The Axial Skeleton

The Skull

The human skull is a complex structure composed of several bones and distinct markings that serve as attachment points for muscles, passageways for nerves and blood vessels, and protection for the brain and sensory organs.

• Frontal Bone: Forms the forehead and the superior part of the eye sockets.

• Parietal Bones: Paired bones forming the superior and lateral aspects of the cranium.

  • Sagittal Suture: Joins the two parietal bones at the midline.

  • Coronal Suture: Separates the frontal bone from the parietal bones.

  • Lambdoid Suture: Separates the parietal bones from the occipital bone.

  • Squamous Suture: Separates the parietal and temporal bones.

• Temporal Bones: Form the sides and base of the cranium.

  • Zygomatic Process: Articulates with the zygomatic bone to form the zygomatic arch.

  • Mandibular Fossa: Depression that articulates with the mandible.

  • Mastoid Process: Attachment site for neck muscles.

  • Styloid Process: Serves as an anchor point for muscles associated with the tongue and larynx.

  • External Acoustic Meatus: Canal leading to the eardrum and middle ear.

  • Jugular Foramen: Passage for the internal jugular vein and cranial nerves.

  • Carotid Canal: Passage for the internal carotid artery.

• Occipital Bone: Forms the posterior part and base of the skull.

  • Foramen Magnum: Large opening for the spinal cord.

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

  • External Occipital Protuberance: Prominent midline bump on the posterior surface.

• Sphenoid Bone: Bat-shaped bone at the base of the skull.

  • Sella Turcica: Saddle-shaped depression housing the pituitary gland.

  • Greater and Lesser Wings: Form part of the cranial floor and orbits.

  • Optic Canal/Foramen: Passage for the optic nerve.

  • Foramen Rotundum, Ovale, Spinosum: Openings for cranial nerves and blood vessels.

• Ethmoid Bone: Forms part of the nasal cavity and the medial wall of the orbit.

  • Cribriform Plate & Foramina: Passage for olfactory nerves.

  • Crista Galli: Attachment for the brain's dura mater.

  • Perpendicular Plate: Forms the superior part of the nasal septum.

  • Superior and Middle Nasal Conchae: Increase surface area in the nasal cavity.

• Maxillae: Upper jaw bones.

  • Palatine Processes: Form the anterior hard palate.

  • Alveolus: Tooth sockets.

• Palatine Bones: Form the posterior hard palate.

• Zygomatic Bones: Cheekbones.

  • Temporal Process: Articulates with the zygomatic process of the temporal bone.

• Nasal Bones: Form the bridge of the nose.

• Vomer: Forms the inferior part of the nasal septum.

• Mandible: Lower jawbone.

  • Mandibular Condyles: Articulate with the mandibular fossa of the temporal bone.

The Vertebral Column

General Structure of a Vertebra

The vertebral column supports the body, protects the spinal cord, and provides attachment points for ribs and muscles. Each vertebra has characteristic features:

• Body (Centrum): Weight-bearing anterior portion.

• Vertebral Foramen: Canal for the spinal cord.

• Vertebral Arch: Encloses the vertebral foramen.

• Spinous Process: Posterior projection for muscle attachment.

• Transverse Processes: Lateral projections for muscle and ligament attachment.

• Superior and Inferior Articular Processes/Facets: Form joints with adjacent vertebrae.

Regional Identification:

• Cervical Vertebrae: Have transverse foramina in each transverse process.

• Thoracic Vertebrae: Superior articular facets face posteriorly.

• Lumbar Vertebrae: Superior articular facets face medially (like hands about to clap).

• Sacrum: Formed by fused sacral vertebrae.

  • Anterior/Posterior Sacral Foramina: Passage for nerves and blood vessels.

  • Median Sacral Crest: Fused spinous processes.

  • Auricular Surfaces: Articulate with the ilium of the pelvis.

  • Sacral Canal: Continuation of the vertebral canal.

• Coccyx: Tailbone, formed by fused coccygeal vertebrae.

The Thoracic Cage

Sternum and Ribs

The thoracic cage protects vital organs and supports the shoulder girdles and upper limbs.

• Sternum: Flat bone at the anterior midline of the thorax.

  • Manubrium: Superior portion.

  • Body (Gladiolus): Main central part.

  • Xiphoid Process: Inferior tip.

  • Costal Cartilages: Connect ribs to sternum.

• Ribs: 12 pairs, each with:

  • Head, Neck, Tubercle, Angle, Shaft, Costal Groove: Features for articulation and muscle attachment.

The Appendicular Skeleton

The Pectoral Girdle

• Clavicle: Collarbone.

  • Sternal End: Articulates with the sternum.

  • Acromial End: Articulates with the scapula.

  • Conoid Tubercle: Ligament attachment site.

• Scapula: Shoulder blade.

  • Acromion, Coracoid Process, Glenoid Cavity, Spine, Subscapular Fossa, Supraspinous Fossa, Infraspinous Fossa, Borders, Angles: Sites for muscle attachment and articulation with the humerus and clavicle.

The Upper Limb

• Humerus: Upper arm bone.

  • Head, Greater/Lesser Tubercle, Intertubercular Sulcus, Capitulum, Trochlea, Epicondyles, Olecranon Fossa, Coronoid Fossa, Radial Fossa, Deltoid Tuberosity: Articulation and muscle attachment sites.

• Radius: Lateral bone of the forearm.

  • Head, Radial Tuberosity, Styloid Process: Articulation and muscle attachment sites.

• Ulna: Medial bone of the forearm.

