Skeletal System Overview

Major Functions of Bones

The skeletal system performs several essential functions that support and protect the human body. Understanding these functions is fundamental to anatomy and physiology.

• Support: Provides structural framework for the body.

• Protection: Shields vital organs (e.g., skull protects brain, rib cage protects heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral/Growth Storage: Stores minerals such as calcium and phosphorus, which can be released into the bloodstream as needed.

• Triglyceride Storage: Fat is stored in yellow bone marrow.

• Blood Cell Formation: Hematopoiesis occurs in red bone marrow, producing red and white blood cells and platelets.

• Hormone Production: Produces hormones such as osteocalcin, which helps regulate bone formation and energy metabolism.

Divisions and Classifications of the Skeleton

Major Divisions

The human skeleton is divided into two main parts:

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage. Provides central support and protection for the brain, spinal cord, and thoracic organs.

• Appendicular Skeleton: Includes the limbs and girdles (shoulder and pelvic), facilitating movement.

Bone Classification by Shape

Bones are classified into four types based on their shapes:

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

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

• Flat Bones: Thin, flat, and slightly curved (e.g., sternum, skull bones).

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

Bone Structure and Growth

Long Bone Structure

Long bones have a shaft (diaphysis), two ends (epiphyses), and membranes. Growth in length occurs at the epiphyseal plate through interstitial growth, maintaining constant thickness.

• Endosteum and Periosteum: Membranes lining the inner and outer surfaces of bones, respectively. Endosteum lines the medullary cavity; periosteum covers the external surface.

• Red Bone Marrow: Found in trabecular cavities of spongy bone in flat bones and the heads of femur and humerus.

Bone Tissue Types

• Compact Bone: Dense outer layer, smooth and solid.

• Spongy Bone: Honeycomb-shaped, contains trabeculae (small needle-like or flat pieces).

Trabeculae

Trabeculae are woven bone structures found in spongy bone, formed when osteoid is laid around blood vessels.

Bone Cells

Bone tissue contains several specialized cell types:

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells responsible for synthesizing bone matrix.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Bone-lining Cells: Flat cells found on bone surfaces where bone remodeling is not occurring.

• Osteoclasts: Large cells that resorb or break down bone matrix.

Microscopic Structure of Bone

The osteon (Haversian system) is the basic structural and functional unit of compact bone. It consists of concentric lamellae (layers) of bone matrix surrounding a central canal (Haversian canal) that contains blood vessels and nerves.

• Volkmann's Canals: Transverse or diagonal canals that connect osteons and allow passage of blood vessels and nerves.

Comparison: Volkmann's canals run perpendicular to Haversian canals and connect them, facilitating vascular and nerve supply throughout bone tissue.

Steps in Fracture Healing

Bone healing after a fracture occurs in four main steps:

1. Hematoma Formation: Blood clot forms at the fracture site.

2. Fibrocartilaginous Callus Formation: Soft callus forms, bridging the broken bone ends.

3. Bony Callus Formation: Hard callus of spongy bone replaces the soft callus.

4. Bone Remodeling: Bone is remodeled to restore its original shape and structure.

Bone Diseases

• Rickets: Softening and weakening of bones in children, usually due to vitamin D deficiency.

• Osteoporosis: Condition characterized by decreased bone mass and increased risk of fractures.

• Paget's Disease: Chronic disorder that can result in enlarged and misshapen bones.

Skull Anatomy

Axial Skeleton and Skull

The axial skeleton consists of 80 bones, including the skull, vertebral column, and thoracic cage. The skull is the most complex bony structure, formed by cranial and facial bones.

• Cranial Bones: Enclose the brain and provide attachment for head and neck muscles.

• Facial Bones: Form cavities for special sense organs, provide openings for air and food, and anchor facial muscles.

Major Sutures of the Skull

Sutures are immovable joints between skull bones, with a serrated, saw-tooth appearance.

• Coronal Suture: Between parietal bones and frontal bone (top).

• Squamous Suture: Between parietal and temporal bones (side).

• Lambdoid Suture: Between parietal bones and occipital bone (back, top).

• Mastoid Suture: Junction of parietal and temporal bones (back, bottom).

• Sagittal Suture: Junction of the two parietal bones (top, back of head).

