Module 7.1: Overview of the Skeletal System

Structure of the Skeletal System and Skeletal Cartilages

The skeletal system consists of about 206 bones and their associated cartilages, providing support, protection, and movement for the body. The skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Forms the body’s longitudinal axis and protects vital organs.

  • Skull: Made up of 22 bones (8 cranial, 14 facial) that encase the brain and form the face.

  • Vertebral Column: Consists of 33 bones; 24 individual vertebrae protect the spinal cord, with the remaining vertebrae fused as the sacrum and coccyx.

  • Thoracic Cage: Composed of 12 pairs of ribs, sternum, and part of the vertebral column; protects the lungs, heart, and other thoracic organs.

• Appendicular Skeleton: Includes bones of the pectoral girdle, upper limb, pelvic girdle, and lower limb. It is primarily suited for movement and muscle attachment.

  • Pectoral Girdle: Clavicle and scapula; anchors the upper limb to the trunk.

  • Upper Limb: Arm (humerus), forearm (radius and ulna), wrist and hand (carpals, metacarpals, phalanges).

  • Pelvic Girdle: Two pelvic bones and the sacrum; collectively called the pelvis, anchors the lower limb to the trunk.

  • Lower Limb: Thigh (femur), leg (tibia and fibula), ankle and foot (tarsals, metatarsals, phalanges).

Bone Markings: Bones have surface features such as depressions, openings, and projections, which serve as sites for muscle attachment, passage of blood vessels and nerves, and articulation with other bones.

Module 7.2: The Skull

Overview of Skull Structure

The skull is composed of 22 bones, divided into cranial and facial bones.

• Cranial Bones (Cranium): 8 bones (frontal, occipital, ethmoid, sphenoid, 2 temporal, 2 parietal) that encase the brain.

• Facial Bones: 14 bones (maxillary, zygomatic, nasal, lacrimal, palatine, inferior nasal conchal, mandible, vomer) that form the framework of the face.

Most skull bones are joined by immovable joints called sutures.

• Cranial Cavity: Formed by cranial bones, surrounds the brain and houses special sense organs.

• Other Cavities: Orbits (eyes), nasal cavity (respiratory tract), oral cavity (digestive tract), paranasal sinuses (air-filled spaces).

The superior portion of the cranial cavity is called the cranial vault or calvarium; the inferior portion is the cranial base.

Major Skull Features and Views

• Anterior View: Shows all facial bones, nasal cavity, and portions of cranial bones.

• Lateral View: Shows all cranial bones, zygomatic arch, and articulation between mandible and temporal bone.

• Posterior, Superior, Inferior Views: Highlight surface markings, sutures, and articulations.

• Medial View: Best for viewing the nasal septum, perpendicular plate of ethmoid, vomer, and paranasal sinuses.

Cavities of the Skull

• Orbit: Houses the eyeball, lacrimal gland, and associated structures. Formed by seven bones: frontal, maxilla, zygomatic, sphenoid, ethmoid, lacrimal, palatine.

• Nasal Cavity: Lined with mucous membranes; divided by the nasal septum (perpendicular plate of ethmoid and vomer). Formed by sphenoid, palatine, maxilla, ethmoid, and inferior nasal conchae.

• Paranasal Sinuses: Air-filled spaces in frontal, sphenoid, ethmoid, and maxillary bones; lighten the skull and enhance voice resonance.

• Oral Cavity: Contains the tongue and most salivary glands; roof formed by palatine processes of maxilla and palatine bones.

Fetal Skull and Fontanels

• Fetal skull bones are not fused; soft spots called fontanels allow for flexibility during birth and brain growth.

• Major fontanels: anterior, posterior, and two sphenoid fontanels.

Hyoid Bone

• Located in the superior neck, does not articulate with other bones.

• Anchored by ligaments and muscles; aids in swallowing and speech.

Module 7.3: The Vertebral Column and Thoracic Cage

Overview of the Vertebral Column

The vertebral column (spine) is composed of about 33 bones called vertebrae, classified by region:

• Cervical Vertebrae: 7 (C1–C7) in the neck.

• Thoracic Vertebrae: 12 (T1–T12) in the thorax.

• Lumbar Vertebrae: 5 (L1–L5) in the lower back.

• Sacrum: 5 fused vertebrae, articulates with the pelvic bones.

• Coccyx: 3–5 fused vertebrae at the base of the spine.

Spaces between moveable vertebrae are called intervertebral foramina, allowing passage of spinal nerves.

Spinal Curvatures

• Primary Curvatures: Thoracic and sacral, present in the fetal period.

• Secondary Curvatures: Cervical (develops as infant lifts head) and lumbar (develops as child walks).

• Abnormal Curvatures:

  • Scoliosis: Abnormal lateral curvature.

  • Lordosis: Exaggerated cervical/lumbar curvature (swayback).

  • Kyphosis: Exaggerated thoracic curvature (hunchback).

Structure of the Vertebrae

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

• Vertebral Foramen: Large opening for the spinal cord.

• Processes:

  • Spinous Process: Posterior projection for muscle attachment.

  • Transverse Processes: Lateral projections for muscle attachment.

  • Superior and Inferior Articular Processes: Form joints between vertebrae.

Regional Vertebral Differences

• Cervical Vertebrae: Smallest, have transverse foramina for arteries. C1 (atlas) supports the skull; C2 (axis) has the dens for rotation.

• Thoracic Vertebrae: Larger, heart-shaped bodies, long spinous processes, costal facets for rib articulation.

• Lumbar Vertebrae: Largest, thick bodies, short spinous processes, support most body weight.

Thoracic Cage

• Composed of the sternum, ribs, and thoracic vertebrae.

• Protects the heart, lungs, and major blood vessels.

Key Terms and Definitions

• Suture: Immovable joint between skull bones.

• Fontanel: Soft spot in the fetal skull where bones have not fused.

• Foramen: Opening in a bone for nerves or blood vessels.

• Process: Projection from a bone for muscle or ligament attachment.

• Sinus: Air-filled cavity within a bone.

Example Table: Types of Vertebrae and Key Features

Formulas and Equations

• Number of Bones in Adult Human Skeleton: $206$

• Number of Vertebrae by Region:

Example Application: Understanding the structure and function of the vertebral column is essential for diagnosing spinal disorders such as scoliosis, lordosis, and kyphosis.