The Skeletal System: Structure, Function, and Anatomy

The Skeletal System

5.1- Subdivisions of the Skeleton

The human skeleton is divided into two main parts, each with distinct anatomical and functional roles.

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage. It forms the longitudinal axis of the body and supports and protects the organs of the head, neck, and trunk.

• Appendicular Skeleton: Includes the bones of the limbs and the girdles (shoulder and pelvic) that attach them to the axial skeleton. It is primarily involved in movement and manipulation of objects.

5.2- Functions of the Skeletal System

The skeletal system performs several vital functions necessary for survival and movement.

• Support: Provides a structural framework for the body and supports soft tissues.

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

• Movement: Serves as levers for muscles to act upon, enabling movement.

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

• Blood Cell Formation (Hematopoiesis): Occurs in the red bone marrow of certain bones.

5.3a- Classification of Bones

There are 2 basic types of osseous or bone tissue:

• Compact bone- is dense, looks smooth and homogenous. It is composed of osteons and provides a lot of support and protection.

• Spongy bones- Have a spiky open appearance like a sponge and it is composed of trabeculae. Spongy bone is found in areas of stress.

b) Bones are classified according to their shapes, which relate to their functions.

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

• Short Bones: Cube-shaped and nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, ribs, skull bones). SANDWICHED COMPACT BONE- SPONGY BONE- COMPACT BONE

• Irregular Bones: Complicated shapes that do not fit other categories (e.g., vertebrae, hip bones).

5.4- Anatomy of a Long Bone

Long bones have distinct anatomical regions that contribute to their function and growth.

• Diaphysis: The shaft or central part of a long bone, composed mainly of compact bone.

• Epiphyses: The expanded ends of the bone, consisting mostly of spongy bone covered by a thin layer of compact bone.

• Metaphysis: The region between the diaphysis and epiphysis, containing the epiphyseal plate (growth plate) in growing bones.

• Periosteum: A dense connective tissue membrane covering the outer surface of the bone.

• Medullary Cavity: The central cavity within the diaphysis, containing yellow bone marrow in adults.

• Endosteum: A thin membrane lining the medullary cavity.

5.5- Microscopic Structure of Compact Bone

Compact bone is organized into structural units called osteons or Haversian systems.

• Osteon (Haversian System): The basic structural unit, consisting of concentric lamellae (layers) of calcified matrix surrounding a central (Haversian) canal.

• Central Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces between lamellae that house osteocytes (bone cells).

• Canaliculi: Tiny channels that connect lacunae, allowing for nutrient and waste exchange between osteocytes and blood vessels.

5.6- Bone Composition: Bone Salts and Organic Matrix

Bones are both hard and flexible due to their unique composition.

• Bone Salts (Inorganic Matrix): Mainly hydroxyapatite (calcium phosphate crystals), providing hardness and resistance to compression.

• Organic Matrix: Primarily collagen fibers, giving bones flexibility and tensile strength.

Example: Removal of minerals makes bone flexible; removal of collagen makes bone brittle.

Bone Formation and Remodeling

Bone development and maintenance involve several processes throughout life.

• Ossification: The process of bone formation, beginning in the fetus. Most bones develop from hyaline cartilage (endochondral ossification), while some flat bones form from fibrous membranes (intramembranous ossification).

• Bone Remodeling: Continuous process where old bone is replaced by new bone tissue, involving osteoclasts (bone-resorbing cells) and osteoblasts (bone-forming cells).

  Example: Bone remodeling helps maintain calcium homeostasis and adapts bone structure to stress.Select the correct order of events that occur during the embryonic and fetal stages of the long bone development observed during the audio tour:

Formation of bone model made of hyaline cartilage, formation of a bone collar, formation of small cavities and invasion of blood vessels, replacement of cartilage by bone in the diaphysis.

Types of Bone Fractures

Fractures are classified by their characteristics and the way the bone breaks.

