Skeletal System: Bone Terminology, Structure, and Skull Anatomy

Skeletal System Overview

Introduction to the Human Skeleton

The human skeleton provides structural support, protection for internal organs, and facilitates movement. It is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Total number of bones in adult body: 206

• Total number of bones in newborn: about 350

Axial vs. Appendicular Skeleton

• Axial skeleton: Skull, auditory ossicles, hyoid bone, sternum, thoracic cage, vertebrae

• Appendicular skeleton: Pectoral girdle and upper limbs, pelvic girdle and lower limbs

• The bones of the appendicular skeleton are appended to the axial skeleton.

Visual identification: In anatomical diagrams, axial skeleton bones are often shown in green, and appendicular skeleton bones in gold.

Bone Terminology

Key Terms and Definitions

Understanding bone terminology is essential for describing bone features, attachments, and articulations. The following table summarizes important terms:

Term	Definition/Description with Example
Process	Any projection or bump on a bone; e.g., mastoid process of the temporal bone
Ramus	An arm-like bar of bone; e.g., ramus of the mandible
Trochanter	Large, blunt, irregularly shaped process; e.g., greater trochanter of the femur
Tuberosity/Tubercle	Rounded projection; tuberosity is larger than a tubercle; e.g., tibial tuberosity
Crest	Narrow, prominent ridge of bone; e.g., iliac crest
Spine	Sharp, slender projection; e.g., spine of the scapula
Head	Expanded articular end of an epiphysis, separated from the shaft by a neck; e.g., head of the femur
Neck	Narrowed region next to the head of a bone
Condyle	Rounded articular projection; e.g., occipital condyles
Trochlea	Process shaped like a pulley; e.g., trochlea of the humerus
Facet	Small, flat articular surface; e.g., facets of vertebrae
Fossa	Shallow depression; e.g., mandibular fossa
Sulcus	Groove; e.g., intertubercular sulcus of the humerus
Notch	Indentation at the edge of a bone; e.g., greater sciatic notch
Meatus	Canal-like passageway; e.g., external acoustic meatus
Sinus	Cavity within a bone, filled with air and lined with mucous membrane; e.g., maxillary sinus
Foramen	Round or oval opening through a bone; e.g., foramen magnum
Fissure	Narrow, slit-like opening; e.g., superior orbital fissure
Canal	Passageway through the bone; e.g., carotid canal

Bone Growth and Adaptation

Bone Growth Where Muscles Pull

• Bone grows where muscles pull it, leading to the development of tuberosities and processes.

• Example: The heel spur develops as the plantar fascia pulls on the calcaneus, and the greater trochanter enlarges as gluteal muscles pull on it.

Bone Changes Over Time

• Prolonged inactivity (e.g., using crutches) can cause significant bone mass loss.

• Electrical fields can stimulate osteoblasts and speed bone recovery (e.g., TENS machine).

• Bone density increases with stress, making processes more prominent, especially with manual labor or weight lifting.

Skull Anatomy

Major Bones of the Skull

The skull is composed of cranial and facial bones, each with distinct features and functions.

Bone	Description/Key Features
Frontal Bone	Forms the forehead, superior part of eye orbits, and floor of anterior cranial fossa. Contains the frontal sinuses.
Parietal Bones	Paired bones forming the superior and lateral portions of the cranium.
Temporal Bones	Paired bones forming the lower sides of the cranium. Markings include the external auditory meatus, zygomatic process, styloid process, mastoid process, jugular foramen, and carotid canal.
Occipital Bone	Forms the back and base of the skull. Contains the foramen magnum and occipital condyles.
Sphenoid Bone	Bat-shaped bone at the skull base. Articulates with all other cranial bones. Contains the sella turcica and optic foramen.
Ethmoid Bone	Forms part of the nasal septum and medial wall of the orbit. Contains the cribriform plate and crista galli.
Sella turcica	"Turkish saddle"; houses the pituitary gland.
Crista galli	"Rooster's comb"; attachment for brain membranes.
Foramen Magnum	Large opening for the spinal cord at the base of the skull.
Jugular Foramen	Passage for the jugular vein and cranial nerves.
Occipital Condyles	Articulate with the first cervical vertebra (atlas).
Styloid Process	Needle-like projection for tongue and neck muscle attachment.
Mastoid Process	Attachment for neck muscles; contains air cells.
External Occipital Protuberance	Bump on the back of the occipital bone.

Skull Sutures and Fontanels

• Coronal suture: Between frontal and parietal bones

• Sagittal suture: Between parietal bones

• Lambdoid suture: Between parietal and occipital bones

• Fontanels: Soft spots in infant skulls where sutures have not yet fused

• Anterior fontanel: Between frontal and parietal bones; closes between 18-24 months

• Posterior fontanel: Between parietal and occipital bones; closes about 2 months after birth

Facial Bones and Landmarks

Bone	Description/Key Features
Nasal Bones	Small rectangular bones forming the bridge of the nose.
Maxilla	Upper jaw; articulates with all other facial bones except the mandible. Forms part of the palate, nasal cavity, and floor of the orbits.
Zygomatic Bones	Cheekbones; form part of the lateral wall and floor of the orbit.
Palatine Bones	Form the posterior part of the hard palate.
Nasal Conchae	Ridges in the nasal cavity that create turbulence and increase surface area.
Vomer	Thin, blade-shaped bone forming the inferior part of the nasal septum.
Mandible	Lower jaw; only movable bone of the skull. Contains the mental protuberance (chin).
Lacrimal Bones	Small bones forming part of the medial wall of each orbit.
Naso-lacrimal Canal	Passage for tears from the lacrimal sac into the nasal cavity.

Bones Forming the Eye Orbit

1. Zygomatic

2. Ethmoid

3. Sphenoid

4. Lacrimal

5. Maxilla

6. Frontal

Additional Information

• Bone Markings: Serve as attachment sites for muscles, passageways for nerves and blood vessels, and articulation points for joints.

• Clinical relevance: Knowledge of bone landmarks is essential for interpreting radiographs and understanding musculoskeletal injuries.