The Skeleton

The Axial Skeleton

The axial skeleton forms the longitudinal axis of the body and consists of 80 bones, accounting for roughly 40% of the bones in the human body. It provides a framework that supports and protects the brain, spinal cord, and organs in the ventral cavity, and offers attachment points for muscles that adjust the position of the head, neck, and trunk, perform respiratory movements, and stabilize parts of the appendicular skeleton.

• Major Functions: Support, protection, muscle attachment, and stabilization.

Bones of the Cranium and Face

• Cranial Bones (8): Frontal, occipital, sphenoid, ethmoid, 2 parietal, 2 temporal.

• Facial Bones (14): 2 zygomatic, 2 maxilla, 2 lacrimal, 2 nasal, 2 palatine, vomer, 2 inferior nasal concha, mandible.

• Additional Bones: 3 pairs of auditory ossicles, hyoid bone.

Significant Features (Markings) of the Skull

• Inferior nasal conchae: Create air turbulence and warm/humidify air.

• External occipital crest: Attachment for ligament stabilizing neck vertebrae.

• Mastoid process: Muscle attachment for head rotation.

• Styloid process: Suspends hyoid bone; muscle attachment.

• Alveolar processes: Sockets for teeth in maxillae and mandible.

• Mental protuberance: Muscle attachment on chin.

• Mandibular angle: Posterior, inferior corner of lower jaw.

• Zygomatic arch: Formed by temporal and zygomatic bones.

• External acoustic meatus: Passage for sound waves to tympanic membrane.

• Frontal sinuses: Variable in size; appear around age 6.

• Petrous part of temporal bone: Encloses inner ear structures.

• Internal acoustic meatus: Passage for blood vessels and nerves to inner ear.

• Mandibular fossa: Articulation site with mandible.

• Occipital condyles: Articulation with first vertebra of neck.

• Foramen ovale: Passage for nerves.

• Carotid canal: Passage for internal carotid artery.

• Jugular foramen: Passage for jugular vein.

• Foramen magnum: Connects cranial cavity with vertebral canal; surrounds brain-spinal cord connection.

Sphenoid, Ethmoid, and Palatine Bones

• Sphenoid: Forms part of cranial floor, unites cranial and facial bones, strengthens skull sides. Contains lesser and greater wings, optic canals, sella turcica (pituitary gland), and sphenoidal sinuses.

• Ethmoid: Forms part of anterior cranial floor, nasal cavity roof, nasal septum, and medial orbit. Includes cribriform plate (olfactory nerves), crista galli (falx cerebri attachment), lateral masses (ethmoidal sinuses), superior and middle nasal conchae, and perpendicular plate (nasal septum).

• Palatine: Forms posterior hard palate and part of orbital floor.

Orbital Complex and Paranasal Sinuses

The orbital complex consists of seven bones forming the eye socket, protecting the eyes and supporting vision. The paranasal sinuses lighten the skull, resonate the voice, and provide mucous epithelium.

• Orbital Complex: Formed by frontal, zygomatic, maxilla, lacrimal, ethmoid, sphenoid, and palatine bones. The lacrimal sulcus drains tears.

• Nasal Complex: Encloses nasal cavities and paranasal sinuses (frontal, sphenoid, ethmoid, palatine, maxillae).

• Paranasal Sinuses: Paired, mucous-lined cavities continuous with the nasal cavity; lighten skull and resonate voice.

Mandible and Associated Bones

• Mandible: Divided into horizontal body (supports lower teeth) and ascending rami. Articulates with temporal bone at temporomandibular joint.

• Condylar process: Articulates with temporal bone.

• Coronoid process: Muscle attachment for jaw closure.

• Alveolar process: Supports lower teeth.

• Mandibular foramen: Passage for nerves and blood vessels to lower teeth.

• Hyoid bone: U-shaped, not attached to other bones; muscle attachment for larynx, pharynx, tongue.

• Auditory ossicles: Middle ear bones conducting sound vibrations.

Skull Differences: Infants, Children, and Adults

• At birth, cranial bones are connected by fibrous connective tissue (fontanelles) allowing brain growth and skull distortion during birth. The anterior fontanel persists until age 2; others close within months. Most skull growth occurs before age 5.

Vertebral Column: Structure and Regions

The vertebral column consists of 26 bones: 24 vertebrae, sacrum, and coccyx. It supports the head, neck, and trunk, transfers weight, protects the spinal cord, and maintains posture.

• Cervical region: 7 vertebrae; supports head.

• Thoracic region: 12 vertebrae; articulates with ribs.

• Lumbar region: 5 vertebrae; largest, supports most weight.

• Sacrum: 5 fused vertebrae; foundation for pelvic girdle.

• Coccyx: 3-5 fused vertebrae; tailbone.