• Simple (Closed) Fracture: Bone breaks cleanly but does not penetrate the skin.

• Compound (Open) Fracture: Broken bone ends penetrate the skin.

• Comminuted Fracture: Bone breaks into several pieces.

• Greenstick Fracture: Bone breaks incompletely, common in children.

• Compression, Spiral, and Impacted Fractures: Other types based on force and direction of break.

Bones and Sutures of the Skull

The skull is composed of several bones joined by immovable joints called sutures.

• Major Skull Bones: Frontal, parietal, temporal, occipital, sphenoid, ethmoid, maxilla, mandible, zygomatic, nasal, lacrimal, palatine, vomer, inferior nasal concha.

• Major Sutures:

  • Coronal Suture: Between frontal and parietal bones.

  • Sagittal Suture: Between the two parietal bones.

  • Lambdoid Suture: Between parietal and occipital bones.

  • Squamous Suture: Between parietal and temporal bones.

Vertebrae: Structure and Regional Differences

Vertebrae share common features but differ by region to accommodate specific functions.

• Parts of a Typical Vertebra: Body (weight-bearing), vertebral arch, vertebral foramen (spinal cord passage), transverse processes, spinous process, superior and inferior articular processes.

• Cervical Vertebrae (C1–C7): Smallest, have transverse foramina, C1 (atlas) and C2 (axis) are specialized.

• Thoracic Vertebrae (T1–T12): Larger, articulate with ribs, have long, downward-pointing spinous processes.

• Lumbar Vertebrae (L1–L5): Largest and strongest, have short, thick spinous processes.

Intervertebral Discs and Spinal Curvatures

Intervertebral discs and spinal curvatures are essential for flexibility and shock absorption.

• Intervertebral Discs: Pads of fibrocartilage between vertebrae that absorb shock and allow movement.

• Spinal Curvatures: Normal curves (cervical, thoracic, lumbar, sacral) help distribute weight and maintain balance.

Abnormal Spinal Curvatures

Abnormal curvatures can affect posture and health.

• Scoliosis: Lateral (sideways) curvature of the spine.

• Lordosis: Exaggerated inward curvature of the lumbar region.

• Kyphosis: Exaggerated outward curvature of the thoracic region (hunchback).

The Thoracic Cage

The thoracic cage protects vital organs and supports the upper body.

• Components: Sternum (breastbone), ribs, and thoracic vertebrae.

True Ribs vs. False Ribs

Ribs are classified based on their attachment to the sternum.

• True Ribs (1–7): Attach directly to the sternum via costal cartilages.

• False Ribs (8–12): Either attach indirectly to the sternum or not at all (ribs 11–12 are also called floating ribs).

Bones of the Shoulder and Pelvic Girdles and Limbs

The appendicular skeleton includes the girdles and limbs, enabling movement and manipulation.

• Shoulder Girdle: Clavicle (collarbone) and scapula (shoulder blade).

• Upper Limb Bones: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic Girdle: Two hip bones (each composed of ilium, ischium, and pubis).

• Lower Limb Bones: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Differences Between Male and Female Pelvis

The pelvis differs between sexes to accommodate childbirth in females.

Feature	Male Pelvis	Female Pelvis
General Structure	Heavier, thicker, more prominent markings	Lighter, thinner, smoother
Pelvic Inlet	Narrow, heart-shaped	Wider, oval-shaped
Pubic Arch Angle	Acute (<90°)	Obtuse (>90°)
Sacrum	Longer, more curved	Shorter, less curved

Example: The wider female pelvis facilitates childbirth.

1. A(n) osteoclast is a cell that breaks down bone matrix.

2. A joint that is slightly moveable is classified as a(n) joint amphiarthrosis

3. The bones between the phalanges and tarsals are the metatarsal.

4. The mallet-shaped protrusion on the inside of the ankle is the medial malleolus

5. A membrane that holds the elbow joint together is called a(n) synovial membrane