Skeletal System is comprised of 206 individual bones in an adult human. The bones of Skeletal System are divided into two type i.e.

• Axial Skeleton

• Appendicular Skeleton

Axial skeleton of the body contains 80 bones in the following regions.

• Skull: Except for Mandible (lower jaw) it has 22 bones fused together.

• Hyoid: It is the only bone not forming a joint with any other bone. It lies inferior to the Mandible.

• Auditory Ossicles: Malleus, Incus and Stapes (bones of middle ear) are altogether known as Auditory Ossicles. These are smallest bones found inside the cavity of Temporal Bone

• Ribs: There 12 pair of ribs together with sternum forming Ribcage.

• Sternum: It is a  thin knife shape bone situated along anterior side of Thoracic Region.

• Vertebral Column: It is comprised of 26 vertebrae and are named by regions Cervical (7 bones), Thoracic (12 bones), Lumbar (5 bones), Sacrum (5 bones) and Coccyx (4 bones).

Appendicular skeleton is comprised of 126 bones in the following regions.

• Upper limbs contains bones of the upper arm.

• Lower Limbs contains bones like Tibia and Fibula of Lower Leg and bones of feet.

• Pelvic Girdle is formed of right and left Hip Bones

• Pectoral Girdle consists of Left and Right Clavicles and Left and Right Scapulae.

Occupying the 30% to 40% of body mass is comprised of non living bone matrix and small bone cells. These bones cells allow following functions.

• Growth and development

• Repairing an injury

• Broken down for the release of stored minerals

TYPES:

All the bones of Skeletal System are of the following type.

• Long Bones are wide and major bones of limbs growing more than any other bone throughout childhood and forming the bulk of height. It has a hollow Medullary Cavity (innermost part of bone shaft) for the storage of Bone Marrow. These bones include Femur, Tibia, Fibula and Metatarsals etc.

• Short Bones are as long as wide having the cube or round shape. For example Wrist Carpal Bones and Foot Tarsal Bones.

• Flat Bones vary in size and shape but all of them has one thin direction and don’t have any Medullary Cavity. For example Occipital Bones of Cranium, Rib bones and Hip bones.

• Irregular Bones are irregular as they are not flat, long or short. For example Vertebrae, Coccyx, and Zygomatic Bones of Skull.

• Sesamoid Bones are formed inside the Tendons running across the joints after birth. They grow to protect the tendons from stress and strain. Patella (Knee cap) and Pisiform bones are the only examples of Sesamoid Bones.

LAYERS:

The bones of Skeletal System are composed of several distinct layered regions.

The outside of the bone is covered by a dense layer of connective tissues called Periosteum. Blood vessels in Periosteum provide blood to cells on the surface of bone. Periosteum goes inside the bone to nourish the cells inside. Periosteum is innervated by Nervous Tissues and Nerve endings enabling it to feel pain on injury.

Deep to Periosteum is Compact Bone layer making up the hard and mineralized part of the bone. Matrix of hard mineral salts is further strengthened by strong Collagen Fibers. Strength of the layer is maintained by the presence of tiny cells known as Osteocytes (star shaped cell in mature bone).

Deep to Compact Bone layer is the Spongy Bone layer. This layer of bone tissue produces thin columns named as Trabeculae having spaces for Red Bone Marrow. They are formed by least amount of mass but still resist outside stresses. Long bones have this Spongy Layer on the end with a hollow Medullary Cavity in middle of Diaphysis (main or midsection of long bone). It contains yellow bone marrow after puberty.

ARTICULATION:

Point of contact between bones, bones and cartilages or between bones and Teeth is called Articulation (Joint). There are three types of articulations i.e.

• Synovial Joint is a common joint with a small gap between bones that allows free movement of joints. This space is filled with Synovial Fluid (viscous Non Newtonian fluid in cavities of Synovial Joints) that helps lubricate the joint.

• Fibrous Joints are present in between tightly joined bones allowing little or no movement. They hold the Teeth in their sockets.

• Cartilaginous Joint are formed where bone come into contact with cartilage or a layer of cartilage between two bones. Gel like consistency of cartilage these joints allow small amount of flexibility of joints.

FUNCTION:

The functions for Skeletal System are as follows.

• Primary function of the Skeletal System is the formation of a solid framework for supporting and protecting the body organs and holding the Skeletal Muscles. Axial Skeleton bones act as a hard cover for protecting internal organs like Brain and Heart from external injuries. Appendicular Skeleton bones gives support and flexion at joints while holding the muscles moving the limbs.

• One of the functions of Skeletal System is to provide attachment points for Skeletal Muscles.

• Bones of Skeletal System has Medullary Cavity containing Red Bone Marrow for production of red blood cells and white blood cells in a process called Hematopoiesis.

• It also contains many important substances for helping in growth and repair of body like Calcium, Osteocalcin (hormone regulating blood sugar and fat), and energy present in form of Lipids in Yellow Bone Marrow and Red bone marrow containing Iron to form Hemoglobin (iron containing oxygen transport metalloprotein) in red blood cells.

CLINICAL SIGNIFICANCE:

Skeletal System suffers many disorders but one of the most common is Osteoporosis. This disease weakens the bone by reducing bone mineral density which leads to risk of Fractures. Another common disorder of Skeletal System is Scoliosis which causes side to side curve in spine. The condition is common in females.