Skull Openings and Cavities

The skull contains 85 openings, including foramina, canals, and fissures, which allow passage of nerves and blood vessels.

• Foramen Magnum: Largest hole in the skull, located at the base, allowing passage of the spinal cord.

• Middle Cranial Fossa: Houses the temporal lobes.

• Posterior Cranial Fossa: Houses the cerebellum.

Other cavities include the middle and internal ear cavities, nasal cavity, and orbits for the eyeballs.

Cranial Bones

The cranium is comprised of eight cranial bones:

• Frontal bone (1)

• Parietal bones (2)

• Occipital bone (1)

• Temporal bones (2)

• Sphenoid bone (1)

• Ethmoid bone (1)

Frontal Bone

• Shell-shaped bone forming the anterior portion of the cranium.

• Vertical part (squamous region) forms the forehead.

• Inferior portion forms the supraorbital margins (ridge of the anterior eyebrows).

• Forms the superior wall of the orbits and most of the anterior cranial fossa.

• Supraorbital Foramen (Notch): Allows passage of the supraorbital artery and nerve.

• Glabella: Area of frontal bone between the orbits.

• Frontal Sinuses: Located just lateral to the glabella.

Parietal Bones and Major Sutures

• Coronal Suture: Parietal and frontal bones.

• Sagittal Suture: Between right and left parietal bones.

• Lambdoid Suture: Parietal and occipital bones.

• Squamous Suture: Parietal and temporal bones.

Occipital Bone

• Forms most of the skull's posterior wall and posterior cranial fossa.

• External occipital protuberance: Protusion just superior to foramen magnum.

Temporal Bones

• Paired bones forming the inferolateral aspects of the skull and parts of the cranial base.

• Squamous: Zygomatic process articulates with the zygomatic bone.

• Tympanic: Surrounds the external acoustic meatus (ear canal).

• Petrous: Houses middle and inner ear cavities.

• Jugular Foramen: Passage for nerves and carotid canal.

• Mastoid and Styloid Processes: Attachment sites for several neck and tongue muscles.

Key Bone Markings and Terms

Bone markings are features on bones that serve as sites of muscle, ligament, and tendon attachment, or as passageways for blood vessels and nerves.

• Tuberosity: Large rounded projection.

• Groove: Furrow.

• Trochanter: Large, blunt, irregularly shaped process (found only on femur).

• Crest: Narrow ridge of bone.

• Spine: Sharp, slender, often pointed projection.

• Process: Any bony prominence.

• Epicondyle: Raised area on or above a condyle.

• Condyle: Rounded articular projection.

• Ramus: Arm-like bar of bone.

• Fissure: Narrow, slitlike opening.

• Foramen: Round or oval opening through a bone.

• Meatus: Canal-like passageway.

• Fossa: Shallow, basinlike depression.

• Sinus: Cavity within a bone, filled with air and lined with mucous membrane.

Additional info: Mastoid process contains cavities (sinuses) called mastoid air cells. Mastoiditis can develop if a middle ear infection spreads into mastoid process area.

Sphenoid Bone

The sphenoid bone is a complex, bat-shaped bone - keystone bone that articulates with all other cranial bones.

• Sphenoid sinuses found within body of sphenoid.

• Body also includes sella turcica prominence that includes hypophyseal fossa area enclosing the pituitary gland.

Sphenoid contains three pairs of processes:

• Greater wings

• Lesser wings

• Pterygoid processes

Sphenoid contains several foramina:

• Optic canals – allow passage of optic nerve

• Superior orbital fissure

• Foramen rotundum and foramen ovale

Ethmoid Bone

• Ethmoid bone – deepest skull bone

• Superior part formed by paired cribriform plates

• Superior orbital fissure

• Forms part of the medial orbit and cribriform plates

• Lateral masses extended medially to form superior and middle nasal conchae

Facial Skeleton

Facial skeleton is made up of 14 bones, 12 of which are paired:

• Mandible

• Maxillary bones (2)

• Zygomatic bones (2)

• Nasal bones (2)

• Lacrimal bones (2)

• Palatine bones (2)

• Vomer

• Inferior nasal conchae (2)

Summary Table: Major Cranial Bones and Sutures