Axial and Appendicular Skeleton: Structure, Terminology, and Key Bones

Axial and Appendicular Skeleton

Definition and Components

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

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage. It forms the central axis of the body and protects vital organs such as the brain, spinal cord, and thoracic organs.

• Appendicular Skeleton: Includes the bones of the limbs and the girdles (pectoral and pelvic) that attach them to the axial skeleton. It is primarily involved in movement and manipulation.

Classification of Bones: Bones are classified by shape: long, short, flat, and irregular.

• Long bones: Have rounded ends for forming joints and provide leverage for movement.

• Short bones: Cube-shaped, found in the wrist and ankle.

• Flat bones: Thin and broad, such as the skull and ribs.

• Irregular bones: Complex shapes, such as vertebrae.

Surface Markings: Bones have various markings for muscle attachment, passage of nerves and blood vessels, and articulation with other bones.

Axial Skeleton

Skull: Structure and Terminology

The skull consists of 22 bones, divided into cranial (8) and facial (14) bones. It protects the brain and forms the structure of the face.

• Cranial bones: Frontal, parietal (2), occipital, sphenoid, ethmoid, temporal (2).

• Facial bones: Nasal (2), maxillae (2), zygomatic (2), mandible, lacrimal (2), palatine (2), inferior nasal conchae (2), vomer.

Sutures: Immovable joints found only between skull bones. Four prominent sutures: coronal, sagittal, lambdoid, squamous.

Fontanels: Soft, membrane-filled spaces in the fetal skull that allow for growth. These are replaced by bone as the child matures.

Major Bones of the Skull and Their Features

• Frontal Bone: Forms the forehead and roofs of the orbits.

• Parietal Bones: Form the superior and lateral walls of the cranium.

• Occipital Bone: Forms the posterior base of the skull; contains the foramen magnum for spinal cord passage.

• Temporal Bones: Form the lateral walls and base of the cranium; house the ear structures.

• Sphenoid Bone: Keystone of the cranium; articulates with all other cranial bones.

• Ethmoid Bone: Forms part of the nasal cavity and orbits; contains the cribriform plate.

• Facial Bones: Maxillae, zygomatic, nasal, lacrimal, palatine, vomer, inferior nasal conchae, mandible.

Unique Features:

• Mandible: Only movable bone of the skull.

• Maxillae: Keystone bone of the face; forms the upper jaw.

• Zygomatic Bones: Cheekbones; articulate with temporal bones.

• Nasal Bones: Form the bridge of the nose.

• Lacrimal Bones: Form part of the medial wall of each orbit.

• Palatine Bones: L-shaped; form the posterior part of the hard palate.

• Vomer: Forms the inferior portion of the nasal septum.

• Inferior Nasal Conchae: Project medially from the lateral walls of the nasal cavity.

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

Hyoid Bone

The hyoid bone is located in the neck and does not articulate with any other bone. It serves as an attachment point for tongue and larynx muscles, aiding in swallowing and speech.

Vertebral Column

General Characteristics

The vertebral column consists of 26 bones and forms the central axis of the body. It protects the spinal cord and supports the head and trunk.

• Regions: 7 cervical, 12 thoracic, 5 lumbar, 1 sacrum (fused), 1 coccyx (fused).

• Functions: Protects spinal cord, supports head, provides attachment for ribs and muscles.

• Curvatures: Cervical and lumbar curves are convex anteriorly; thoracic and sacral curves are convex posteriorly.

General Structure of a Typical Vertebra:

• Body: Weight-bearing portion.

• Vertebral arch: Encloses the vertebral foramen.

• Processes: Spinous, transverse, superior and inferior articular.

Special Vertebrae:

• Cervical Vertebrae (C1-C7): Smallest, lightest. C1 (Atlas) supports the skull; C2 (Axis) has the dens for rotation.

• Thoracic Vertebrae (T1-T12): Articulate with ribs; spinous processes are long and pointed.

• Lumbar Vertebrae (L1-L5): Largest, strongest; support most body weight.

• Sacrum: Five fused vertebrae; forms the posterior wall of the pelvis.

• Coccyx: Three to five fused vertebrae; forms the tailbone.

Thoracic Cage

Structure and Bones

The thoracic cage protects vital organs and supports the shoulder girdles and upper limbs. It consists of the sternum, ribs, and thoracic vertebrae.

• Sternum: Flat bone at the midline; consists of manubrium, body, and xiphoid process.

• Ribs: 12 pairs; true ribs (1-7) attach directly to the sternum, false ribs (8-12) do not. Ribs 11 and 12 are floating ribs.

Appendicular Skeleton

Pectoral Girdle and Upper Limb

The appendicular skeleton enables movement and manipulation. The pectoral girdle attaches the upper limbs to the axial skeleton.

• Pectoral Girdle: Clavicle and scapula.

• Clavicle: Collar bone; acts as a brace for the shoulder.

• Scapula: Shoulder blade; provides attachment for muscles and articulates with the humerus.

Upper Limb Bones:

• Humerus: Arm bone; articulates with the scapula at the shoulder and with the radius and ulna at the elbow.

• Radius and Ulna: Forearm bones; radius is lateral, ulna is medial.

• Carpals, Metacarpals, Phalanges: Wrist, hand, and finger bones.

Pelvic Girdle and Lower Limb

The pelvic girdle attaches the lower limbs to the axial skeleton and supports the weight of the body.

• Pelvic Girdle: Two hip bones (coxal bones), sacrum, and coccyx.

• Femur: Thigh bone; largest and strongest bone in the body.

• Patella: Kneecap; protects the knee joint.

• Tibia and Fibula: Leg bones; tibia is medial and weight-bearing, fibula is lateral and slender.

• Tarsals, Metatarsals, Phalanges: Ankle, foot, and toe bones.

Table: Comparison of Axial and Appendicular Skeleton

Feature	Axial Skeleton	Appendicular Skeleton
Main Components	Skull, vertebral column, thoracic cage	Limb bones, pectoral girdle, pelvic girdle
Function	Protection, support	Movement, manipulation
Number of Bones	80	126
Examples	Frontal bone, vertebrae, ribs	Femur, humerus, scapula

Key Terms and Definitions

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

• Suture: Immovable joint between skull bones.

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

• Sinus: Air-filled cavity within a bone.

Formulas and Equations

• Bone Mass Percentage:

$\text{Bone mass percentage} = \frac{\text{Bone mass}}{\text{Total body mass}} \times 100$

• Number of Vertebrae:

$7 \; (\text{cervical}) + 12 \; (\text{thoracic}) + 5 \; (\text{lumbar}) + 1 \; (\text{sacrum}) + 1 \; (\text{coccyx}) = 26 \; \text{vertebrae}$

Additional info:

• Fontanels are important in infants for allowing brain growth and passage through the birth canal.

• Paranasal sinuses help humidify and warm inhaled air, lighten the skull, and enhance voice resonance.

• The vertebral column's curvatures help absorb shock and maintain balance.