The Anatomy of the Skeletal System

The skeletal system comprises 206 bones and has two main parts—the axial skeleton and the appendicular skeleton. The skeletal system includes your bones, ligaments that attach bone to bone, and cartilage that provides padding between your bones.

This article discusses the anatomy of the skeletal system—what it's made of, how it's organized, conditions that affect it, and tests that assess it.

Skeletal System: Labeled Diagram of Major Organs

In addition to the bones, organs of the skeletal system include ligaments that attach bones to other bones and cartilage that provides padding between bones that form joints throughout your body.

The bones are divided into two main categories—the axial skeleton, which contains the bones that support the middle of your body, and the appendicular skeleton, which includes bones that make up your appendages—arms and legs—and bones that attach your limbs to your axial skeleton.

The Skeletal System Tour

Axial Skeleton

The axial skeleton forms the "axis" that runs down the center of the body. There are 80 bones that make up the axial skeleton.

Skull (Cranium)

Your skull (cranium) is made up of cranial and facial bones. Cranial bones protect your brain, while facial bones make up your facial structure. Skull bones include the following:

Cranial bones include:

• Frontal
• Parietal
• Occipital
• Temporal
• Sphenoid
• Ethmoid

Facial bones include:

• Maxillae (upper jaw)
• Vomer
• Mandible (lower jaw)
• Lacrimal
• Zygomatic (cheekbones)
• Nasal
• Palatine
• Inferior nasal conchae

Auditory Ossicles

The auditory ossicles consist of a total of six tiny bones, with three in each ear. They are located in the inner ear and are structures that help create sound in your body. The auditory ossicles include the following:

• Malleus
• Incus
• Stapes

Hyoid Bone

The hyoid bone is a horseshoe-shaped bone located in the throat. It is part of bodily functions like speaking, swallowing, and airway maintenance.

Vertebral column

The vertebral column (spine) protects your spinal cord, supports your head, and allows bodily movement. It contains the sacrum (made up of four bones) and coccyx (the tailbone, which is made up of five bones), and 24 vertebrae, including:

• Cervical vertebrae: Seven bones in the neck region
• Thoracic vertebrae: Twelve bones attached to the ribs
• Lumbar vertebrae: Five bones in the low back region

Thorax

The thorax contains the sternum (breastbone) and the thoracic (rib) cage. The thoracic cage comprises 12 pairs of ribs connecting to the thoracic vertebrae and the sternum. Your rib cage protects your heart.

Appendicular Skeleton

The appendicular skeleton includes 126 bones that comprise your appendages—your arms and legs—and the bones that attach your limbs to your axial skeleton.

Upper Extremities

Your upper extremities refer to your shoulders and arms. Bones in the upper extremities include:

• Scapula (shoulder blade)
• Clavicle (collarbone)
• Humerus (upper arm)
• Radius and ulna (forearm bones)
• Carpals (eight tiny bones in the wrist)
• Metacarpals (in the palm)
• Phalanges (bones of the fingers)

Lower Extremities

Bones in the lower extremities make up your hips and legs and include:

• Pelvis
• Femur (thigh bone)
• Patella (kneecap)
• Tibia and fibula (lower leg bones)
• Tarsals (eight tiny bones in the ankle)
• Metatarsals (in the middle of the foot)
• Phalanges (bones of the toes)
  

What Is the Purpose of the Skeletal System?

The primary purpose of the skeletal system is to give the body its shape and to provide attachment points for the muscles that move the body.

Other purposes of the skeletal system include:

• Storing minerals (such as calcium) and fats
• Producing red blood cells
• Protecting internal organs

Skeletal System Associated Conditions

Various conditions and injuries can affect the skeletal system. Examples include:

• Fractures (broken bones)
• Ligament sprains
• Osteoarthritis
• Osteoporosis
• Osteopenia
• Osteogenesis imperfecta
• Scoliosis
• Rickets
• Osteomalacia
• Cancer
• Paget's disease of bone
• Osteomyelitis
• Avascular necrosis (osteonecrosis)
• Marfan syndrome
• Ehlers-Danlos syndrome
• Osteopetrosis
• Ankylosing spondylitis
• Bone marrow diseases

Skeletal System Tests

Many different tests can help diagnose conditions that affect the skeletal system.

Imaging Tests

Healthcare providers use various imaging tools to get detailed pictures of your bones. Depending on the reason for imaging, a healthcare provider will conduct one or more of the following tests:

• X-rays: This common test can help diagnose conditions that affect the bones and joints, such as fractures or arthritis.
• Computed tomography (CT scan), computerized axial tomography (CAT scan): This test provides three-dimensional pictures that help diagnose fractures that aren't clear on X-rays, or other bone conditions, such as cancer.
• Magnetic resonance imaging (MRI): This type of imaging often helps diagnose conditions that affect soft tissues of the skeletal system (ligaments, cartilage).
• Bone scintigraphy (bone scan): These scans can provide detailed information about a bone injury or condition, such as the staging of bone cancer.
• Positron emission tomography (PET scan): This test, which uses an injected radioactive tracer, can help stage bone (and other types of) cancer.
• Bone density test: These tests are primarily for determining how dense bones are—the key factor in diagnosing osteoporosis.
  

Other Skeletal System Tests

Healthcare providers may perform additional tests if they need further information about your bones or skeletal system. These tests may include:

• Joint aspiration: This test involves removing a sample of fluid from a joint to help diagnose infection.
• Biopsy: For the skeletal system, this procedure can involve removing a small sample of bone or bone marrow so that it can be tested for conditions such as cancer.
• Blood tests: These tests help diagnose infections that can affect the skeletal system.
  

Summary

The skeletal system is made up of your bones, ligaments, and cartilage. Though its main function is to provide structural support for the body, it also stores important minerals—such as calcium—forms red blood cells, and protects your internal organs. The skeletal system can break down into two main categories—the axial skeleton, which forms the "long axis" of the body, and the appendicular skeleton, which forms your arms and legs.

Many different injuries and diseases can affect the skeletal system. Imaging procedures, such as X-rays, MRI, and bone density tests, and other tests, such as blood work or tissue biopsy, can help to diagnose these conditions.