Anatomy => Skeleton
Skeleton, term applied to all the rigid or semirigid structures supporting the soft tissues of an animal's body and providing leverage for muscular action. In vertebrates, the skeleton is known as the endoskeleton and is formed within the body. Some invertebrate animals, such as insects and crustaceans, have skeletons known as exoskeletons on the outside of the body.
A form of exoskeleton is the shell of calcium or silica secreted by certain protozoans known as foraminiferans. Commercial sponges have an exoskeleton consisting of spongin, which is a tough, elastic substance. Coelenterates secrete a wide variety of exoskeletal substances, ranging from the elastic covering of the jellyfish to the stony material deposited by coral. The familiar shells of most mollusks are composed of calcium carbonate and an organic ground substance known as conchiolin. Among insects, each of the three principal divisions of the body-the head, the thorax, and the abdomen-is enclosed in a framework of horny plates. The plates of each primary division are separated from those of the next division by elastic tissue that permits flexibility of motion. The appendages are enclosed by sheaths projecting from the exoskeleton; elastic tissue similar to that between the plates joins the segments of the appendages and attaches them to the body.
Vertebrates have a more or less rigid group of structures composed of cartilage or bone or of a combination of these two connective tissues. The most primitive of these structures is the notochord, which is a backbone of cartilage occurring in fishes. Animals higher on the evolutionary scale have an axial skeleton, consisting of the skull, spinal column, and ribs, and an appendicular skeleton, made up of the pelvic and pectoral girdles and the appendages.
In higher animals, the skeleton formed in the embryo is initially cartilaginous; bone and calcium are deposited as the organism matures. In humans, the process of bone hardening, or ossification, is completed at about the age of 25. The last bone to ossify is the breastbone.
The total number of bones in any animal varies with its age; many bones fuse together during the ossification process. The average number of distinct skeletal structures in a young human is 200, exclusive of the 6 ossicles found in the ears. The human skeleton is subject to a number of pathological conditions, most important of which are fracture and a deficiency disease that is known as rickets.