Comprehensive Study Notes on the Human Skeleton: Axial and Appendicular Divisions

AXIAL SKELETON

Skull: Cranium

The cranium encloses and protects the brain, forming the upper part of the skull. It consists of several bones with distinct features and functions.

• Frontal (1): Forms the forehead, roof of orbits, and nasal cavity. Contains the frontal paranasal sinuses (air-filled spaces that develop as two bones fuse in childhood).

• Parietal (2): Located posterior to the frontal bone; one on each side of the skull.

• Temporal (2): Inferior to parietal bones and anterior to occipital; forms parts of the sides and base of the cranium. Contains:

  • Squamous portion: Superior, flat portion; houses middle and inner ear structures.

  • Petrous portion: Rocky part at the base; houses the middle and inner ear.

  • External auditory/acoustic meatus: Tube-like passage to the ear.

  • Mastoid process: Posterior/inferior to external auditory meatus; attachment for neck muscles; contains air spaces (mastoiditis if infected).

  • Styloid process: Slender projection; attachment for muscles and ligaments of the pharynx and tongue.

• Occipital (1): Forms the posterior skull and most of the skull base. Features:

  • Foramen magnum: Large opening for the spinal cord and brainstem connection.

  • Occipital condyles: Articulate with the first cervical vertebra (atlas).

  • External occipital protuberance: Midline projection posterior to the foramen magnum.

• Sutures: Immovable joints (synarthroses) where cranial bones interlock. Types include:

  • Lambdoidal: Separates occipital and parietal bones.

  • Squamosal: Separates temporal and parietal bones.

  • Sagittal: Separates left and right parietal bones.

  • Coronal: Separates frontal and parietal bones.

• Wormian (sutural) bones: Small bones within sutures; variable in number.

Sphenoid and Ethmoid Bones

• Sphenoid (1): Spans the width of the skull; forms part of the cranial floor. Features:

  • Sella turcica: Houses the pituitary gland.

  • Greater/lesser wings: Lateral masses; butterfly-shaped.

• Ethmoid (1): Anterior to sphenoid; forms part of the orbits, nasal cavity, and septum. Features:

  • Cribriform plate: Contains olfactory foramina for olfactory nerves.

  • Crista galli: Attachment for brain meninges.

  • Perpendicular plate: Forms superior part of nasal septum.

  • Nasal conchae: Superior and middle scroll-like bones that increase surface area in the nasal cavity.

Fontanels

• Soft spots in the fetal skull; allow for compression during birth and brain growth. Major fontanels:

  • Frontal/anterior

  • Occipital/posterior

  • Mastoid/posterolateral

  • Sphenoid/anterolateral

Facial Bones

• Maxillae (2): Upper jaw; forms part of the orbits, nasal cavity, and hard palate. Contains maxillary sinuses.

• Palatine (2): Form posterior hard palate and part of the orbit and nasal cavity.

• Vomer (1): Forms lower part of nasal septum.

• Lacrimal (2): Small bones in the medial orbit wall; house the nasolacrimal duct.

• Nasal (2): Form the bridge of the nose.

• Mandible (1): Lower jaw; only movable skull bone. Features:

  • Body: Horizontal portion.

  • Ramus: Vertical extension.

  • Mandibular condyle: Articulates with temporal bone (TMJ).

  • Coronoid process: Muscle attachment.

  • Mental foramen: Passage for nerves and blood vessels.

  • Alveolar processes: Sockets for teeth.

• Hyoid (1): U-shaped bone in the neck; does not articulate with other bones; supports tongue and larynx.

Thoracic Cage

• Sternum (1): "Breastbone"; consists of:

  • Manubrium: Superior portion; articulates with clavicle and rib 1.

  • Body: Middle portion; articulates with ribs.

  • Xiphoid process: Inferior tip; attachment for abdominal muscles.

• Ribs (12 pairs):

  • True ribs (1-7): Attach directly to sternum.

  • False ribs (8-12): Do not attach directly; ribs 11-12 are "floating ribs" (no anterior attachment).

  • Head: Articulates with vertebral body.

  • Tubercle: Articulates with vertebral transverse process.

