The Skeleton: Structure, Regions, and Key Features

Introduction to the Skeleton

The human skeleton is a complex framework of bones that supports and protects the body, facilitates movement, and houses critical organs. It is divided into two main divisions: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Forms the longitudinal axis of the body and includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Comprises the bones of the limbs and girdles that attach them to the axial skeleton.

Axial Skeleton

Overview and Major Functions

The axial skeleton consists of 80 bones, accounting for roughly 40% of the bones in the human body. Its primary functions include:

• Supporting and protecting the brain, spinal cord, and organs in the ventral cavity

• Providing attachment points for muscles that adjust the head, neck, and trunk, perform respiratory movements, and stabilize the appendicular skeleton

Bones of the Skull

The skull is divided into cranial and facial bones, with additional associated bones. The cranial bones protect the brain, while the facial bones form the structure of the face and support sensory organs.

• Cranial Bones (8): Frontal, occipital, sphenoid, ethmoid, 2 parietal, 2 temporal

• Facial Bones (14): 2 zygomatic, 2 maxilla, 2 lacrimal, 2 nasal, 2 palatine, vomer, 2 inferior nasal concha, mandible

• Associated Bones: 3 pairs of auditory ossicles, hyoid bone

Skull Sutures and Markings

Sutures are immovable joints that connect the bones of the skull. Key sutures include the coronal, sagittal, squamous, and lambdoid sutures. Important bone markings include:

• External occipital crest: Attachment for ligament stabilizing neck vertebrae

• Mastoid process: Muscle attachment for head rotation

• Styloid process: Attachment for muscles and hyoid suspension

Facial and Cranial Bone Features

• Inferior nasal conchae: Create air turbulence and warm/humidify air

• Alveolar processes: Sockets for teeth in maxillae and mandible

• Zygomatic arch: Formed by temporal and zygomatic bones

• External acoustic meatus: Opening for sound waves to reach the tympanic membrane

Internal and Inferior Skull Features

• Frontal sinuses: Variable in size, appear around age 6

• Petrous part of temporal bone: Encloses inner ear structures

• Foramen magnum: Connects cranial cavity with vertebral canal

• Occipital condyles: Articulate with the first cervical vertebra

Sphenoid, Ethmoid, and Palatine Bones

• Sphenoid: Forms part of the cranial floor, unites cranial and facial bones, contains optic canals and sella turcica (pituitary gland seat)

• Ethmoid: Forms part of the anterior cranial floor, roof of nasal cavity, and nasal septum; contains cribriform plate (olfactory nerves), crista galli, and ethmoidal labyrinth (sinuses)

• Palatine: Forms posterior hard palate and part of the orbit floor

Orbital and Nasal Complexes

• Orbital complex: Formed by 7 bones; houses the eyes and associated structures

• Lacrimal sulcus: Groove for tear drainage

• Paranasal sinuses: Air-filled chambers in frontal, sphenoid, ethmoid, palatine, and maxillae; lighten skull, resonate voice, and provide mucous membrane surface

Mandible and Associated Bones

• Mandible: Lower jaw; supports lower teeth, articulates with temporal bone at temporomandibular joint

• Hyoid bone: U-shaped, not attached to other bones; supports muscles of larynx, pharynx, and tongue

• Auditory ossicles: Malleus, incus, stapes; conduct sound vibrations in the middle ear

Developmental Differences in the Skull

Infant skulls have fontanelles—areas of fibrous connective tissue—that allow for brain growth and passage through the birth canal. The anterior fontanelle is the largest and persists until about age 2. Most skull growth occurs before age 5.

Vertebral Column

Structure and Regions

The vertebral column consists of 26 bones: 24 vertebrae, the sacrum, and the coccyx. It supports the head and trunk, protects the spinal cord, and transfers weight to the appendicular skeleton.

