Learning Objectives

At the end of this chapter, students should be able to:

• Discuss bone tissue.
• Explain the general features and surface markings of bones.
• Discuss the skeleton and its function:
• Axial skeleton
• Appendicular skeleton
• Explain joints, types of joints, and their movements.

Selected Key Terms

• Appendicular skeleton
• Myeloid tissue
• Articulation
• Orbits
• Axial skeleton
• Ossicles
• Bone cell
• Ossification
• Cranium
• Perichondrium
• Diaphysis
• Periosteum
• Endosteum
• Skull
• Epiphyseal plate
• Surface marking
• Epiphysis
• Sutures
• Fontanel
• Tendons
• Intervertebral disc
• Thorax
• Metaphysis
• Vertebrae

Overview of the Skeletal System

• The skeletal system includes bones, joints, cartilages, and ligaments.
• It provides support, protection, movement, storage, and blood cell formation.

Functions of the Skeletal System

1. Support: Forms the internal framework that supports and anchors all soft organs.
2. Protection: Bones protect soft body organs.
3. Movement: Skeletal muscles attached to the bones use them as levers to move the body.
4. Storage: Bones store minerals like calcium and phosphorus and fat in their internal cavities.
5. Blood Cell Formation: Occurs within the marrow cavities of certain bones.

Bone

• Specialized connective tissue that is strong and light.
• Living bone is moist and continually resorbed, reformed, and remodeled.

Types of Bone

1. Long Bones: Longer than they are wide (e.g., arms and legs).
2. Short Bones: Roughly equal in length, width, and thickness (e.g., wrists and ankles).
3. Flat Bones: Thin or curved (e.g., ribs, scapulae, sternum, cranium).
4. Irregular Bones: Do not fit into other categories (e.g., vertebrae, facial bones, hipbones).
5. Sesamoid Bones: Embedded in tendons (e.g., patella, pisiform).
6. Accessory Bones: Often found in the feet and appear as extra bones or broken on X-ray.

Gross Anatomy of a Typical Long Bone

• Diaphysis: Tubular shaft with walls of compact bone tissue.
• Epiphysis: Spherical ends of the bone.
• Metaphysis: Separates diaphysis from epiphysis.
• Medullary Cavity: Contains yellow marrow.
• Spongy Epiphyses: Filled with red bone marrow (myeloid tissue).
• Endosteum: Lines the medullary cavity.
• Periosteum: Covers the outer surface of the bone except at joints.

Bone Tissue

• Composed of cells in a matrix of ground substances and fibers.
• Contains inorganic salts like calcium phosphate and calcium carbonate.
• Compact Bone: Forms the outer sheet, contains osteons with central and perforating canals.
• Spongy (Cancellous) Bone: Open, interlaced pattern with trabeculae.

Bone Cells

1. Osteogenic Cells: Small, spindle-shaped cells found in periosteum and endosteum, can transform into osteoblasts.
2. Osteoblasts: Found in growing bones, synthesize and secrete unmineralized ground substance.
3. Osteocytes: Main cells of fully developed bones, derived from osteoblasts.
4. Osteoclasts: Multinuclear giant cells that resorb bone.
5. Bone-lining Cells: Found on the surface of bones, derived from inactive osteoblasts.

Bone Development and Growth

• Ossification: Process of bone development.
• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., skull vault, flat bones).
• Endochondral Ossification: Bone tissue replaces hyaline cartilage (e.g., long bones).

Functions of Bone

• Support and protection of internal organs.
• Storage and supply of calcium and phosphate.
• Manufacture of red and white blood cells.

The Skeleton

• Surface markings include bumps, holes, and ridges for muscle, tendon, ligament attachment, and passage of blood and lymph vessels and nerves.

Divisions of the Skeletal System

• Axial Skeleton: Bones around the axis (e.g., skull, vertebral column, rib cage).
• Appendicular Skeleton: Bones of appendages and girdles (e.g., arms, legs, shoulder, pelvic girdles).

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Bone Counts

• Total Axial Bones: 80
• Total Appendicular Bones: 126

Skull

• Total Bones: 22 (skull bones, ossicles of the ears, and hyoid bone)
• Cranial Bones: 8
• Facial Bones: 14

Axial Skeleton

• Skull: Contains 22 bones, composed of cranial and facial bones.

Cranial Bones (Description and Function)

1. Ethmoid (1): Base of cranium, forms roof of nasal cavity and septum, attachment for brain membranes.
2. Frontal (1): Forehead and brow areas, contains frontal sinuses and supraorbital foramen.
3. Occipital (1): Posterior part of cranium, contains foramen magnum and occipital condyles, protects posterior brain.
4. Parietal (2): Sides and roof of cranium, protect top and sides of brain.
5. Sphenoid (1): Base of cranium, houses pituitary gland, contains foramina for cranial nerves.
6. Temporal (2): Sides and base of cranium, contains ear ossicles, mastoid sinuses.

Sutures

• Coronal Suture: Between frontal and parietal bones.
• Sagittal Suture: Between parietal bones.
• Lambdoidal Suture: Between parietal and occipital bones.
• Squamosal Suture: Between parietal and temporal bones.

Fontanels (Infant Skull)

1. Anterior Fontanel: Diamond-shaped, closes 18-24 months after birth.
2. Posterior Fontanel: Diamond-shaped, closes 2 months after birth.
3. Antrolateral Fontanels: Paired, irregular, close at 3rd month.
4. Postrolateral Fontanels: Paired, irregular, begin closing at 1-2 months, completed by 12 months.

