Skeletal System - Axial Skeleton
Chapter 7: Axial Skeleton
Skeletal System Overview
Axial Skeleton: Composed of the bones forming the longitudinal axis of the skeleton.
Total Bones: 80
Components:
Skull (includes cranium and facial bones)
Vertebral column (composed of vertebrae, sacrum, coccyx)
Thorax (includes ribs and sternum)
Auditory ossicles
Hyoid bone
Appendicular Skeleton: Bones of the extremities and pelvic and pectoral girdles.
Total Bones: 126
Functions of Skeleton:
Provides framework for the body
Serves as an attachment site for muscles, allowing movement
There are 206 bones found in the adult human body (infants have more bones as some bones fuse later in development)
Bone Types Summary
Sutural Bones (Wormian):
Accessory bones found within the skull, located between cranial bones.
Long Bones:
Longer than wide; examples include the femur, humerus, tibia, and phalanges.
Structure: Compact bone in diaphysis and epiphyses.
Short Bones:
Length and width are relatively equal; examples include carpals and tarsals.
Structure: Compact bone covers spongy bone.
Flat Bones:
Thin and flat; examples include cranial bones, ribs, sternum, and scapula.
Structure: Compact bone forms two layers sandwiching spongy bone.
Irregular Bones:
Have complex shapes; examples include vertebrae and pelvic bones.
Structure: Variable amount of compact bone covering spongy bone.
Sesamoid Bones:
Shape resembles a sesame seed; examples include patella (kneecap) and pisiform bone.
Structure: Compact bone covers spongy bone; imbedded within tendons protecting them from wear and reinforcing them to decrease stress.
Bone Markings
Types of Bone Markings:
Depressions or Grooves:
Pathways along bone surfaces where blood vessels and nerves enter bone.
Elevations or Projections:
Sites where tendons and ligaments attach, also at joints with other bones.
General Description of Bone Markings
Elevation and Projection Terms:
Process: Any projection or bump.
Ramus: An extension of bone making an angle with the rest of the structure.
Trochanter: A large, rough projection.
Tuberosity: A smaller, rough projection.
Tubercle: A small, rounded projection.
Crest: A prominent ridge.
Line: A low ridge.
Spine: A pointed or narrow process.
Head: The expanded articular end of an epiphysis, separated by a neck.
Condyle: A smooth, rounded articular process.
Trochlea: A smooth, grooved articular process shaped like a pulley.
Facet: A small, flat articular surface.
Depression Terms
Fossa: A shallow depression.
Sulcus: A narrow groove.
Openings Terms
Foramen: A rounded passageway for blood vessels or nerves.
Canal: A duct or channel.
Meatus: A passageway through a bone.
Fissure: An elongated cleft or slit.
Sinus: A chamber within a bone, normally filled with air.
Structure of Flat Bones
Example: Parietal bone of the skull.
Structure: Resembles a sandwich, having spongy bone between two layers of compact bone.
Specific Terms: Inside the cranium, the spongy bone layer between compact bones is referred to as the "diploë".
The Skull
Overall Composition: 22 bones joined by sutures (immovable joints).
Cranial Bones (8): Protect the cranial cavity and enclose the brain.
Facial Bones (14): Framework of the face and protect the openings to respiratory and digestive systems.
Functions of Facial Bones
Protect and support special sense organs for vision, taste, smell, hearing, and balance.
Support teeth and facial and jaw muscles.
Cranial Bones
Types of Cranial Bones:
Occipital Bone
Parietal Bones (2)
Frontal Bone
Temporal Bones (2)
Sphenoid Bone
Ethmoid Bone
Cavities of the Skull:
Cranial cavity, nasal cavity, buccal cavity, middle and inner ear cavities, orbits (eye sockets).
Paranasal Sinuses: Open into the nasal cavity; lined with mucous membranes that allow for resonance and lighten weight of facial bones.
Sinuses include:
Frontal sinuses
Ethmoid sinuses
Sphenoid sinuses
Maxillary sinuses
Facial Bones
Maxillae (2)
Form the upper jaw; support upper teeth. Includes:
Infraorbital foramen: openings for nerve and blood vessels.
