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Skeletal system

skeletal system definition and examples

Skeletal system
n., [ˈskɛl ɪ tl ˈsɪstəm]
Definition: An organ system for structural support and protection

What is the Skeletal System?

How to define a skeleton? The skeletal system is the main framework that gives your body its shape. Bones and connective tissues including cartilages, tendons, and ligaments are the main components of the skeletal system. It is referred to as the musculoskeletal system.

How many bones are in a human body? The adult human skeletal system has about 206 bones. It forms about 20% of the human body weight. The skeletal system structure differs according to gender where the male body skeleton is longer and has a higher bone mass. Animal skeletons are found either inside their bodies (endoskeleton) or outside their bodies (exoskeleton).

The main function of the skeletal system is to support the body shape. what is the purpose of the skeletal system? The skeletal system’s main function is to control the body movement in correlation with muscles to perform daily activities as well as protect vital organs.

Is bone a connective tissue? Bone is connective tissue, the main functional unit of the skeletal system is the bone. The bone system consists of three layers; they are:

  • Periosteum: it’s the outside membrane of the bone anatomy that protects the bone from any damage.
  • Compact bone: it’s the white smooth layer that comes after the periosteum.
  • Spongy bone: it forms the core and the inner part of the bone. This part contains holes in which the bone marrow is stored.

What are bones made of? Human bones are generally formed of proteins and some minerals like calcium and phosphate.
How does the skeletal system work? The skeletal system organs work with the aid of muscles, together, they form a system called the musculoskeletal system. Moreover, other tissue aid in the movement of the skeletal system such as:

  • Cartilage: it is a substance that is found between bones to facilitate movement and decrease friction between two bones while moving.
  • Joints: It’s the point of attachment between two bones.

There are three types of joints as follows:

  1. Immovable joints: where bones are immovable like skull bones.
  2. Partly movable joints: it allows limited movement like rib cage bones.
  3. Movable joints: it allows a free or wide range of motion, such as knee and shoulder joints.
    · Ligaments: refer to the connective tissue which holds bones together at the joints.
    · Tendons: a structure that connects bones with muscles to ease movement of the body.
body organ diagram
Figure 1: Bbody organ diagram. Image Credit: Wikimedia

Axial Skeleton Anatomy

What are the 6 main parts of the skeletal system? The human skeletal system consists of two skeletal system parts. One of them contains the axial skeleton bones including the skull bones, vertebral column, and the thoracic cage. The other is the appendicular skeleton, which consists of the pectoral girdle, the pelvic girdle, the upper and lower limbs.

Skull bones

The Human skull has a unique structure to support the face and protect the brain. Skull bones are connected to each other by fibrous joints. Ossification of fibrous joints connects skull bones together in order to provide sufficient protection to the brain.
There are two groups of bones forming the skull, the cranium, and the bones of the face. The face is generally formed of 14 bones which are connected together to form the orbits of the eyes, nasal, mouth cavities, and sinuses.

Auditory ossicles

The ear consists of the external, middle, and inner ear, the middle ear contains the auditory ossicles and the tympanic membrane. The auditory ossicles play an important role in the transmission of sound waves. The ossicles are divided into three parts, the malleus, the incus, and the stapes.


It is a small U-shaped bone located in the anterior side, at the base of the mandible, in the posterior side, of the fourth cervical vertebra, and superior to the thyroid cartilage. It is located in the anterior part of the neck attached to muscles from the pharynx, larynx, floor of the mouth, and tongue.

Vertebral column

The vertebral column is one of the most important bony structures found in the human body. Since it has many significant roles in the body such as:

  • Protection of the spinal cord from damage
  • Supporting and bearing the weight of the body above the pelvis
  • Facilitating the movement of the body

The vertebral column has about 33 bones (vertebrae), which are separated by cartilage discs. Each region of the vertebral column has vertebrae of a different structure, it is divided into five regions which are as follows:

  • The cervical region has 7 bones.
  • The thoracic region includes 12 bones.
  • The Lumbar region includes 5 bones.
  • The sacrum region contains 4 bones and the coccyx.

The coccyx is a small bone at the end of the vertebral column.