Facial Bones (Description and Function)

1. Inferior Nasal Conchae (2): Lateral walls of nasal cavities, thin and curved.
2. Lacrimal (2): Medial wall of orbit, contains tear duct depression.
3. Mandible (1): Lower jaw, strongest facial bone, part of temporomandibular joint.
4. Maxillae (2): Upper jaw and part of hard palate, contains maxillary sinuses.
5. Nasal (2): Supports bridge of nose.
6. Palatine (2): Posterior part of hard palate, floor of nasal cavity.
7. Vomer (1): Forms part of nasal septum.
8. Zygomatic (2): Cheekbones, part of eye sockets.
9. Hyoid (1): Suspended from temporal bone, attachment for muscles used in speaking and swallowing.
10. Ossicles of Ear (6): Tiny bones in the ear (malleus, incus, stapes) that convey sound vibrations.

Orbits

• Roof: Frontal and sphenoid bones.
• Lateral Wall: Zygomatic and sphenoid bones.
• Floor: Maxilla, zygomatic, and palatine bones.
• Medial Wall: Maxilla, lacrimal, ethmoid, and sphenoid bones.

Vertebral Column

• Composition: 26 vertebrae in adults (previously 33).
• Functions: Supports head, protects spinal cord, attachment for ribs and back muscles.

Intervertebral Discs

• Composition: Annulus fibrosus (outer fibrous ring) and nucleus pulposus (inner soft core).
• Function: Absorb shock, permit movement.

Normal Curves

• Concave Curves: Thoracic and sacral (primary curves).
• Convex Curves: Cervical and lumbar (secondary curves).

Typical Vertebra

• Parts: Body, vertebral arch, transverse processes, spinous processes.
• Function: Protect spinal cord, support body weight, attachment for muscles.

Vertebral Column Bones

1. Cervical Vertebrae (7): Atlas (C1), Axis (C2), support head movement.
2. Thoracic Vertebrae (12): Articulate with ribs, allow limited movement.
3. Lumbar Vertebrae (5): Support back muscles, allow bending.
4. Sacrum: Fused vertebrae, supports pelvis.

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Coccyx:

• Description: Triangular tailbone.
• Attachment: United with the sacrum by intervertebral (3 to 5 fused bones) disk.
• Embryonic Vestige: Represents a vestige of an embryonic tail.
• Vertebrae Count: In children, there are 33 separate vertebrae. The 9 in the sacrum and coccyx are not yet fused.

The Thorax:

• Definition: Refers to the chest.
• Components: Bony cage formed by the sternum (breast bone), costal cartilage, ribs, and bodies of the thoracic vertebrae.
• Sternum (Breast Bone):
• Location: Flat, narrow bone located in the median line of the anterior thoracic wall.
• Dimensions: Approximately 15 cm (6 inches) long.
• Portions:
  • Manubrium: Superior portion; has a jugular (supra sternal) notch.
  • Body: Middle and largest portion; articulates with 2nd to 10th ribs.
  • Xiphoid Process: Inferior and smallest portion; consists of hyaline cartilage in infancy and childhood, ossifying by age 40.
• Junctions:
  • Manubrium: Articulates with the 1st and 2nd ribs.
  • Clavicular Notch: On each side of the jugular notch, articulates with the medial end of the clavicle.

Ribs:

• Total: 12 pairs.
• Length: Increase from 1st through 7th and decrease from 8th through 12th.
• Attachment:
• True Ribs (1st-7th): Directly attached to sternum by costal cartilage.
• False Ribs (8th-10th): Vertebrochondral ribs; cartilage attaches to cartilage of the 7th rib.
• Floating Ribs (11th-12th): Do not attach anteriorly to the sternum.
• Typical Rib Structure:
• Head: Projection at the posterior end; articulates with vertebrae.
• Neck: Constricted portion just lateral to the head.
• Tubercle: Knob-like structure that articulates with the transverse process of the vertebra and attaches to trunk muscles.
• Body (Shaft): Main part of the rib.
• Costal Angle: Site where the rib changes direction; contains the costal groove for protection of thoracic nerves and blood vessels.

Appendicular Skeleton:

• Upper Extremities:
• Bones Count: 64 bones.
• Support: Connected and supported by the axial skeleton through the shoulder joint and muscles from the vertebral column, ribs, and sternum to the shoulder girdle.
• Clavicle (2): Collarbone; holds shoulder joint and arm away from thorax.
• Scapula (2): Shoulder blade; site of muscle attachment.
• Humerus (2): Longest, largest bone of the upper limb; forms ball-and-socket joint with the scapula.
• Radius (2): Larger forearm bone; forms hinge joint at elbow.
• Carpals (16): Short bones in wrist; allow slight gliding movement.
• Metacarpals (10): Long bones in hand; aid in thumb opposition and hand cupping.
• Phalanges (28): Miniature long bones in fingers; allow stable grips.
• Lower Extremities:
• Bones Count: 62 bones.
• Support: Connected to the axial skeleton via the hip girdle.
• Hipbone (Coxal) (2): Formed by fusion of ilium, ischium, and pubis; forms socket for ball-and-socket joint with the femur.
• Femur (2): Longest, strongest bone; supports body and forms ball-and-socket joint with pelvic bones.
• Patella (2): Kneecap; increases leverage for quadriceps muscle.
• Fibula (2): Smaller lower leg bone; provides strength to the ankle joint.
• Tibia (2): Larger lower leg bone; supports body weight and transmits it from femur to talus.
• Tarsals (14): Short bones in the ankle; form arches of the foot and bear body weight.
• Metatarsals (10): Long bones in foot; form the sole and arches of the feet.
• Phalanges (28): Miniature long bones in toes; provide stability during locomotion.

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