Overview of the Skeletal System and Axial Skeleton: Study Notes

Module 7.1: Overview of the Skeletal System

Structure of the Skeletal System and Skeletal Cartilages

The skeletal system consists of approximately 206 bones and their associated cartilages, which provide structural support and protection 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. It includes:

  • 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, while the remaining vertebrae are fused (sacrum and coccyx).

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

• Appendicular Skeleton: Includes bones of the pectoral girdle, upper limb, pelvic girdle, and lower limb. Primarily suited for movement and support.

  • 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 sacrum; 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 various surface features, including depressions, openings, and projections, which serve as sites for articulation, passage of nerves and blood vessels, and attachment of ligaments and tendons.

• Depressions: Allow bones to articulate with other bones.

• Openings: Permit passage of structures and allow travel through bones.

• Projections: Sites for ligament and tendon attachment.

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, temporal (2), parietal (2).

• Facial Bones: 14 bones: maxillary (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal concha (2), mandible, vomer.

Key Features of the Skull

• All facial bones are visible in the anterior view.

• The vomer and inferior nasal conchae are visible within the nasal cavity.

• Portions of ethmoid, sphenoid, and frontal bones are visible in the cranial cavity.

• The lateral view shows the zygomatic arch and articulation between mandible and temporal bone.

• The posterior, superior, and inferior views highlight surface markings and sutures.

• The medial view (midsagittal section) shows the nasal septum and paranasal sinuses.

Skull Cavities

• Orbit: Houses the eyeball, lacrimal gland, and associated structures. Formed by seven bones.

• Nasal Cavity: Lined with mucous membranes; divided by the nasal septum. Formed by several bones including the ethmoid, vomer, and maxilla.

• Paranasal Sinuses: Frontal, sphenoid, ethmoid, and maxillary sinuses; reduce skull weight and enhance voice resonance.

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

Fetal Skull and Fontanels

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

• Major fontanels:

  • Anterior fontanel: Between frontal and parietal bones.

  • Posterior fontanel: Between parietal and occipital bones.

  • Sphenoid fontanels: On the sides of the skull.

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) consists of an average of 33 bones called vertebrae, classified by structure and location.

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

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

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

• Sacral Vertebrae: 5 fused vertebrae (sacrum).

• Coccygeal Vertebrae: 3–5 fused vertebrae (coccyx).

Spinal Curvatures

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

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

• Abnormal Curvatures:

  • Scoliosis: Abnormal lateral curvature.

  • Lordosis: Exaggerated cervical/lumbar curvature.

  • Kyphosis: Exaggerated thoracic curvature (hunchback).

Structure of the Vertebrae

All vertebrae share common structural features that support spinal cord protection and movement.

• Body (Centrum): Main weight-bearing surface.

• Vertebral Foramen: Large opening for spinal cord.

• Processes:

  • Spinous Process: Projects posteriorly; muscle attachment.

  • Transverse Processes: Project laterally; muscle attachment.

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

Special Cervical Vertebrae

• Atlas (C1): Supports the skull; allows nodding motion.

• Axis (C2): Has the dens (odontoid process); allows rotation of the head.

Thoracic and Lumbar Vertebrae

• Thoracic Vertebrae: Larger than cervical; heart-shaped bodies; facets for rib articulation.

• Lumbar Vertebrae: Largest and heaviest; adapted for weight-bearing.

Thoracic Cage

The thoracic cage consists of the sternum, ribs, and thoracic vertebrae. It protects the heart and lungs and supports the shoulder girdles.

• Sternum: Flat bone at the center of the chest.

• Ribs: 12 pairs; true ribs (1–7), false ribs (8–12), floating ribs (11–12).

Table: Major Divisions and Features of the Human Skeleton

Division	Main Components	Function
Axial Skeleton	Skull, vertebral column, thoracic cage	Protection of vital organs, support
Appendicular Skeleton	Pectoral girdle, upper limb, pelvic girdle, lower limb	Movement, support, muscle attachment

Key Terms and Definitions

• Foramen: An opening in a bone for nerves and blood vessels.

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

• Suture: An immovable joint between skull bones.

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

• Sinus: Air-filled cavity within a bone.

Formulas and Equations

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

• Number of Cervical Vertebrae: $7$

• Number of Thoracic Vertebrae: $12$

• Number of Lumbar Vertebrae: $5$

Summary

The skeletal system provides the framework for the body, protects vital organs, and enables movement. Understanding the structure and function of bones, joints, and associated features is essential for the study of anatomy and physiology.