Each vertebra consists of an anterior part called the vertebral body, the vertebral lateral, and the posterior site called the vertebral arch, the vertebral arch is considered an enclosed hole in vertebrae. Each vertebral arch in all lined vertebrae forms a canal through which the spinal cord passes. The superior and inferior parts of vertebrae are covered with hyaline cartilage and each vertebra is separated from the following vertebra by discs called intervertebral discs to minimize friction between vertebrae.

Thoracic cage

The thoracic or the rib cage forms the chest portion of the body. How many ribs are in the human body? The rib cage consists of 12 pairs of ribs which are considered flat bones examples. The human ribs are connected to 12 thoracic vertebrae. They are numbered for T1 to T12. The thoracic cage has a significant role in the protection of the lungs and the heart. The twelve pairs of ribs are connected anteriorly to the sternum, which is an elongated bone located at the middle of the chest. The sternum consists of three regions, the manubrium which is a U-shaped wide bone, the body of the sternum, and the xiphoid which is considered the inferior tip of the sternum.

female rib cage diagram with organs
Figure 2: Female rib cage diagram with organs showing rib cage with organs included and protected inside it.
Image credit: Wikimedia

Skeletal System Diagram

Below is a diagram of the skeletal system, with labeled parts.

human skeleton
Figure 3: Human skeleton labeled showing all bones of the body in the form of a skeleton diagram
Image Credit: Wikimedia


Or watch this vid about the skeletal system.


Appendicular Skeleton Anatomy

The appendicular skeleton bones consist of the upper and lower extremities. the appendicular system consists of 126 bones

Pectoral girdle

The pectoral girdle includes an S-shaped bone called the clavicle and the scapula which is found at the back of the shoulder. It is responsible for the connection between the upper limbs and the body axis.

Upper limbs

Upper limbs include bones of the shoulder, arms, and hands. It consists of:

  • Shoulder girdle “pectoral girdle”: consists of the clavicle and the scapula bones
  • Arms: formed of an upper arm bone called the humerus and two forearm bones called ulna and radius which are considered long bone examples
  • Wrist bones: it has 8 bones called Scaphoid, Lunate, Triquetrum, Pisiform, Trapezoid, Capitate, Hamate, and hand bones which are considered short bone examples
  • Hand: consists of short bones called metacarpals and phalanges

Pelvic girdle

The pelvic girdle connects between the vertebral column and legs. It is formed of complex bones joined together anteriorly at the pubic symphysis and posteriorly at the sacrum. It consists of three bones, the ilium which has the width of the hip, the ischium, and the pubic bone. Additionally, it has a cup-shaped pocket-forming the hips joint and carrying the head of the femur bone (thigh bone). The thigh bone is one of the flat bones.

Lower limbs

Lower limbs include bones of the pelvis, legs, and feet. It consists of:

  • The pelvic girdle: consist of the ilium, ischium, and pubic bones
  • Leg: it consists of the large femur, intermediate tibia, and smaller fibula bones.
  • Tarsal bones: which include talas, calcaneus, cuboid, cuneiform, and navicular bones.
  • Foot: consists of tarsals, metatarsals, and phalanx which are considered small bones in the body.

What is the Function of the Skeletal System?

What does the skeletal system do? The Skeletal system function is mainly supporting the body movement along with other 5 functions of the skeletal system including:

  • Movement: together with the muscular system and body joints, skeleton bones are responsible for body movement.
  • Production of blood cells: bone marrow is found in large bones such as thigh bones. There are two types of bone marrow, including red bone marrow which contains blood stems and yellow bone marrow contains a large percentage of fat and stem cells.
  • Protection of vital organs: where the skull protects the brain, the vertebral column protects the spinal cord, and the thoracic cage protects the heart and lungs.
  • Storage of minerals: bones can store many minerals like calcium and vitamin D which increase bone density.
  • Posture: as it gives the correct shape to the body.

What kinds of Conditions Affect the Skeletal System?

There are many conditions that may affect the appendicular and axial skeleton such as:


Fractures are conditions where part of the body’s bones are broken. Fractures affect bones due to accidents “stress” like trauma, car accidents, falling, and other stressors that may affect bones or diseases ” pathological conditions” such as Osteoporosis. Osteoporosis is one of the most common medical conditions that lead to bone fractures. Osteoporosis is a condition where the bone density decreases so it becomes fragile and unable to function normally.

