The Skeletal System: Structure, Classification, and Anatomy

Chapter 6: The Skeletal System

Classification of Bones by Shape

Bones are classified according to their shapes, which relate to their functions and locations in the body.

• Long Bones: Longer than they are wide; primarily act as levers. Examples: humerus, radius, ulna, femur, tibia, fibula.

• Short Bones: Nearly equal in length and width; provide stability and support with little movement. Examples: carpals (wrist), tarsals (ankle), talus.

• Flat Bones: Thin, flattened, and usually curved; protect internal organs and provide surfaces for muscle attachment. Examples: sternum, ribs, scapulae, most cranial bones.

• Irregular Bones: Complex shapes that do not fit other categories. Examples: vertebrae, sacrum, coccyx, hip bones.

• Sesamoid Bones: Small, round bones embedded within tendons. Example: patellae.

Structure of a Long Bone

Long bones have a specialized structure to support their function in movement and weight-bearing.

• Proximal Epiphysis: The upper rounded end of the bone, often involved in joint formation.

• Diaphysis: The long, central shaft composed mainly of dense compact bone for strength.

• Distal Epiphysis: The lower end of the bone, also involved in joint formation.

• Articular Cartilage: Smooth hyaline cartilage covering the epiphyses, reducing friction and absorbing shock at joints.

• Spongy Bone: Found mainly in the epiphyses; contains trabeculae aligned along stress lines.

• Epiphyseal Line: Remnant of the growth plate; ossified in adults, indicating cessation of growth.

• Periosteum: Dense connective tissue covering the bone, rich in nerves and blood vessels.

• Compact Bone: Dense outer layer providing strength and protection.

• Medullary Cavity: Central cavity in the diaphysis, containing yellow bone marrow (fat storage); lined by endosteum.

• Perforating (Sharpey's) Fibers: Collagen fibers anchoring periosteum to bone.

• Nutrient Arteries: Blood vessels supplying the bone.

Microscopic Structure of Bone

Bone tissue is organized into structural units that provide strength and facilitate nutrient delivery.

• Osteon (Haversian System): The fundamental unit of compact bone, consisting of concentric lamellae around a central canal.

• Lamellae: Layers of bone matrix within osteons.

• Interstitial Lamellae: Remnants of old osteons found between new ones.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Transverse channels connecting blood and nerve supply of periosteum to central canals.

• Trabeculae: Lattice-like structures in spongy bone, providing strength with minimal weight.

• Periosteum and Endosteum: Membranes covering outer and inner bone surfaces, respectively.

Bone Markings

Bones have various markings that serve as sites for muscle, ligament, and tendon attachment, joint formation, or passage of nerves and blood vessels.

• Attachment Sites (for ligaments/tendons): Tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process.

• Joint Formation: Head, facet, condyle, ramus.

• Depressions and Openings: Groove, fissure, foramen, notch, meatus, sinus, fossa.

Axial vs. Appendicular Skeleton

The human skeleton is divided into two main parts:

• Axial Skeleton: Forms the central axis; includes the skull, vertebral column, and thoracic cage (ribs and sternum).

• Appendicular Skeleton: Includes the limbs and girdles (pectoral and pelvic) that attach them to the axial skeleton.

Axial Skeleton

The axial skeleton consists of the skull, vertebral column, and thoracic cage.

• Skull: 8 cranial bones and 14 facial bones, joined by immovable sutures (coronal, lambdoidal, sagittal, squamosal, intermaxillary).

• Hyoid Bone: The only bone not articulating directly with another bone; supports the tongue.

• Auditory Ossicles: Malleus, incus, stapes—tiny bones in the middle ear.

• Sinuses: Air-filled cavities in frontal, ethmoid, sphenoid, and maxillary bones; lighten skull and enhance voice resonance.

Selected Skull Bones and Features

• Frontal Bone: Forms forehead; features include supraorbital margins, foramen, notch, superciliary arch, lacrimal fossa, zygomatic process, metopic suture.

• Occipital Bone: Foramen magnum, occipital condyles, external occipital crest/protuberance, nuchal lines, jugular foramen.

• Ethmoid Bone: Crista galli, cribriform plate, olfactory foramina, perpendicular plate, nasal conchae, ethmoidal sinuses.

• Sphenoid Bone: Greater/lesser wings, sella turcica, optic foramen, orbital fissure, foramen rotundum/ovale/spinosum, sphenoidal sinus, pterygoid processes.

• Temporal Bones: Squamous/petrous portions, mastoid/styloid processes, zygomatic process, mandibular fossa, auditory canals, carotid canal, jugular foramen.

• Facial Bones: Maxillae, palatine, lacrimal, zygomatic, nasal, vomer, inferior nasal conchae, mandible (with body, ramus, condyles, coronoid process, mental foramen, alveolar process).

