Human Anatomy - Axial Skeleton

Human Anatomy - Axial Skeleton

• The skeleton consists of bones, cartilage, joints, and ligaments.
• Joints are also called articulations.
• There are 206 named bones, grouped into two divisions: axial and appendicular.
• The axial skeleton has 80 bones: the skull, vertebral column, and thoracic cage.
• The appendicular skeleton has 126 bones: the upper and lower limbs.

The Skull

• The skull is the body's most complex bony structure, formed by cranial and facial bones.
• Cranial bones enclose and protect the brain, provide attachment sites for some head and neck muscles.
• Cranium is divided into cranial vault and the base.
• Facial bones form the anterior aspect of the skull.
• They form the framework of the face.
• They form cavities for sense organs like sight, taste, and smell.
• They provide openings for the passage of air and food.
• They hold the teeth in place and anchor muscles of the face.

Cranial Bones

• There are 8 large cranial bones.
• Frontal bone: Forms the forehead, roof of the nasal cavity, and orbits. Contains sinuses above the eyes.
• Occipital bone: Forms the back and base of the cranium. Foramen magnum (opening at the bottom) and occipital condyles (rounded processes).
• Temporal bone: Forms side walls and part of the floor of the cranium. External auditory meatus, mastoid process, and zygomatic process are part of the temporal bone.
• Parietal bones: Form sides and roof of the cranium.
• Sphenoid bone: Forms part of the base and sides of the cranium and floors and sides of orbits. Includes the sella turcica (houses the pituitary gland) and contains sinuses.
• Ethmoid bone: Forms part of the roof of the nasal cavity, cranial floor and orbital walls, and superior and middle nasal conchae. Includes a cribriform plate (opening for olfactory nerves) and contains sinuses.

Sutural Bones

• Small bones that occur within sutures (joints between cranial bones).
• Irregular in shape, size, and location, found in some but not all people.

Facial Bones (14)

• Unpaired: Mandible (lower jaw) and vomer.
• Paired: Maxilla (upper jaw), zygomatic bones (cheek bones), nasal bones (bridge of nose), lacrimal bones (inner wall of eye orbits), palatine bones (roof of the mouth), and inferior nasal conchae (nasal cavity).

Maxillary Bones

• Articulates with all other facial bones except the mandible.
• Contains maxillary sinuses.
• Forms part of the inferior orbital fissure.
• Is the "keystone" bone of the face.

Mandible

• The lower jawbone, the largest and strongest facial bone.
• The only movable bone of the skull.
• Composed of a horizontal body and two upright rami.

Fetal Skull

• Infant face is very small compared to the skull at birth.
• The skull has fontanels (soft spots) which are areas where the bones are incompletely fused.
• This allows for skull compression during birth and brain growth.

Hyoid Bone

• Lies inferior to the mandible.
• The only bone with no direct articulation with any other bone.
• Acts as a movable base for the tongue.

Vertebral Column (Spine)

• Extends from the skull to the pelvis.
• Formed of 26 irregular bones connected and held in place by ligaments, resulting in a flexible and strong S-shaped structure.
• It surrounds and protects the spinal cord.
• Serves as attachment sites for muscles of the neck and back.
• Organized into Cervical, Thoracic, Lumbar, Sacrum, and Coccyx.
• Cervical (7 vertebrae), Thoracic (12 vertebrae), Lumbar (5 vertebrae), Sacrum (5 fused vertebrae), Coccyx (3-5 fused vertebrae).

Intervertebral Discs

• Cushion-like pads.
• Composed of nucleus pulposus (gelatinous inner sphere) and anulus fibrosus (outer rings of ligament and inner rings of fibrocartilage).
• Absorbs compressive stresses.

General Structure of Vertebrae

• All vertebrae have similar structures and features: body, vertebral arch, vertebral foramen, spinous process, transverse process, superior and inferior articular processes, and intervertebral foramina.

Cervical Vertebrae

• Smallest vertebrae in the vertebral column.
• Short, and stumpy transverse processes.
• C2-C6 have bifid spinous processes.
• C7 has a large spinous process ending in a tubercle.
• All 7 cervical vertebrae have transverse foramina, allowing the passage of vertebral arteries and veins.

First Two Cervical Vertebrae (Atlas and Axis)

• Atlas (C₁): No vertebral body or spinous process, large round vertebral foramen, articulates with occipital condyles, permitting nodding "yes".
• Axis (C₂): Prominent dens (superior projection on body), dens bound to atlas by transverse ligament, permits rotation as in shaking head "no".

Thoracic Vertebrae

• Twelve thoracic vertebrae.
• Each one slightly larger as they move inferiorly.
• Heart-shaped body.
• Long, slender spinous process projecting posteriorly and inferiorly.
• Costal facets on vertebral body for rib articulation.
• T1-T10 have costal facets on transverse processes.

Lumbar Vertebrae

• Largest vertebrae in the vertebral column (transmit most weight).
• 5 (L1-L5).
• Thicker body than thoracic vertebrae.
• No costal facets.
• Stumpy spinous processes.

Sacrum and Coccyx

• Sacrum: Formed by the fusion of 5 vertebrae, begins fusing from fetal period, completed by age 25-30, superiorly articulates with L5, inferiorly with the coccyx.
• Coccyx: Tailbone, formed by fusion of 3-5 vertebrae.

Thoracic Cage (Bony Thorax)

• Provides bony support to the walls of the thoracic cavity.
• Protects the heart, lungs, and thymus.
• Composed of thoracic vertebrae, ribs, and the sternum.
• Attachment point for muscles involved in breathing, maintaining the position of the vertebral column, and movements of the pectoral girdle and upper limbs.

Thoracic Cage (Sternum and Ribs)

• Sternum: Flat bone, fusion of manubrium, body, and xiphoid process, attached to the first seven pairs of ribs.
• Ribs: Twelve pairs, articulate with the vertebral column posteriorly, curve down and anteriorly. True ribs (first seven pairs) attach to the sternum by individual costal cartilages; false ribs (next five) attach either indirectly or not at all (floating ribs). Intercostal spaces are between ribs, filled with intercostal muscles for breathing.

Explore the structure of the human axial skeleton, which comprises 80 bones including the skull, vertebral column, and thoracic cage. This quiz covers key details about cranial and facial bones, their functions, and the organization of joints within the axial skeleton.