• Parts of a Vertebra: Vertebral foramen (spinal cord), intervertebral foramen (spinal nerves), articular processes (joints), vertebral body (weight-bearing), intervertebral disk (shock absorption), pedicles and laminae (vertebral arch), transverse and spinous processes (muscle attachment).

Distinctive Features of Cervical, Thoracic, and Lumbar Vertebrae

• Cervical: Smallest; transverse foramen for arteries/veins; atlas (C1) allows nodding; axis (C2) with dens for rotation; C7 is vertebra prominens.

• Thoracic: 12 vertebrae; each articulates with ribs; bodies increase in size inferiorly.

• Lumbar: 5 largest vertebrae; thick bodies; common site for compression fractures in elderly.

• Sacrum: 5 fused vertebrae; protects pelvic organs; sacral hiatus, cornu, promontory, apex.

Thoracic Cage: Structure and Function

The thoracic cage supports the thoracic cavity walls, protects vital organs, and provides muscle attachment points for respiration, posture, and upper limb movement.

• Sternum: Manubrium (clavicle/rib 1), body (ribs 2-7), xiphoid process (muscle attachment).

• Ribs: 12 pairs; vertebrosternal (1-7, direct attachment), vertebrochondral (8-10, indirect), floating (11-12, no attachment).

• Intercostal spaces: Access points for surgery.

The Appendicular Skeleton

Overview

The appendicular skeleton includes the bones of the limbs and the girdles that connect them to the trunk.

Pectoral Girdle

• Clavicle: Sternal end articulates with sternum; acromial end with scapula.

• Scapula: Acromion (clavicle articulation), glenoid cavity (humerus articulation), scapular spine (muscle attachment).

Bones of the Arm and Forearm

• Humerus: Head (glenoid cavity), surgical neck (fracture site), epicondyles (muscle attachment), olecranon/coronoid/radial fossae (articulations).

• Radius: Lateral, radial tuberosity (biceps attachment), interosseous membrane (connects to ulna), styloid process (wrist stabilization).

• Ulna: Olecranon (elbow), trochlear notch (humerus articulation), ulnar head (wrist articulation).

Wrist and Hand

• Carpus: 8 carpal bones (2 rows of 4).

• Metacarpals: 5 bones supporting the hand.

• Phalanges: 14 per hand; thumb (2), other fingers (3 each).

Pelvic Girdle

• Coxal bones: Each formed by fusion of ilium (superior), ischium (inferior/posterior), pubis (inferior/anterior).

• Lateral view: Ischial tuberosity (sitting), iliac crest (muscle attachment), acetabulum (femur articulation), greater sciatic notch (sciatic nerve passage).

• Medial view: Pubic symphysis (cartilage pad), obturator foramen (muscle attachment).

Pelvis Structure and Sex Differences

• Pelvis: Two coxal bones, sacrum, coccyx; joined by ligaments at sacro-iliac joints (union of axial and appendicular skeletons).

• Pelvic brim: Plane from sacral promontory to pubic symphysis.

• Greater (false) pelvis: Superior to brim; does not contain pelvic organs.

• Lesser (true) pelvis: Inferior to brim; contains pelvic organs; outlet is inferior opening.

• Sex differences: Female pelvis is wider, shallower, larger pelvic inlet/outlet, and less curved sacrum/coccyx for childbirth.

Thigh and Leg Bones

• Femur: Longest, strongest bone; neck (narrow region), greater trochanter (muscle attachment), linea aspera (muscle attachment).

• Patella: Quadriceps tendon (superior), patellar ligament (inferior).

• Tibia: Larger, medial; tibial tuberosity (patellar ligament), medial malleolus (ankle prominence).

• Fibula: Thinner, lateral; lateral malleolus (ankle), interosseous membrane (connects to tibia).

Ankle and Foot

• Tarsal bones: 7 bones; calcaneus (heel, weight-bearing), talus (transmits weight from tibia).

• Metatarsals: 5 bones forming the foot's distal portion.

• Phalanges: Hallux (big toe, 2 bones), other toes (3 bones each).

• Arches: Longitudinal (anterior-posterior, distributes weight), transverse (side-to-side, medial part more curved).

Medical Terminology Associated with the Axial Skeleton

• Laminectomy: Removal of vertebral lamina to relieve herniated disc symptoms.

• Spinal fusion: Stabilization of vertebrae with graft/device, often after fracture or disc removal.

• Whiplash: Neck injury from rapid hyperextension/flexion, common in car accidents.

• Clubfoot (talipes equinovarus): Inherited deformity; foot twisted inferiorly/medially, high arch.

• Genu valgum (knock-knee): Knees close together, ankles apart due to lateral tibia angulation.

• Genu varum (bowleg): Knees apart, lower limbs bowed medially.