Human Skeleton Quiz: Bones and Ossification

Number of Bones in the Human Skeleton

• An adult human skeleton typically has 206 bones.
• The number of bones changes throughout a person's life, increasing during childhood and adolescence and decreasing with age due to bone fusion.
• The upper limbs contain 62 bones in total.
• There are 34 single bones and 86 paired bones in an adult skeleton.

Types of Bones and their Ossification

• Bones are classified into several types based on their shape and structure:
  • Long bones (e.g., femur, humerus)
    • Have a shaft (diaphysis) and two expanded ends (epiphyses).
    • The shaft contains a medullary cavity filled with bone marrow.
    • They typically ossify in cartilage.
  • Short bones (e.g., carpals, tarsals)
    • Are cubical or scaphoid in shape.
    • Have three or four articular surfaces.
    • They ossify in cartilage.
  • Flat bones (e.g., skull bones, ribs, sternum)
    • Consist of two lamellae of compact bone with a spongy layer between them.
    • Most ossify in membrane, except for the sternum and ribs which ossify in cartilage.
  • Irregular bones (e.g., vertebrae, facial bones)
    • Have complex shapes and do not fit into the other categories.
    • They can ossify in cartilage or membrane.
  • Sesamoid bones (e.g., patella)
    • Are small, round bones embedded in tendons or ligaments.
    • They ossify in cartilage.
• Some bones exhibit mixed ossification, meaning they are formed by a combination of cartilage and membrane ossification.
  • Bones of the base of the skull, bones of the face, and the mandible are examples of this mixed ossification.
  • The clavicle is an irregular bone that also undergoes mixed ossification.

Bone Structure and Composition

• Compact bone
  • Is hard and ivory-like.
  • Forms the cortical covering of all types of bones.
  • Forms the shaft of long bones.
  • Resists bending and twisting by compression forces.
  • Does not contain bone marrow.
• Cancellous bone (also known as spongy bone)
  • Is formed of a trabecular meshwork (interconnected beams of bone).
  • Contains spaces filled with bone marrow.
  • Forms the interior of all types of bones.
  • Is covered by compact bone.
  • Does not form the shaft of long bones.

Features of Bones

• Elevations:
  • Rounded elevations: Head, Condyle, Trochlea, Capitulum
  • Irregular elevations: Tubercle, Tuberosity, Trochanter
  • Linear elevations: Line, Lip, Crest, Ridge
• Depressions:
  • Rounded depressions: Pit, Fovea, Impression, Fossa
  • Elongated depression: Sulcus
  • Depression on the border of a bone: Notch

Periosteum

• The periosteum is a fibrous membrane that covers the outer surface of all bones, except the articular surfaces.
• It is adherent to the bone by Sharpey's fibers.
• It is innervated and sensitive.
• Contains an outer fibrous layer and an inner cellular layer.
• Nourishes the outer part of the bony cortex.
• Is responsible for bone growth in thickness.
• Receives attachments from the joint capsule, tendons of muscles, ligaments, and fascial septa.

Bone Growth

• Growth in length of bones is due to the activity of the epiphyseal cartilage.
• The epiphyseal cartilage is located between the epiphysis and the metaphysis.
• As the epiphyseal cartilage grows, it is replaced by bone, leading to the lengthening of the bone.
• Growth in thickness of bones is due to the activity of the periosteum.

Bone Marrow

• Red bone marrow is the main site of blood cell production.
• It is present in all types of bones at birth.
• Over time, it is gradually replaced by yellow bone marrow in the medullary cavities of long bones.
• Yellow bone marrow is primarily composed of fat cells.

Blood Supply of Bones

• Nutrient artery: Supplies the medullary cavity, bone marrow, metaphysis, and the deep 2/3 of the compact cortex of the shaft.
• Periosteal arteries: Supply the outer 1/3 of the bone cortex.
• Metaphyseal arteries: Supply the metaphysis.
• Epiphyseal arteries: Supply the epiphysis.

Cartilage

• Cartilage is a non-vascular and insensitive connective tissue that provides support and flexibility.
• Articular cartilage is found at the ends of bones where they form joints. It is not covered by perichondrium.
• Repair of injured articular cartilage is difficult due to its avascular nature.

Hyaline Cartilage

• Hyaline cartilage is the most common type of cartilage.
• It is homogenous, bluish-white in color, translucent, and rich in collagenous fibers.
• It is liable to calcification.

Special Features of Bones

• The clavicle is the only long bone that does not have a medullary cavity.
• The patella (kneecap) is a sesamoid bone located in the tendon of the quadriceps femoris muscle.

Test your knowledge on the human skeleton, including the number of bones in adults and children, types of bones, and their ossification processes. This quiz covers key aspects of anatomy and how bones evolve throughout life.