The Skeletal System

Overview and Divisions

The skeletal system, or skeleton, is a complex framework composed of bones, cartilage, and ligaments. It provides structural support, protection, and facilitates movement. The skeleton accounts for approximately 20% of total body mass and is divided into two main regions:

• Axial Skeleton: Forms the central axis of the body.

• Appendicular Skeleton: Includes the limbs and structures that attach them to the axial skeleton.

Key Point: The axial skeleton supports and protects the organs of the head, neck, and trunk, while the appendicular skeleton enables movement and interaction with the environment.

Axial Skeleton

Major Regions and Functions

The axial skeleton consists of 80 bones divided into three main regions:

• Skull

• Vertebral Column

• Thoracic Cage

Functions of the axial skeleton include:

• Forming the longitudinal axis of the body

• Supporting the head, neck, and trunk

• Protecting the brain, spinal cord, and thoracic organs

Skull: Structure and Function

The skull is the most complex bony structure in the body and consists of two sets of bones:

• Cranium: Protects the brain and provides sites of attachment for head and neck muscles.

• Facial Bones: Form the framework of the face, house cavities for special sense organs (sight, taste, smell), and provide openings for air and food passage.

Most skull bones are flat and firmly locked together by immovable joints called sutures, which have a saw-tooth appearance.

Cranial Bones

The cranium is composed of eight bones:

• Frontal Bone (forehead)

• Parietal Bones (two—left and right)

• Occipital Bone (back of head)

• Temporal Bones (two—left and right, temples)

• Sphenoid Bone

• Ethmoid Bone

These bones form articulations (sutures) with each other, creating the protective cranial cavity for the brain.

Major Openings and Foramina

The skull contains numerous foramina (openings), which serve as passageways for the spinal cord, major blood vessels, and the 12 cranial nerves. These structures are essential for communication between the brain and the rest of the body.

Additional info: Students should be able to distinguish between foramina, canals, and fissures, but are not required to memorize all specific contents for each opening at this level.

Facial Bones

The facial skeleton is made up of 14 bones, most of which are paired. These bones form the structure of the face and provide cavities for the sense organs and openings for air and food passage.

• Mandible: Lower jawbone, largest and strongest facial bone.

• Maxillae (2): Upper jawbones, contain sockets for upper teeth.

• Zygomatic Bones (2): Cheekbones.

• Nasal Bones (2): Bridge of the nose.

• Lacrimal Bones (2): Medial walls of orbits, house lacrimal sacs for tear drainage.

• Palatine Bones (2): Form part of the hard palate.

• Vomer: Forms part of the nasal septum.

• Inferior Nasal Conchae (2): Form part of the lateral walls of the nasal cavity.

Key Sutures and Fontanels

Sutures are immovable joints that connect the bones of the skull. In infants, the skull bones are connected by fontanels (soft spots), which allow for growth and development of the brain and skull.

• Coronal Suture: Between frontal and parietal bones

• Sagittal Suture: Between parietal bones

• Lambdoid Suture: Between parietal and occipital bones

• Squamous Suture: Between parietal and temporal bones

Additional info: Fontanels ossify as the child grows, forming the adult sutures.

Appendicular Skeleton

Overview

The appendicular skeleton consists of the bones of the limbs and the girdles that attach them to the axial skeleton. It is essential for movement and manipulation of the environment.

• Pectoral Girdle: Attaches upper limbs to the trunk

• Pelvic Girdle: Attaches lower limbs to the trunk

• Upper Limbs: Arms, forearms, wrists, and hands

• Lower Limbs: Thighs, legs, ankles, and feet

Summary Table: Axial vs. Appendicular Skeleton

Key Terms and Definitions

• Bone: Rigid organ that constitutes part of the vertebrate skeleton.

• Cartilage: Flexible connective tissue found in joints, ear, nose, and other structures.

• Ligament: Connective tissue that connects bones to other bones.

• Suture: Immovable joint between skull bones.

• Fontanel: Soft spot on an infant's skull where bone formation is not complete.

• Foramen: Opening or hole in a bone for passage of nerves and blood vessels.

Example: Clinical Application

Damage to the foramina of the skull can result in neurological deficits due to impaired passage of cranial nerves. Understanding the location and function of these openings is crucial for clinical diagnosis and surgical procedures.

Formulas and Equations

While the skeletal system does not involve many direct mathematical formulas, the following equation is relevant for bone mass estimation:

Additional Info

• Students should be familiar with the names, locations, and functions of major bones and bone markings.

• Understanding the differences between axial and appendicular skeletons is foundational for further study in anatomy and physiology.