Muscular and Skeletal System of Human Beings

Irregular bones are irregular and complicated. Often this irregular shape is due to their many centers of ossification or because they contain bony sinuses. The bones of the spine, pelvis and some bones of the skull are irregular bones. Examples include the ethmoid and sphenoid bones. 

Chondroitin sulfate is an important structural component of cartilage and provides much of its resistance to compression. Along with glucosamine, chondroitin sulfate has become a widely used dietary supplement for treatment of osteoarthritis.

The latissimus dorsi is the larger, flat dorso-lateral muscle on the trunk. Latissimi dorsi are commonly known as lats, especially among bodybuilders. These muscles are responsible for extension, adduction and internal rotation of the shoulder joint.

The female inlet is larger and oval in shape, while the male sacral promontory projects further (i.e. the male inlet is more heart-shaped).

In a saddle joint, the opposing surfaces are reciprocally concave-convex. The best example of saddle joints are first carpo metacarpal joints (Articulationes carpometacarpeae), joints between femur and patella and the calcaneocuboid joints.

Osteopetrosis is an extremely rare inherited disorder whereby the bones harden and become denser. It can cause bones to dissolve and break.

The tibialis posterior is the most central of all the leg muscles, and is located in the posterior compartment of the leg. It is the key stabilising muscle of the lower leg.

A fibroblast is a type of cell that synthesises the extracellular matrix and collagen, the structural framework (stroma) for animal tissues and plays a critical role in wound healing. Fibroblasts are the most common cells of connective tissue in animals.

Within each column, the walls between the lacunae break down and the chondrocytes die. This converts each column into a longitudinal channel, which is immediately invaded by blood vessels and marrow from the marrow cavity. Osteoblasts line up along the walls of these channels and begin depositing concentric lamellae of matrix, while osteoclasts dissolve the temporarily calcified cartilage.

While the muscle fiber does not have a smooth endoplasmic reticulum, it contains a sarcoplasmic reticulum. The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the calcium ions needed to cause a muscle contraction.

Osteoblasts are bone cells that are responsible for the formation of new bone. The osteoblasts are much smaller; they only have one nucleus. Osteoblasts also work in groups to form new bone. Osteoblasts are important because it allows the bones to be made, remodeled and repaired.

Osteogenesis imperfecta is a congenital bone disorder characterised by brittle bones that are prone to fracture. People with OI are born with defective connective tissue, or without the ability to make it, usually because of a deficiency of Type-I collagen.

In contrast to skeletal muscle, cardiac muscle requires extracellular calcium ions for contraction to occur. Like skeletal muscle, the initiation and upshoot of the action potential in ventricular muscle cells is derived from the entry of sodium ions across the sarcolemma in a regenerative process.

An extracapsular ligament has its origin on the flexor retinaculum and is inserted on the ulnopalmar tubercle of the first metacarpal. It is taut in abduction, extension and pronation and often found elongated in connection to CMC joint arthritis. 

The sternoclavicular articulation is a synovial double-plane joint. It is composed of two portions separated by an articular disc which is made from fibrocartilage. This disk also allows motion between the sternum (manubrium) and itself during protraction and retraction of the scapula.