  • Trochlear Notch, Olecranon, Styloid Process, Coronoid Process: Articulation and muscle attachment sites.

• Carpal Bones: Eight wrist bones arranged in two rows.

  • Proximal Row (lateral to medial): Scaphoid, Lunate, Triquetrum, Pisiform

  • Distal Row (lateral to medial): Trapezium, Trapezoid, Capitate, Hamate

• Metacarpals: Numbered I (thumb) to V (little finger).

• Phalanges: Each finger has three (proximal, middle, distal), except the thumb, which has two.

The Pelvic Girdle

• Hip (Coxal) Bone: Formed by the fusion of ilium, ischium, and pubis.

  • Acetabulum: Socket for the femoral head.

  • Obturator Foramen: Large opening for nerves and blood vessels.

  • Ischial Tuberosity, Iliac Crest, Sacroiliac Joint, Pubic Symphysis, Greater/Lesser Pelvis, Pelvic Inlet/Outlet: Support and articulation features.

The Lower Limb

• Femur: Thigh bone, longest in the body.

  • Head, Greater/Lesser Trochanter, Linea Aspera, Medial/Lateral Condyles: Articulation and muscle attachment sites.

• Tibia: Medial bone of the leg.

  • Tibial Tuberosity, Medial Malleolus, Condyles, Anterior Crest: Articulation and muscle attachment sites.

• Fibula: Lateral bone of the leg.

  • Head, Lateral Malleolus: Articulation and muscle attachment sites.

• Tarsal Bones: Seven ankle bones.

  • Proximal Row: Navicular, Talus, Calcaneus

  • Distal Row: Medial (1st) Cuneiform, Intermediate (2nd) Cuneiform, Lateral (3rd) Cuneiform, Cuboid

• Metatarsals: Numbered I (big toe) to V (little toe).

• Phalanges: Same arrangement as fingers; big toe has two, others have three.

Microscopic Structure of Bone: The Osteon Model

The osteon (Haversian system) is the fundamental functional unit of compact bone, providing strength and structure.

• Periosteum: Dense connective tissue covering the bone's outer surface.

• Osteon: Cylindrical structure consisting of concentric lamellae around a central canal.

• Central Canal: Contains blood vessels and nerves.

• Lamellae: Concentric (around central canal) and circumferential (around the bone's outer surface) layers of bone matrix.

• Perforating (Sharpey) Fibers: Anchor periosteum to bone.

• Lacuna with Osteocyte: Small spaces housing bone cells (osteocytes).

• Endosteum: Membrane lining the internal bone surface.

• Bone Marrow: Soft tissue in medullary cavities.

• Perforating (Volkmann) Canals: Connect central canals of adjacent osteons.

Cell Model: Key Organelles

Cells contain specialized structures (organelles) that perform essential functions for survival and homeostasis.

• Nucleus: Contains genetic material (DNA).

• Nucleolus: Site of ribosome synthesis.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids and detoxifies chemicals.

• Golgi Complex: Modifies, sorts, and packages proteins and lipids.

• Golgi Vesicles: Transport materials from the Golgi apparatus.

• Centrioles & Centrosome: Organize microtubules during cell division.

• Mitochondrion: Site of ATP (energy) production.

• Plasma Membrane: Regulates entry and exit of substances.

• Lysosome: Contains digestive enzymes for waste breakdown.

Skin Model: Layers and Structures

The skin is the largest organ, consisting of multiple layers and specialized structures for protection, sensation, and thermoregulation.

• Merocrine Sweat Gland: Produces watery sweat for cooling.

• Apocrine Sweat Gland: Found in axillary and genital areas; secretes thicker fluid.

• Sebaceous Gland: Secretes sebum (oil) into hair follicles.

• Tactile (Meissner) Corpuscle: Detects light touch.

• Lamellar (Pacinian) Corpuscle: Detects deep pressure and vibration.

• All 5 Layers of the Epidermis: (from superficial to deep) Stratum corneum, lucidum (in thick skin), granulosum, spinosum, basale.

• Hair (Pilus) and Follicle: Hair production and growth.

• Piloerector Muscle: Causes hair to stand (goosebumps).

• Dermis: Connective tissue layer beneath the epidermis.

• Dermal Papillae: Projections that increase surface area for attachment.

• Epidermal Ridges: Form fingerprints.

• Hypodermis/Subcutaneous Fat: Insulation and energy storage.

Major Joints and Ligaments

Knee Joint

• Anterior Cruciate Ligament (ACL): Prevents anterior displacement of the tibia.

• Posterior Cruciate Ligament (PCL): Prevents posterior displacement of the tibia.

• Patellar Ligament: Connects the patella to the tibia.

• Fibular (Lateral) Collateral Ligament: Stabilizes the lateral aspect.

• Tibial (Medial) Collateral Ligament: Stabilizes the medial aspect.

• Lateral and Medial Menisci: Cartilage pads that cushion the joint.

• Quadriceps Tendon: Connects quadriceps muscles to the patella.

Shoulder Joint

• Acromioclavicular Ligament: Connects the acromion of the scapula to the clavicle.

• Coracoclavicular Ligaments (2): Connect the coracoid process to the clavicle.

• Coracoacromial Ligament: Connects the coracoid process to the acromion.

• Glenohumeral Ligaments (3): Reinforce the anterior capsule of the shoulder joint.

• Coracohumeral Ligament: Strengthens the superior part of the capsule.

Summary Table: Major Bone Markings and Their Functions

Additional info:

• Students should be able to identify bones and markings on models, diagrams, or actual specimens.

• Understanding the function of each marking aids in remembering their locations and clinical significance.