The Skeletal System: Structure, Landmarks, and Major Bones

The Skeletal System

Overview of the Skeletal System

The human skeletal system provides structural support, protects internal organs, enables movement, and serves as a reservoir for minerals. The adult human skeleton typically consists of 206 bones, while a newborn has approximately 270 bones due to unfused elements.

• Bone Count: 206 in adults; 270 in infants (bones fuse with age).

• Functions: Support, protection, movement, mineral storage, blood cell production.

Bone Terminology and Landmarks

Bones have distinct surface markings that serve as sites for muscle attachment, articulation, and passage of nerves and blood vessels. Understanding these terms is essential for identifying anatomical features.

• Facet: Shallow convex or concave surface where two bones articulate.

• Fossa: Indentation in a bone into which another structure fits.

• Sulcus/Groove: Large indentation along which a narrow structure (tendon, nerve, or blood vessel) travels.

• Meatus: A tunnel or canal through a bone.

• Fissure: A narrow slit through a bone or between adjacent bones.

• Foramen: A hole in a bone for blood vessels or nerves.

• Condyle: Rounded end of a bone that articulates with another bone.

• Crest: Ridge or projection of a bone.

• Head: Rounded projection from a bone’s epiphysis.

• Tubercle/Tuberosity: Small (tubercle) or large (tuberosity) rounded bony projection.

• Epicondyle: Small projection usually proximal to a condyle.

• Process: Prominent bony projection.

• Spine: Sharp process of a bone.

• Protuberance: Outgrowth of a bone.

• Trochanter: Large projection found only on the femur.

• Line: Long, narrow ridge on a bone.

• Sinus: Hollow, air-filled cavity in bone.

Bones of the Axial Skeleton

The axial skeleton forms the central axis of the body and includes the skull, vertebral column, sternum, and ribs.

Skull

• Cranial Bones: Encase the brain. In infants, cranial bones are joined by membranous areas called fontanels, which ossify to form suture joints (coronal, sagittal, lambdoid, squamous).

  • Frontal bone

  • Parietal bones

  • Temporal bones (mastoid process, styloid process, external auditory meatus)

  • Occipital bone (foramen magnum)

  • Sphenoid bone (bat or butterfly shaped)

  • Ethmoid bone

• Facial Bones: Form the structure of the face.

  • Maxillae

  • Palatine bones

  • Zygomatic bones (zygomatic arch)

  • Lacrimal bones

  • Vomer

  • Mandible

  • Nasal bones

  • Nasal concha

Hyoid Bone

A U-shaped bone in the neck that supports the tongue and is not directly articulated with other bones.

Auditory Ossicles

Three small bones in the middle ear (malleus, incus, stapes) that transmit sound vibrations.

Vertebral Column

The vertebral column protects the spinal cord and supports the head and body. Vertebrae are separated by intervertebral discs.

• Cervical Vertebrae: 7 vertebrae in the neck. The first (Atlas) and second (Axis) have unique structures for head movement.

• Thoracic Vertebrae: 12 vertebrae in the chest region.

• Lumbar Vertebrae: 5 vertebrae in the lower back.

• Sacrum: Triangular bone formed by fusion of 4-5 vertebrae in adulthood.

• Coccyx: 3-5 fused vertebrae forming the tailbone.

Body Plan of Vertebrae: Each vertebra typically has a vertebral foramen, body, transverse process, and spinous process.

Sternum

Also known as the breastbone, the sternum consists of three parts: manubrium, body, and xiphoid process.

Ribs

• True Ribs (1-7): Attach directly to the sternum via their own costal cartilage.

• False Ribs (8-10): Attach to the cartilage of the 7th rib, which then attaches to the sternum.

• Floating Ribs (11-12): Do not attach to the sternum.

Rib Body Plan: Each rib has a head, body (shaft), and sternal end.

Bones of the Appendicular Skeleton

The appendicular skeleton includes the bones of the limbs and girdles that attach them to the axial skeleton.

Pectoral Girdle

• Clavicle: Has an acromial end (lateral) and sternal end (medial).

• Scapula: Features include acromion process, coracoid process, spine, glenoid cavity, supraspinous fossa, and infraspinous fossa.

Arm, Wrist, and Hand

• Humerus: Head, deltoid tuberosity, coronoid fossa, trochlea, olecranon fossa.

• Radius: Head, radial tuberosity, styloid process.

• Ulna: Olecranon process, trochlear notch, coronoid process.

Carpal Bones

Arranged in two rows (proximal and distal) from thumb to pinky:

Proximal Row	Distal Row
Scaphoid (navicular)	Trapezium
Lunate	Trapezoid
Triquetrum	Capitate
Pisiform	Hamate

• Metacarpals: Bones of the hand located in front of the carpal bones.

• Phalanges: Each finger has proximal, middle, and distal phalanges (thumb lacks a middle phalanx).

Pelvic Girdle

Supports the trunk and legs, attaching the lower limbs to the axial skeleton. The coxal bone (os coxae or innominant bone) is formed by the fusion of three bones:

• Ilium: Superior portion.

• Ischium: Posteroinferior portion; ischial tuberosity supports weight when sitting.

• Pubis: Smallest portion; pubic arch is important for determining skeletal gender.

Key landmarks: acetabulum (hip socket), obturator foramen (large opening).

Leg

• Femur: Head, neck, greater and lesser trochanters, medial and lateral condyles.

• Patella: Kneecap.

• Tibia: Medial and lateral condyles, medial malleolus.

• Fibula: Head, lateral malleolus.

Tarsal Bones

Ankle bones include:

• Calcaneus (heel bone, largest tarsal)

• Talus (forms ankle joint)

• Navicular

• Medial, intermediate, and lateral cuneiforms

• Cuboid

• Metatarsals: Bones of the foot.

• Phalanges: Toes have proximal, middle, and distal phalanges (big toe lacks a middle phalanx).

Summary Table: Major Bone Groups and Key Features

Region	Major Bones	Key Features/Landmarks
Skull	Cranial, Facial	Sutures, processes, foramina
Vertebral Column	Cervical, Thoracic, Lumbar, Sacrum, Coccyx	Body, foramen, processes
Thorax	Sternum, Ribs	Manubrium, costal cartilage
Pectoral Girdle	Clavicle, Scapula	Acromion, glenoid cavity
Upper Limb	Humerus, Radius, Ulna, Carpals, Metacarpals, Phalanges	Condyles, tuberosities, processes
Pelvic Girdle	Ilium, Ischium, Pubis	Acetabulum, obturator foramen
Lower Limb	Femur, Patella, Tibia, Fibula, Tarsals, Metatarsals, Phalanges	Trochanters, condyles, malleoli

Example: The foramen magnum in the occipital bone allows passage of the spinal cord from the brain to the vertebral column.

Additional info: The arrangement and fusion of bones in the skeleton are critical for movement, protection, and support. Knowledge of bone landmarks is essential for clinical practice, such as identifying fracture sites or performing surgical procedures.