The human skeleton is comprised of the total set of bones that provide the human body a multifunctional structure. The adult human skeletal system contains 206 bones! It also consists of the **joints**, **cartilage**, **tendons**, and ligaments that connect them.
The main functions of the skeletal system are:
Support of the body
Provide protection for internal organs
Act as a site for the production of blood cells and the storage of minerals, particularly calcium
Growth and development
The human skeletal system is divided into two principal subdivisions: the **axial** and **appendicular** skeleton.
The **axial skeleton** forms the central axis of the human body and is composed of 80 bones that include the bones of the skull, the ossicles of the middle ear, the hyoid bone of the throat, the vertebral column, and the thoracic cage. It also provides protection and support for the brain, spinal cord and vital organs, in addition to providing a surface for muscles to attach to.
The **appendicular skeleton** consists of 126 bones that include both the upper and lower limbs and bones of the shoulders and pelvis.
## **Components of the Skeletal System** ##
The skeletal system is composed of four main fibrous and mineralized connective tissues : bones, ligaments, tendons, and joints.
**Bone**: A rigid form of connective tissue that is part of the skeletal system of vertebrates and is composed principally of calcium.
**Ligament**: A small band of dense, white, fibrous elastic tissue. Ligaments connect the ends of bones together in order to form a joint. They also assist in holding organs in place.
**Tendon**: A tough, flexible and inelastic band of fibrous connective tissue that connects muscles to bones.
**Joint**: Joints hold the skeleton and support movement. They can be grouped together by function and structure, such as ball-and-socket, hinge, and pivot joints.
## **Types of Bones** ##
There are five types of bones in the human skeletal system: long, short, flat, irregular, and sesamoid.
**Long Bone**: Helps to facilitate movement and support the weight of the body. Long bones are characterized by a long tubular shaft and an articular surface at each end of the bone where ligaments and tendons attach. These bones include the major bones of the arms and legs such as the humerus and femur, tibia and fibula, and the radius and ulna.
**Short Bone**: Helps to provide stability and movement within the ankle and wrist joints. They provide little to no movement. Short bones are roughly cube shaped and are as long as they are wide. Examples of this type of bone include the carpals and metacarpals in the wrists and ankles.
**Flat Bone**: The primary purpose of this type of bone is to protect internal organs such as the brain, heart and lungs. It also provides a large surface area for muscles to attach to. Examples of this type of bone include the cranium (skull), the thoracic cage (sternum and ribs) and the ilium (pelvis).
**Irregular Bone**: These types of bones vary in size and structure with the shape usually being very complex. Irregular bones serve different functions depending on location. For example, vertebrae protect the spinal cord and together make up the spinal column. Another example would be the hyoid bone which helps to maintain tracheal and pharyngeal support.
**Sesamoid Bone**: The function of this bone is to protect tendons and diminish friction and wear on joint surfaces. This type of bone is usually small and round and is found in the hands, feet, and knees. A common example of a sesamoid bone is the patella (kneecap).
When you consider the relationship between the axial and appendicular skeleton and the soft tissues of the body, it becomes clear that the primary functions of the skeleton are support, protection, and motion. Of these functions, support is the oldest and most primitive; similarly, the axial skeleton was the first to evolve.
A distinguishing characteristic of the human skeleton in comparison to other mammals is erect posture. Our bodies are, to some extent, similar to a tall building with legs. Enormous advantages have been gained from this type of posture with the most prevalent being the freeing of our arms and hands for other functions.
## **Different Types of Joints** ##
### **By Function** ###
**Synarthrosis:** These immovable and strong joints include skull sutures, the articulations between the teeth and the mandible, and the joint found between the first pair of ribs and the sternum.
**Amphiarthrosis:** These joints allow slight movement and include the distal joint between the tibia and the fibula and the pubic symphysis of the pelvic girdle.
**Diarthrosis:** These joints allow full movement and include many bone articulations in the upper and lower limbs. Examples of these include the elbow, shoulder, and ankle.
### **By Structure** - see illustrations below as well ###
**Fibrous:** Thick connective tissues composed of collagen fibers found between the articulations of fibrous joints.
**Cartilaginous:** Joints that join bones with cartilage.
**Synovial:** These are characterized by the presence of an articular capsule between the two joined bones. Bone surfaces at synovial joints are protected by a coating of articular cartilage.
## **Some Examples of Joints** ##
## **Watch** ##
Watch some of the amazing things our skeletal system allows humans to do!
- [Contact juggling](https://www.youtube.com/watch?v=0rqeR-iLKYA)
## **References** ##
- Encyclopaedia Britannica
- [BC Campus Open Textbooks: Welcome to Anatomy and Physiology](https://opentextbc.ca/anatomyandphysiology/chapter/9-1-classification-of-joints/)