7.1 & 7.2

Skeletal System Overview

  • Definition: The skeletal system forms the rigid internal framework of the body.

    • Composed of:

    • Bones

    • Cartilages

    • Ligaments

  • Functions of the skeletal system:

    • Supports the weight of the body.

    • Allows for body movements.

    • Protects internal organs.

Cartilage

  • Definition: Cartilage provides flexible strength and support for body structures such as:

    • Thoracic cage

    • External ear

    • Trachea and larynx

  • Functions at joints:

    • Unites adjacent bones.

    • Provides cushioning between bones.

Ligaments

  • Definition: Ligaments are strong connective tissue bands that hold bones at moveable joints together.

  • Functions:

    • Prevent excess movement of joints to avoid injury.

Muscles

  • Role: Muscles are firmly attached to the skeleton via connective tissue structures called tendons.

  • Function: As muscles contract, they pull on bones to produce movements of the body.

  • Importance: Without a skeleton, fundamental movements like standing, running, or feeding oneself would be impossible.

Bone Structure and Function

  • Each bone serves a particular function; thus, bones vary in:

    • Size

    • Shape

    • Strength

  • Example: Bones in the lower back and lower limbs are thick and strong to support body weight.

  • Muscle attachment sites:

    • Size of a bony landmark related to muscle strength.

    • Bones must have enlarged bony landmarks to resist strong pulling forces from muscles.

  • Dynamic nature of bones:

    • Bones can modify their strength and thickness in response to changes in:

    • Muscle strength

    • Body weight

    • Muscle attachment sites may thicken with increased muscle strength from workouts.

    • Weight-bearing bones thicken with body weight or activity.

    • Conversely, reductions in muscle strength or body weight lead to bone thinning, as seen during hospital stays or limb immobilization.

Divisions of the Skeletal System

  • Total bones in the adult human skeleton: 206.

  • Higher number in younger individuals due to fusion of bones during development.

Major Functions of the Skeleton

  • Providing a rigid internal structure that supports body weight against gravity.

  • Allowing muscles to act upon it to produce bodily movements.

  • Specialization of the lower skeleton for stability during walking/running;

  • Upper skeleton specialization for greater mobility and range of motion.

  • Protective functions:

    • Protects internal organs like the brain, spinal cord, heart, lungs, and pelvic organs.

  • Storage functions:

    • Serves as the primary storage site for minerals such as calcium and phosphate.

    • Contains bone marrow that stores fat and produces blood cells.

Divisions of the Skeleton

  • The skeleton is subdivided into two major divisions:

    • Axial Skeleton:

    • Comprises 80 bones.

    • Includes bones of the head, neck, chest, and back.

    • Protects vital organs (e.g., brain, spinal cord, heart, lungs).

    • Appendicular Skeleton:

    • Comprises 126 bones.

    • Includes all bones of the upper and lower limbs plus bones that attach limbs to the axial skeleton.

The Axial Skeleton

  • Protects the brain and vital internal organs.

  • Attachment site for muscles moving the head, neck, and trunk.

  • Consistent components:

    • Skull (22 bones)

    • Vertebral column (24 vertebrae + sacrum and coccyx)

    • Thoracic cage (12 pairs of ribs + sternum)

The Skull

  • Comprises:

    • Cranium (brain case): Protects the brain.

  • Subdivided into:

    • Facial bones: Underlie facial structures and support structures of the face, including the nasal cavity and teeth.

    • Brain case: Rounded structure surrounding the brain, housing the inner and middle ear parts.

  • Number of bones in the adult skull: 22 (21 immobile and the mandible is movable).

Anterior Skull View

  • Dominated by:

    • Openings of orbits (eye sockets)

    • Nasal cavity

    • Upper and lower jaws

  • Bone components:

    • Orbits: Bony sockets housing eyeballs and related structures.

    • Anteriorly defined by the supraorbital margin.

    • Supraorbital foramen: Passage for sensory nerve to forehead skin.

    • Infraorbital foramen: Emergence point for sensory nerve supplying anterior face.

Nasal Cavity

  • Divided by the nasal septum:

    • Upper septum: Formed by the perpendicular plate of the ethmoid bone.

    • Lower septum: Formed by the vomer bone.

    • Nasal conchae: Bony plates that project from lateral walls of nasal cavity:

    • Inferior nasal concha: Independent bone.

    • Middle nasal concha: Part of ethmoid bone.

    • Superior nasal concha: Smaller, located above middle concha.

Lateral Skull View

  • Features:

    • Dominated by the braincase and upper/lower jaws with teeth.

    • Zygomatic arch: Formation by:

    • Temporal process of zygomatic bone (anterior)

    • Zygomatic process of temporal bone (posterior).

    • Serves as a site for muscle attachment during biting/chewing.

Brain Case Components

  • Houses and protects the brain, occupying the cranial cavity.

  • Boundaries:

    • Superiorly by the calvaria (skullcap).

    • Base of the skull: Complex area with openings for cranial nerves and blood vessels.

    • Subdivided into: anterior cranial fossa, middle cranial fossa, and posterior cranial fossa.

Bones of the Braincase

  • Eight total bones:

    • Paired parietal and temporal bones.

    • Unpaired frontal, occipital, sphenoid, and ethmoid bones.

Parietal Bone

  • Form most of the upper lateral side of the skull.

  • Pairing: Right and left parietal bones join at the top of the skull.

  • Bound by:

    • Anterior: Frontal bone

    • Inferior: Temporal bone

    • Posterior: Occipital bone

Temporal Bone

  • Forms lower lateral side of skull and is divided into various regions:

    • Squamous portion: The flattened upper part.

    • Zygomatic process: Anteriorly projecting component.

    • Mastoid portion: Houses small cavities for ear structures.

Important Features of Temporal Bone:

  • External acoustic meatus: Ear canal opening.

  • Internal acoustic meatus: Connects with inner ear cavities.

  • Mandibular fossa: Depression for mandible attachment, part of temporomandibular joint.

  • Articular tubercle: Anterior smooth ridge associated with the mandible.

  • Styloid process: Downward projection for muscle attachment related to the neck.

  • Stylomastoid foramen: Small exit for cranial nerve to facial muscles.

  • Carotid canal: Zig-zag tunnel for major artery supplying the brain.