Fetal Skull (Fontanels)

• Fontanels: Soft spots in the fetal skull (frontal/anterior, posterior/occipital, anterolateral/sphenoid, posterolateral/mastoid) that allow for brain growth and skull flexibility during birth.

Vertebral Column

The vertebral column supports the body, protects the spinal cord, and provides attachment points for ribs and muscles.

• Regions: Cervical (C1–C7), thoracic (T1–T12), lumbar (L1–L5), sacrum (S1–S5, fused), coccyx (1–3/5, partially fused).

• Cervical Vertebrae: Have transverse foramina; C1 (atlas) supports the skull, C2 (axis) has the dens for rotation, C7 is vertebra prominens.

• Thoracic Vertebrae: Have costal facets for rib articulation.

• Lumbar Vertebrae: Large, robust bodies for weight-bearing.

Vertebral Curvatures

• Normal Curvatures: Primary (thoracic, sacral) and secondary (cervical, lumbar) curves provide balance and flexibility.

• Abnormal Curvatures: Kyphosis (excessive thoracic curve), lordosis (excessive lumbar curve), scoliosis (lateral curvature).

General Vertebral Anatomy

• Body (Centrum): Weight-bearing portion.

• Neural Arch: Formed by pedicles and laminae, enclosing the vertebral foramen.

• Processes: Spinous, transverse, superior/inferior articulating processes for muscle and ligament attachment and articulation with adjacent vertebrae.

Thoracic Cage: Ribs and Sternum

The thoracic cage protects vital organs and supports the shoulder girdles and upper limbs.

• Ribs (12 pairs): True ribs (1–7, attach directly to sternum), false ribs (8–12, indirect or no attachment), floating ribs (11–12, no anterior attachment).

• Sternum: Manubrium, body, xiphoid process.

Appendicular Skeleton

The appendicular skeleton enables movement and manipulation of the environment.

Upper Limb

• Pectoral Girdle: Clavicle (sternal/acromial ends, conoid tubercle), scapula (acromion, coracoid process, spine, fossae, borders, angles, glenoid cavity, tubercles).

• Humerus: Head, tubercles, intertubercular groove, neck, deltoid tuberosity, condyle, fossae, epicondyles, trochlea, capitulum.

• Ulna: Olecranon process, trochlear notch, coronoid process, radial notch, head, styloid process.

• Radius: Head, neck, tuberosity, styloid process, ulnar notch.

• Carpals: Proximal (scaphoid, lunate, triquetrum, pisiform), distal (trapezium, trapezoid, capitate, hamate).

• Metacarpals: I–V.

• Phalanges (Hand): Proximal, middle, distal.

Lower Limb

• Pelvic Girdle: Pelvis, false/true pelvis, pelvic brim/inlet/outlet, arcuate line, obturator foramen.

• Os Coxae: Ischium (spine, notch, tuberosity, ramus), ilium (crest, auricular surface, fossa, notches, spines), pubis (symphysis, tubercle, rami).

• Femur: Head, fovea capitis, neck, trochanters, intertrochanteric line/crest, linea aspera, pectineal line, gluteal tuberosity, supracondylar ridges, popliteal surface, condyles, epicondyles, intercondylar fossa, patellar surface.

• Patella: Base, apex, facets.

• Tibia: Intercondylar eminence, condyles, tuberosity, anterior crest, medial malleolus.

• Fibula: Head, interosseous border, lateral malleolus.

• Tarsals: Calcaneus, talus, navicular, cuboid, medial/intermediate/lateral cuneiforms.

• Metatarsals: I–V.

• Phalanges (Foot): Proximal, middle, distal.

Male vs. Female Pelvis

Feature	Male	Female
Pelvic Inlet Shape	Narrow, heart-shaped	Wide, oval
Ischial Spine Orientation	Pointed medially	Pointed laterally
Pelvic Width/Depth	Narrow, deep	Wide, shallow
Pubic Angle	Less than 90°	Greater than 90°
Pubic Ramus Thickness	Thicker	Thinner

Arches of the Feet

The foot has longitudinal and transverse arches that distribute body weight and provide leverage and shock absorption during movement.

• Medial Longitudinal Arch

• Lateral Longitudinal Arch

• Transverse Arch

Differences in Growth Rates

Different parts of the skeleton grow at different rates, influencing body proportions during development. For example, the head is proportionally larger in infants, while limbs and trunk lengthen more rapidly during childhood and adolescence.

Additional info: The skeletal system provides support, protection, movement, mineral storage, and blood cell formation (hematopoiesis). Disorders such as osteoporosis, fractures, and congenital malformations can affect bone structure and function.