Anatomy of the Coccyx

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The Skeletal System

• The skeletal system consists of 206 named bones, most of which are paired.
• The skeletal system can be divided into two main parts:
  • Axial skeleton
  • Appendicular skeleton

Axial Skeleton

• Consists of 80 bones
• Includes bones of the:
  • Skull
    • Cranium (8 bones)
    • Face (14 bones)
  • Hyoid bone (1 bone)
  • Auditory ossicles (6 bones)
  • Vertebral column (26 bones)
  • Thorax
    • Sternum (1 bone)
    • Ribs (24 bones)

Appendicular Skeleton

• Consists of 126 bones
• Includes bones of the:
  • Pectoral (shoulder) girdles
    • Clavicle (2 bones)
    • Scapula (2 bones)
  • Upper limbs
    • Humerus (2 bones)
    • Ulna (2 bones)
    • Radius (2 bones)
    • Carpals (16 bones)
    • Metacarpals (10 bones)
    • Phalanges (28 bones)
  • Pelvic (hip) girdle
    • Pelvic or coxal (2 bones)
  • Lower limbs
    • Femur (2 bones)
    • Patella (2 bones)
    • Fibula (2 bones)
    • Tibia (2 bones)
    • Tarsals (14 bones)
    • Metatarsals (10 bones)
    • Phalanges (28 bones)

Types of Bones

• Long bones
  • Have greater length than width
  • Consist of a shaft plus two ends which are often expanded
  • Examples: femur, humerus, tibia, fibula, ulna, radius### Types of Bones
• Limb bones, except patella, wrist, and ankle bones, are classified into five types: long bones, short bones, flat bones, irregular bones, and sesamoid bones.
• Short bones are roughly cube-shaped, nearly equal in length and width, and are found in the wrist and ankle.
• Flat bones are thin, flattened, and usually curved, and are found in the sternum, scapulae, ribs, and most skull bones.
• Irregular bones have complex shapes and are found in the vertebrae and hip bones.
• Sesamoid bones are a special type of short bone that form in a tendon, vary in size and number in different individuals, and are found in the patella and pisiform.
• Sutural bones are small bones located in the sutures (joints) between certain cranial bones.

Bone Textures

• Every bone has two layers: compact bone and spongy bone.
• Compact bone is the dense outer layer that looks smooth and solid to the naked eye.
• Spongy bone is the internal layer consisting of small needle-like or flat pieces called trabeculae.
• In living bone, the spaces between the trabeculae are filled with red or yellow bone marrow.

Structure of a Typical Long Bone

• The diaphysis is the long, cylindrical, main portion of the bone.
• The epiphysis is the proximal and distal ends of the bone.
• The metaphysis is the region between the diaphysis and the epiphyses.
• Articular cartilage is a thin layer of hyaline cartilage covering the part of the epiphysis, reducing friction at movable joints.
• Periosteum is a tough connective tissue sheath associated with blood supply, protecting the bone, assisting in fracture repair, helping nourish bone tissue, and serving as an attachment point for ligaments and tendons.
• Medullary cavity or marrow cavity is a hollow, cylindrical space within the diaphysis containing fat yellow bone marrow and blood vessels.
• Endosteum is a thin membrane lining the medullary cavity, containing a single layer of bone-forming cells and a small amount of connective tissue.

Structure of Short, Irregular, and Flat Bones

• Short, irregular, and flat bones share a simple design with an outer thin plate of compact bone, periosteum covering the compact bone, inner spongy bone, and endosteum covering the spongy bone.
• They lack a diaphysis and epiphyses and a medullary cavity.
• Spongy bone of a typical flat bone is called the diploë.

Bone Surface Markings

• Projections (e.g., heads, trochanter, spine, tubercle, crest, line, tuberosity, condyle, ramus) help to form joints and serve as attachment points for connective tissue.
• Depressions (e.g., fossae, fissure, groove) and openings (e.g., foramina, sinus, meatus) allow the passage of soft tissues (blood vessels, nerves, ligaments, and tendons) and form joints.

Functions of Bones

• Bones provide a structural framework that supports the body.
• They protect the internal organs from injury.
• Bones are used as levers to move the body and its parts.
• Bones act as a reservoir for minerals (calcium and phosphate).
• Blood cell formation (hemopoiesis) occurs in the red marrow.
• Yellow bone marrow stores triglycerides.

The Skull

• The skull is the bony framework of the head that rests on the superior end of the vertebral column.
• Cranial bones form the cranial cavity, enclosing and protecting the brain, and provide attachments for muscles of the head and neck.
• Facial bones form the framework of the face, contain cavities for the special sense organs, provide openings for air and food passages, secure the teeth, and anchor the facial muscles of expression.

Cranial Bones

• Paired cranial bones include the parietal bones (form superior and lateral aspects of the skull) and temporal bones (form inferolateral aspects of the skull and part of the cranial floor).
• Unpaired cranial bones include the frontal bone (forms the forehead, roofs of the orbits, and part of the cranial floor), occipital bone (forms the posterior aspect and base of the skull), and sphenoid bone (forms the posterior aspect and base of the skull and is called the keystone of the cranial floor).