Introduction to the Skeletal System and the Axial Skeleton

Types of Cartilage

The skeletal system contains three main types of cartilage, each with distinct locations and functions:

• Hyaline cartilage: Most abundant; found in articular surfaces, costal cartilages, nose, trachea, and larynx.

• Elastic cartilage: Contains elastic fibers; found in the ear and epiglottis.

• Fibrocartilage: Highly compressible; found in intervertebral discs, pubic symphysis, and menisci.

Example: The meniscus in the knee is made of fibrocartilage, providing shock absorption.

Bone Classification and Structure

Bones are classified by shape and structure:

• Long bones: Longer than wide (e.g., femur, humerus)

• Short bones: Cube-shaped (e.g., carpals, tarsals)

• Flat bones: Thin, flattened (e.g., sternum, skull)

• Irregular bones: Complex shapes (e.g., vertebrae, hip bones)

• Sesamoid bones: Embedded in tendons (e.g., patella)

Structure of a Long Bone:

• Diaphysis: Shaft

• Epiphysis: Ends

• Articular cartilage: Covers joint surfaces

• Periosteum: Outer covering

• Endosteum: Inner lining

• Medullary cavity: Contains marrow

• Compact bone: Dense outer layer

• Spongy bone: Internal network

• Microscopic structure: Osteon (Haversian system), lamellae, canaliculi, lacunae, central canal, perforating (Volkmann's) canals

Example: The femur is a long bone with a diaphysis and two epiphyses.

Bone Composition

• Inorganic salts: Mainly hydroxyapatite (calcium phosphate)

• Organic matrix: Collagen fibers and proteins

Formula:

The Axial Skeleton

Skull: Cranial Bones (8 bones total)

• Frontal bone: Supraorbital foramen, coronal suture

• Parietal bone (paired): Sagittal suture

• Temporal bone (paired): Squamous suture, external acoustic meatus, zygomatic process, mastoid process, mandibular fossa, styloid process, carotid canal, jugular foramen

• Occipital bone: Lambdoid suture, foramen magnum, occipital condyles, external occipital protuberance

• Sphenoid bone: Sella turcica

• Ethmoid bone: Cribriform plate, crista galli

Skull: Facial Bones (14 bones total)

• Mandible: Body, ramus, condylar process, coronoid process, mandibular foramen, alveolar margin

• Maxilla (paired): Infraorbital foramen, alveolar margin

• Palatine bone (paired): Hard palate only

• Zygomatic bone (paired)

• Nasal bone (paired)

• Vomer bone

• Inferior nasal conchae (paired)

• Paranasal sinuses: Frontal, ethmoid, sphenoid, maxillary

• Hyoid bone: Not attached to other bones; supports tongue

Vertebral Column

• Vertebra (typical): Body, vertebral foramen, transverse process, spinous process, articular processes, intervertebral foramen

• Cervical vertebrae (7 total): Transverse foramen, bifid spinous process

• Atlas (C1): No spinous process

• Axis (C2): Odontoid process (dens)

• Thoracic vertebrae (12 total): Heart-shaped body, downward spinous process, articulate with ribs

• Lumbar vertebrae (5 total): Block-like body, short spinous process

• Sacrum: Median sacral crest, sacral foramina, sacral canal, sacral hiatus

• Coccyx: Tailbone

Bony Thorax

• Sternum: Manubrium, body, xiphoid process, jugular notch

• Ribs: 1-7 vertebrosternal, 8-12 vertebrochondral (11-12 are floating), costal cartilage

The Appendicular Skeleton

Upper Appendicular Bones: Shoulder Girdle

• Clavicle

• Scapula: Spine, acromion, coracoid process, glenoid cavity, supraspinous fossa, infraspinous fossa, subscapular fossa, borders (lateral, medial, superior), superior & inferior angles

Upper Appendicular Bones: Upper Extremity

• Humerus: Head, anatomical neck, greater & lesser tubercles, intertubercular sulcus, trochlea, capitulum, olecranon fossa, radial fossa, deltoid tuberosity

• Radius: Head, neck, styloid process, radial tuberosity

• Ulna: Olecranon process, coronoid process, trochlear notch, styloid process

• Carpals (8): Scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate

• Mnemonic: "Stop Letting Those People Touch The Cadaver's Hand" (proximal row: scaphoid, lunate, triquetrum, pisiform; distal row: trapezium, trapezoid, capitate, hamate)

• Metacarpals: 1-5

• Phalanges: 14 (3 per finger, 2 in thumb)

Lower Appendicular Bones: Pelvic Girdle

• Coxal bone: Formed from ilium, ischium, pubis

• Ilium: Iliac crest, iliac fossa, auricular surface, anterior superior/inferior spine, posterior superior/inferior iliac spine, greater sciatic notch

• Ischium: Ischial tuberosity, ischial spine, lesser sciatic notch

• Pubis: Pubic symphysis, pubic arch, pubic tubercle, inferior ramus, obturator foramen, acetabulum

• Male vs. Female pelvis: Female pelvis is wider, shallower, with a larger pelvic inlet and outlet

Lower Appendicular Bones: Lower Extremity

• Femur: Head, fovea capitis, neck, greater & lesser trochanters, linea aspera, lateral & medial condyles, intercondylar fossa, patella surface

• Tibia: Medial & lateral condyles, tibial tuberosity, anterior border, medial malleolus

• Fibula: Head, lateral malleolus

• Tarsals (7): Talus, calcaneus, navicular, cuboid, medial/intermediate/lateral cuneiform

• Metatarsals: 1-5

• Phalanges: 14 (3 per toe, 2 in big toe)

Articulations and Body Movements

Classification of Joints

Joints are classified structurally and functionally:

• Structural: Fibrous, Cartilaginous, Synovial

• Functional: Synarthroses (immovable), Amphiarthroses (slightly movable), Diarthroses (freely movable)

Fibrous Joints

• Sutures: Skull bones

• Gomphosis: Tooth in socket

• Syndesmosis: Ligament connection (e.g., distal tibiofibular joint)

Cartilaginous Joints

• Synchondrosis: Hyaline cartilage (e.g., epiphyseal plate)

• Symphysis: Fibrocartilage (e.g., pubic symphysis, intervertebral discs)

Synovial Joints

• Types: Plane, Hinge, Pivot, Condyloid, Saddle, Ball and Socket

• Structures: Joint cavity, articular cartilage, ligaments, bursae, menisci, articular capsule (with synovial membrane)

Joint Movements

• Flexion vs. Extension: Decreasing vs. increasing joint angle

• Abduction vs. Adduction: Moving away from vs. toward midline

• Rotation: Movement around axis

• Circumduction: Circular movement

• Inversion vs. Eversion: Sole of foot inward vs. outward

• Dorsiflexion vs. Plantar Flexion: Foot upward vs. downward

• Pronation vs. Supination: Palm down vs. palm up

Joint Disorders

• Sprain: Ligament injury

• Strain: Muscle/tendon injury

• Dislocation: Bone displaced from joint

Difference between Tendon and Ligament:

• Tendon: Connects muscle to bone

• Ligament: Connects bone to bone

Table: Joint Classification

Additional info: Academic context and definitions have been expanded for clarity and completeness.