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 found in the limbs (e.g., humerus, radius, ulna, femur, tibia, fibula).

• Short Bones: Nearly equal in length and width; provide support and stability (e.g., carpals, tarsals, talus).

• Flat Bones: Thin, flattened, and often curved; protect internal organs (e.g., sternum, ribs, scapulae, cranial bones).

• Irregular Bones: Complex shapes that do not fit other categories (e.g., vertebrae, sacrum, coccyx, hip bones).

• Sesamoid Bones: Small, round bones embedded within tendons (e.g., patellae).

Structure of a Long Bone

Long bones have a specialized structure that supports movement and strength.

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

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

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

• Articular Cartilage: Hyaline cartilage covering the epiphyses, reducing friction and absorbing shock.

• Spongy Bone: Found mainly in the epiphyses, containing trabeculae aligned along stress lines.

• Epiphyseal Line: Remnant of the growth plate; indicates cessation of growth in adults.

• 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 containing yellow bone marrow; lined by endosteum.

• 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: Connect blood vessels of periosteum to those in central canals.

• Trabeculae: Lattice-like structures in spongy bone, providing support and reducing weight.

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

Bone Markings

Bones have various markings that serve as sites for muscle and ligament 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.

• 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.

• Hyoid Bone: Does not articulate with other bones; supports the tongue.

• Auditory Ossicles: Malleus, incus, stapes (middle ear bones).

• Sinuses: Air-filled cavities in certain skull bones (frontal, ethmoid, sphenoid, maxillary).

Major Sutures of the Skull

• Coronal, lambdoidal, sagittal, squamosal, intermaxillary.

Selected Cranial Bones and Features

• Frontal Bone: Orbital surface, 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, foramina (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 and Features

• Maxillae: Frontal process, infraorbital foramen, zygomatic process, palatal/alveolar processes, incisive fossa, maxillary sinus.

• Palatine: Posterior hard palate, part of the orbit.

• Lacrimal: Forms part of the orbit; contains lacrimal canal for tear drainage.

• Zygomatic: Cheekbone.

• Nasal, Vomer, Inferior Nasal Conchae, Mandible: Various features including body, ramus, condyles, coronoid process, mental foramen, alveolar process.

Fetal Skull (Fontanels)

• Frontal (anterior), posterior (occipital), anterolateral (sphenoid), posterolateral (mastoid) fontanels allow for skull flexibility during birth and brain growth.

Spinal Column (Vertebral Column)

The vertebral column supports the body and protects the spinal cord.

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

• Cervical Vertebrae: Transverse foramina, atlas (C1), axis (C2) with dens, vertebra prominens (C7).

• Thoracic Vertebrae: Costal facets for rib articulation.

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

Vertebral Curvatures

• Normal: Primary (thoracic, sacral), secondary (cervical, lumbar).

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

General Vertebral Anatomy

• Body (centrum), neural arch, lamina, pedicles, vertebral foramen, spinous/transverse processes, superior/inferior articulating processes.

Ribs and Sternum

The thoracic cage protects vital organs and supports respiration.

• 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

Composed of the limbs and girdles that attach them to the axial skeleton.

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, condyles, epicondyles, 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 help distribute body weight and provide leverage during walking.

• Medial Longitudinal Arch

• Lateral Longitudinal Arch

• Transverse Arch

Differences in Growth Rates

Some parts of the body grow at different rates, affecting proportions during development (e.g., head size relative to body in infants vs. adults).

Additional info: The skeletal system provides support, protection, movement, mineral storage, and blood cell formation (hematopoiesis). Disorders such as osteoporosis, fractures, and abnormal curvatures can affect skeletal health.