The skeletal system comprises the bones of the body, as well as the supporting tissues that connect them. Bones are considered living organs and are made up of minerals, cells, and protein fibers. They have several layers, including the hard outside casing referred to as compact bone, the porous, flexible inner layer of spongy bone, and the deepest layer, bone marrow, which produces red blood cells. Bones serve several purposes, including providing a framework for the body itself, protecting organs, producing red blood cells via the marrow, and storing calcium, iron, and fat.
Human babies are born with around 300 bones, but some fuse together during growth and development, so an adult human has 206 bones. The skeletal system accounts for 30%–40% of the average person’s mass.
There are five types of bones found in the body:
Long ― Long bones make up most of the limbs. Like their name suggests, they are longer than they are wide. Examples of long bones include the tibia and femur of the legs and radius and ulna of the arms.
Flat ― Flat bones are thin bones used by the body for protection or muscular attachment. Examples include the bones of the cranium and pelvis, as well as the sternum and ribs.
Short ― Short bones are wider than they are long. They provide stability but little movement. Examples include the carpal bones in the wrist and the tarsals found in the foot.
Sesamoid ― Sesamoid bones are found inside tendon or muscle. An example is the patella of the knee.
Irregular ― Irregular bones have a unique shape that doesn’t allow them to be classified in any of the other four bone types. Examples include the hyoid bone of the neck, and the vertebrae of the spine.
Tendons ― Tendons connect muscle to bone and help aid in movement.
Ligaments ― Ligaments connect bone to bone. They are responsible for keeping structures stable.
Cartilage ― Cartilage is a firm but flexible connective tissue found between bones. It is located in various locations through the body, including the nose, ears, rib cage, and bronchial tubes.
The muscular system works in tandem with the skeletal system to allow movement in the body. There are about 700 muscles in the body, grouped into three muscle types:
Cardiac muscle is an involuntary muscle located in the heart. The muscle is striated, meaning it has a striped appearance when examined under a microscope.
Smooth muscle is an involuntary, non-striated muscle type found throughout the body in areas such as the blood vessels, uterus, bladder, and eye. It is unique in that it has the ability to stretch much further than the other two muscle types.
Skeletal muscles are voluntary muscles; they can be moved at will. Skeletal muscles are striated and most are attached to bones by tendons. They are typically named because of their location, function, shape, or origin. For example, the trapezius muscle in the back is named because of its trapezium, or diamond, shape. However, the transverse abdominis is so named because of its location in the abdomen.
Muscles have a bundle-within-a-bundle structure. On the outermost level, every muscle in the body is surrounded by fascia, a layer of connective tissue. Underneath this superficial layer of fascia, there are three deeper layers:
Epimysium ― The outermost layer of deep fascia surrounds the whole muscle.
Perimysium ― This layer of deep fascia surrounds a bundle of skeletal muscle fibers called a fascicle.
Endomysium ― This layer of deep fascia surrounds the individual muscle cell.
Inside individual myocytes, there is a fluid called the (also called the muscle cell or muscle fiber) sarcoplasm, which is the cell’s cytoplasm. Within the myocyte, there are also myofibrils, which are very long chains of sarcomeres. A sarcomere is the basic function unit of muscle, and it is comprised of two important proteins, actin and myosin. The actions of these proteins in conjunction with ATP cause muscles to move.