Study Notes: The Skeletal System – Axial Skeleton and Bone Structure

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The Skeletal System

Introduction

The skeletal system provides the structural framework for the human body, protects vital organs, supports movement, and serves as a reservoir for minerals. This section focuses on the axial skeleton and the anatomy of bone tissue.

Bone Classification and Shapes

Types of Bones

Long Bones: Longer than they are wide; primarily found in the limbs (e.g., femur, humerus).
Short Bones: Approximately equal in length and width; found in the wrist and ankle (e.g., carpals, tarsals).
Flat Bones: Thin, flattened, and often curved; provide protection (e.g., sternum, cranial bones).
Irregular Bones: Complex shapes that do not fit other categories (e.g., vertebrae, some facial bones).

Gross Anatomy of Bone

Major Structures

Compact Bone: Dense outer layer providing strength and protection.
Spongy Bone: Porous inner layer containing trabeculae; houses red bone marrow.
Epiphysis: The expanded ends of long bones; contains spongy bone and red marrow.
Diaphysis: The shaft of a long bone; composed mainly of compact bone surrounding the medullary cavity.
Articular Cartilage: Hyaline cartilage covering joint surfaces; reduces friction and absorbs shock.
Marrow (Medullary) Cavity: Central cavity in the diaphysis; contains yellow bone marrow in adults.
Epiphyseal Line: Remnant of the epiphyseal plate in adults; marks where bone growth occurred.
Epiphyseal Plate: Hyaline cartilage plate in growing bones; site of lengthwise growth in children.
Red Bone Marrow: Site of hematopoiesis (blood cell formation); found in spongy bone.
Yellow Bone Marrow: Stores fat; found in the medullary cavity of long bones in adults.

Microscopic Anatomy of Bone

Bone Tissue Structure

Periosteum: Dense connective tissue membrane covering the outer surface of bones; contains nerves and blood vessels.
Endosteum: Thin membrane lining the medullary cavity and spaces within spongy bone.
Osteon (Haversian System): Structural unit of compact bone; consists of concentric lamellae around a central canal.
Concentric Lamellae: Rings of bone matrix within an osteon.
Interstitial Lamellae: Layers of bone matrix between osteons.
Circumferential Lamellae: Layers of bone matrix that encircle the entire bone beneath the periosteum.
Central (Haversian) Canal: Channel in the center of each osteon; contains blood vessels and nerves.
Perforating (Volkmann’s) Canal: Canals perpendicular to the central canal; connect blood and nerve supply of periosteum to those in central canals.
Lacunae: Small spaces between lamellae housing osteocytes.
Osteocytes: Mature bone cells that maintain bone tissue.
Canaliculi: Tiny channels connecting lacunae; allow for nutrient and waste exchange between osteocytes.
Trabeculae: Lattice-like structures in spongy bone; provide structural support and house marrow.

Chemical Composition of Bone

Organic and Inorganic Components

Organic Components: Include collagen fibers and ground substance; provide flexibility and tensile strength.
Inorganic Components: Mainly hydroxyapatite (calcium phosphate crystals); provide hardness and resistance to compression.

Formula for Hydroxyapatite:

General Bone Markings

Types and Functions

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

Marking	Description
Canal (Meatus)	Passageway through a bone
Condyle	Rounded articular projection
Crest	Narrow, prominent ridge
Epicondyle	Raised area above a condyle
Facet	Smooth, flat articular surface
Fissure	Narrow, slit-like opening
Foramen	Round or oval opening through a bone
Fossa	Shallow depression
Fovea	Small pit or depression
Groove (Sulcus)	Furrow
Head	Enlarged, often rounded end of a bone
Line	Narrow ridge, less prominent than a crest
Neck	Narrowed region below the head
Notch	Indentation at the edge of a bone
Process	Any bony prominence
Protuberance	Projection of bone
Sinus	Cavity within a bone
Spine	Sharp, slender projection
Trochanter	Large, blunt, irregular projection (femur only)
Tubercle	Small, rounded projection
Tuberosity	Large, rounded projection

Axial Skeleton

Overview

The axial skeleton forms the central axis of the body and consists of the skull, vertebral column, rib cage, and associated structures.

Skull

Cranial Bones: Protect the brain. Includes the frontal, parietal, occipital, temporal, sphenoid, and ethmoid bones.
Facial Bones: Form the structure of the face. Includes maxilla, zygomatic, mandible, vomer, nasal, inferior nasal conchae, lacrimal, and palatine bones.

Vertebral Column

Cervical Vertebrae: 7 vertebrae in the neck region. Includes the atlas (C1) and axis (C2).
Thoracic Vertebrae: 12 vertebrae in the upper back; articulate with ribs.
Lumbar Vertebrae: 5 vertebrae in the lower back.
Sacral Vertebrae: 5 fused vertebrae forming the sacrum.
Coccyx: 3-5 fused vertebrae forming the tailbone.

Bone Markings of Vertebrae

Spinous Process: Posterior projection for muscle attachment.
Transverse Process: Lateral projections for muscle and ligament attachment.
Vertebral Foramen: Opening for the spinal cord.
Body (Centrum): Weight-bearing portion of the vertebra.
Transverse Foramina: Openings in cervical vertebrae for vertebral arteries.
Dens: Odontoid process of the axis (C2); allows head rotation.

Rib Cage

Ribs (Costae): 12 pairs; protect thoracic organs.
True Ribs: First 7 pairs; attach directly to the sternum.
False Ribs: Pairs 8-10; attach indirectly to the sternum.
Floating Ribs: Pairs 11-12; do not attach to the sternum.
Sternum: Consists of the manubrium, body, and xiphoid process.
Hyoid Bone: U-shaped bone in the neck; supports the tongue.

Bone Markings of Cranial and Facial Bones

Cranial Bones

Frontal Bone: Supraorbital foramen, supraorbital margin, glabella, frontal squama, frontal sinuses.
Parietal Bones: Coronal, sagittal, lambdoid, and squamous sutures.
Temporal Bones: Styloid process, mastoid process, external acoustic meatus, jugular foramen, carotid canal, mandibular fossa, zygomatic process.
Occipital Bone: External occipital crest, external occipital protuberance, hypoglossal canal, foramen magnum, occipital condyles.
Sphenoid: Greater wing, lesser wing, sella turcica.
Ethmoid: Crista galli, cribriform plate, olfactory foramina, perpendicular plate.

Facial Bones

Mandible: Mandibular body, coronoid process, mandibular condyle, mandibular notch, mandibular foramen, mental foramen, alveolar process.
Maxillary Bones: Alveolar process, infraorbital foramen, palatine process.
Palatine Bones, Zygomatic Bones, Nasal Bones, Inferior Nasal Conchae, Lacrimal Bones, Vomer: Each contributes to the structure and function of the face and nasal cavity.

Example: The Atlas and Axis

Atlas (C1): Supports the skull; allows nodding motion.
Axis (C2): Has the dens (odontoid process); allows rotation of the head.

Additional info: The above notes expand on the checklist by providing definitions, context, and examples for each anatomical term and structure, ensuring a comprehensive understanding suitable for college-level study.