Metabolic bone diseases

Bones consist of proteins and minerals like calcium and phosphate. Minerals are affected by Thyroid gland hormones since the increase in thyroid hormone leads to withdrawing calcium from bones to the bloodstream. Consequently, the bone density decreases and it becomes weaker. Moreover, Vitamin D levels can affect bone health since it helps in calcium absorption.


Arthritis is a condition where the joint is inflamed. There are many forms of arthritis, the most common is osteoarthritis, other conditions are significant such as rheumatoid arthritis and gout. Arthritis may be attributed to many reasons such as increased stress, repeated use of the joint, inflammation, deposition of crystals, or immune response in the joint.


Bone cancer is rare. However, it is a serious condition. Bone cancer mostly affects the pelvis and long bones of arms and legs. There are several types of bone cancers such as chondrosarcoma, Ewing sarcoma, and osteosarcoma. Osteosarcoma is the most common type of bone cancer; it is commonly found in long bones. Symptoms include pain, swelling, fatigue, and weakness.

Spinal curvatures

The spinal cord or backbone is formed of bones stacked and separated by discs between them. Generally, there are slight curves in the spinal that help it to absorb stress. But if these curves become deeper than normal that can lead to some pathological conditions as follows:

  • Lordosis: it’s a condition when curves are inward at the lower back.
  • Kyphosis: it’s the condition that the upper back gets abnormally rounded.
  • Scoliosis: it’s when the curve is S-shaped or C-shaped.
Biology definition and takeaways:
Skeletal system is an organ system whose primary function is for structural support and protection. It consists basically of bones, cartilages, ligaments, and tendons. An organ system, (sometimes simply system), is a group of organs that work together to carry out a particular task. In humans and other animals, the organ systems are integumentary system, lymphatic system, muscular system, nervous system, reproductive system, urinary system, respiratory system, skeletal system, and immune system. The skeleton pertains to all the bones and cartilages of an organism. It may be an endoskeleton or an exoskeleton. An endoskeleton is one in which skeletal structures are found within the body. An exoskeleton is a type of skeleton that is found outside the body of the organism. Most vertebrates have an endoskeleton. In humans, the skeleton is an endoskeleton-type, consisting of 206 bones. The tiniest bones are found in the middle ear whereas the largest bone is the femur. Animals such as crabs, shrimps, insects, and many other invertebrates have an exoskeleton.

Try to answer the quiz below to check what you have learned so far about the skeletal system.


Choose the best answer. 

1. Skeleton found inside the body
2. How many bones are there in a human body?
3. Connects bone to another bone
4. Which of the following is part of the appendicular skeleton?
5. A condition where the bone density decreases so it becomes fragile and unable to function normally

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  • El Demellawy, D., Davila, J., Shaw, A., & Nasr, Y. (2018, September). Brief review on metabolic bone disease. Academic forensic pathology. Retrieved February 22, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6490580/
  • Encyclopædia Britannica, inc. (n.d.). Pelvis. Encyclopædia Britannica. Retrieved February 22, 2022, from https://www.britannica.com/science/pelvis
  • Marks, H. (n.d.). Spine curvature disorders: Causes, symptoms, diagnosis, treatment. WebMD. Retrieved February 22, 2022, from https://www.webmd.com/back-pain/guide/types-of-spine-curvature-disorders
  • Mayo Foundation for Medical Education and Research. (2020, March 10). Bone cancer. Mayo Clinic. Retrieved February 22, 2022, from https://www.mayoclinic.org/diseases-conditions/bone-cancer/symptoms-causes/syc-20350217
  • Osteoporosis: Symptoms, causes, tests & treatment. Cleveland Clinic. (n.d.). Retrieved February 22, 2022, from https://my.clevelandclinic.org/health/diseases/4443-osteoporosis
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  • The vertebral column. The Vertebral Column – Joints – Vertebrae – Vertebral Structure. (n.d.). Retrieved February 22, 2022, from https://teachmeanatomy.info/back/bones/vertebral-column/


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