Vertebral Column

The vertebral column ("backbone") supports the body and protects the spinal cord. It consists of 33 vertebrae, separated by intervertebral disks.

• Regions:

  • Cervical (7): Neck region; C1 (atlas) and C2 (axis) are specialized for head movement.

  • Thoracic (12): Chest region; articulate with ribs.

  • Lumbar (5): Lower back; largest vertebrae.

  • Sacral (5, fused): Sacrum; forms posterior pelvic wall.

  • Coccygeal (4, fused): Coccyx; "tailbone".

• Normal Curvatures: Cervical and lumbar (anterior); thoracic and sacral (posterior).

• Key Features:

  • Body: Weight-bearing portion.

  • Vertebral foramen: Canal for spinal cord.

  • Spinous process: Posterior projection.

  • Transverse process: Lateral projections.

  • Intervertebral foramen: Passage for spinal nerves.

APPENDICULAR SKELETON

Pectoral Girdle

• Clavicle (2): "Collarbone"; connects arm to trunk; stabilizes shoulder.

• Scapula (2): "Shoulder blade"; features:

  • Spine: Ridge on dorsal surface.

  • Acromion: Lateral process; articulates with clavicle.

  • Coracoid process: Anterior projection; muscle attachment.

  • Glenoid fossa: Articulates with humerus.

Upper Limb

• Humerus (2): Arm bone; features:

  • Head: Articulates with glenoid fossa.

  • Greater/lesser tubercles: Muscle attachment.

  • Deltoid tuberosity: Attachment for deltoid muscle.

  • Olecranon fossa: Receives ulna during extension.

• Radius (2): Lateral forearm bone (thumb side); features:

  • Head: Articulates with capitulum of humerus.

  • Radial tuberosity: Insertion for biceps brachii.

  • Styloid process: Distal projection; ligament attachment.

• Ulna (2): Medial forearm bone (pinky side); features:

  • Trochlear notch: Articulates with humerus.

  • Olecranon process: Elbow prominence.

  • Styloid process: Distal projection; ligament attachment.

• Carpals (8 per hand): Wrist bones.

• Metacarpals (5 per hand): Palm bones.

• Phalanges (14 per hand): Finger bones; proximal, middle, distal (thumb has 2).

Pelvic Girdle

• Os Coxae (2): Hip bones; each formed by fusion of ilium, ischium, and pubis.

  • Ilium: Superior, flared portion; iliac crest is palpable.

  • Ischium: Inferior, posterior; ischial tuberosity supports weight when sitting.

  • Pubis: Anterior, inferior; pubic symphysis is the midline joint.

  • Acetabulum: Socket for femoral head.

  • Obturator foramen: Large opening for nerves and vessels.

Lower Limb

• Femur (2): Thigh bone; largest and strongest bone. Features:

  • Head: Articulates with acetabulum.

  • Neck: Connects head to shaft; common fracture site.

  • Greater/lesser trochanters: Muscle attachment.

  • Medial/lateral condyles: Articulate with tibia.

• Patella (2): Kneecap; sesamoid bone in quadriceps tendon.

• Tibia (2): Medial leg bone; weight-bearing. Features:

  • Medial/lateral condyles: Articulate with femur.

  • Tibial tuberosity: Attachment for patellar ligament.

  • Medial malleolus: Forms medial ankle.

• Fibula (2): Lateral leg bone; non-weight-bearing. Lateral malleolus forms lateral ankle.

• Tarsals (7 per foot): Ankle bones; includes talus (articulates with tibia/fibula) and calcaneus (heel).

• Metatarsals (5 per foot): Foot bones; numbered 1-5 from great toe.

• Phalanges (14 per foot): Toe bones; proximal, middle, distal (great toe has 2).

Selected Table: Types of Cranial Sutures

Suture	Bones Separated
Coronal	Frontal & Parietal
Sagittal	Left & Right Parietal
Lambdoidal	Parietal & Occipital
Squamosal	Parietal & Temporal

Additional info:

• Synarthroses: Immovable joints, such as cranial sutures.

• Fontanels: Allow for rapid brain growth in infants; ossify by age 2.

• TMJ (Temporomandibular Joint): Only movable joint in the skull.

• Sesamoid bone: A bone embedded within a tendon (e.g., patella).