• Cervical region: 7 vertebrae; supports the head

• Thoracic region: 12 vertebrae; articulates with ribs

• Lumbar region: 5 vertebrae; largest and strongest

• Sacrum: 5 fused vertebrae; foundation for pelvic girdle

• Coccyx: 3-5 fused vertebrae; tailbone

Parts of a Vertebra

• Vertebral foramen: Passage for spinal cord

• Intervertebral foramen: Passage for spinal nerves

• Vertebral body: Weight-bearing portion

• Articular, transverse, and spinous processes: Muscle and ligament attachment, articulation with adjacent vertebrae

• Intervertebral disc: Cartilaginous pad for shock absorption and movement

Regional Comparison of Vertebrae

Distinctive Features of Cervical and Thoracic Vertebrae

• Cervical vertebrae: Smallest, have transverse foramina; atlas (C1) allows nodding, axis (C2) allows rotation; C7 is vertebra prominens

• Thoracic vertebrae: Articulate with ribs, bodies increase in size inferiorly

Lumbar Vertebrae and Sacrum

• Lumbar vertebrae: Largest, thickest bodies, support most weight

• Sacrum: Five fused vertebrae, protects pelvic organs, features sacral hiatus, sacral promontory, and articulates with coccyx

Thoracic Cage

Structure and Function

The thoracic cage protects vital organs and supports the shoulder girdles and upper limbs. It consists of the sternum, ribs, costal cartilages, and thoracic vertebrae.

• Sternum: Manubrium, body, xiphoid process

• Ribs: 12 pairs; true ribs (1-7), false ribs (8-12), floating ribs (11-12)

Appendicular Skeleton

Overview

The appendicular skeleton includes 126 bones of the limbs and girdles. It is essential for movement and manipulation of the environment.

Pectoral Girdle

• Clavicle: Articulates with sternum (medially) and scapula (laterally)

• Scapula: Features include acromion, glenoid cavity, and scapular spine

Arm and Forearm Bones

• Humerus: Head articulates with scapula; features include surgical neck, epicondyles, olecranon fossa, radial fossa, coronoid fossa

• Radius: Lateral bone; radial tuberosity, styloid process

• Ulna: Medial bone; olecranon, trochlear notch, ulnar head

Wrist and Hand

• Carpal bones: 8 bones in two rows

• Metacarpals: 5 bones

• Phalanges: 14 bones per hand; thumb (pollex) has 2, others have 3

Pelvic Girdle and Pelvis

• Coxal bones: Each formed by fusion of ilium, ischium, and pubis

• Acetabulum: Socket for femur head

• Greater sciatic notch: Passage for sciatic nerve

• Pubic symphysis: Cartilaginous joint between pubic bones

• Obturator foramen: Large opening for nerves and blood vessels

Pelvis Structure and Sex Differences

• Pelvic brim: Separates true and false pelvis

• Female pelvis: Broader, shallower, wider pelvic inlet and outlet, adapted for childbirth

• Male pelvis: Narrower, deeper, more robust

Thigh and Leg Bones

• Femur: Longest, strongest bone; features include neck, greater trochanter, linea aspera

• Patella: Kneecap; protects knee joint

• Tibia: Larger, medial lower leg bone; tibial tuberosity, medial malleolus

• Fibula: Thinner, lateral; lateral malleolus, does not bear weight

Ankle and Foot

• Tarsal bones: 7 bones; calcaneus (heel), talus (ankle joint)

• Metatarsals: 5 bones

• Phalanges: 14 bones per foot; hallux (big toe) has 2, others have 3

• Arches: Longitudinal and transverse arches distribute body weight and provide support

Medical Terminology Associated with the Axial Skeleton

• Laminectomy: Surgical removal of a vertebral lamina, often to relieve herniated disc symptoms

• Spinal fusion: Surgical stabilization of vertebrae using bone grafts or devices

• Whiplash: Neck injury from rapid hyperextension and hyperflexion

• Clubfoot (talipes equinovarus): Inherited deformity with twisted foot

• Genu valgum: Knock-knee; knees close together, ankles apart

• Genu varum: Bowleg; knees apart, lower limbs bowed medially