Alveolar processes: where teeth articulate.
Palatine process: forms part of the hard palate.
Palatine Bones (2): Form the posterior 1/3 of the hard palate.
Nasal Bones (2): Form the bridge of the nose.
Zygomatic Bones (2): Form the cheekbones; articulates with the temporal bone to create the zygomatic arch.
Lacrimal Bones (2): Smallest facial bones, part of the medial walls of the orbits.
Mandible (1): Lower jaw, movable bone.
Vomer (1): Forms part of the nasal septum.
Inferior Nasal Conchae (2): Projections in the nasal cavity that increase air turbulence.
Mandible Overview
Components of Mandible:
Body: Main horizontal portion.
Rami: Vertical portions.
Angle: Junction of the body and rami.
Mandibular Condyle: Articulates with temporal bone forming the TMJ (temporomandibular joint).
Coronoid Process: Muscle attachment site (temporalis muscle).
Mental Foramen: Passageway for nerves and blood vessels to the lips/chin.
Nerve Supply
Supplied by the trigeminal nerve branches such as the masseteric nerve.
Developmental Aspects of the Skeleton
Fontanelles: Areas of incomplete ossification in infants that will close as they grow.
Anterior Fontanelle: Closes within 2 years.
Occipital Fontanelle: Closes within 2 months.
Sphenoidal and Mastoid Fontanelles: Close at approximately 3 months to 1 year.
Hyoid Bone
Structure: U-shaped; does not articulate with other bones.
Connects to the styloid process via ligaments.
Attaches muscles involved in swallowing and speech.
Vertebral Column
Total Number of Vertebrae: 26 in adults; 33 in children due to sacral and coccygeal vertebrae fusing.
Purpose:
Supports body, protects spinal cord, attachments for muscles, serves as anchor for ribs and pelvic girdle.
Curvature and Functionality
Normal Curves:
Thoracic and Sacral Curves: Concave, primary curves present during fetal development.
Cervical and Lumbar Curves: Convex, secondary curves developed after birth to support upright posture.
Abnormal Curvatures
Scoliosis: Lateral bending of the spine often found in adolescents.
Kyphosis (Hunchback): Exaggerated thoracic curvature due to factors like osteoporosis.
Lordosis (Swayback): Excessive lumbar curvature.
Overview of Vertebrae
Basic Structure:
C1-C7: Cervical (7), T1-T12: Thoracic (12), L1-L5: Lumbar (5).
Specialized Structures:
Cervical Vertebrae: Small with transverse foramen for blood supply to the brain.
Thoracic vertebrae: Larger with facets for rib articulation.
Lumbar Vertebrae: Largest, support the most weight; broad and robust with short spinous processes.
Sacrum and Coccyx
Sacrum: Formed by the fusion of five vertebrae; supports pelvic organs, connects with the pelvic girdle, and contains nerves.
Coccyx: Four fused vertebrae; attachment for ligaments and pelvic floor muscles; susceptible to injuries.
Coccygeal cornua articulate with sacral cornua.
Caudal Anesthesia
Administered through the sacral hiatus for lower body pain management, less risk of affecting the spinal cord.
Thorax Overview
Components: Thoracic cage includes ribs, sternum, and thoracic vertebrae; protects the thoracic organs and serves attachment sites for respiratory muscles.
Sternum Characteristics
Composed of three parts: Manubrium, Body, and Xiphoid Process.
The Xiphoid process is a landmark during CPR, which must be handled carefully to avoid injury.
Ribs Overview
Total of 12 pairs of ribs divided into true ribs (1-7), false ribs (8-12), and floating ribs (11-12).
Ribs articulate posteriorly with thoracic vertebrae and anteriorly with the sternum via costal cartilage.
Ribs protect vital organs and their ossification helps determine age in forensic examinations.
Conclusion
The axial skeleton is foundational to human anatomy, providing structure, protection, and functional capacity for movement, respiration, and interaction with the environment.
Understanding its components and their functions is crucial for studies in anatomy, medicine, and related fields.