarsal bones, the foot contains five metatarsals, and the toes contain 14 phalanges. The foot also has three arches (medial longitudinal, lateral longitudinal, and transverse) for support and shock absorption.

Study Boost: Mnemonics and Visual Analogies

• PEST OF 6: Parietal, Ethmoid, Sphenoid, Temporal, Occipital, Frontal (cranial bones)

• Breakfast at 7, Lunch at 12, Dinner at 5: 7 cervical, 12 thoracic, 5 lumbar vertebrae

• The trapezium is by the thumb (carpals)

• TIBia = Thick, Inner Bone; FibuLA = Lateral bone