A skeleton is a supportive framework of an animal body. In vertebrates, it's an internal system composed of connective tissues like bone, which supports and protects organs. Bones form in two ways. Most bones start as cartilage models that are replaced by bone. Some, like the clavicle, develop in fibrous tissue without a cartilage stage. Besides support, skeletons protect, aid movement, and help with sensory functions. Some skeletons rely on fluid pressure for support, known as a hydrostatic skeleton. The bones of the skull protect the brain, while the rib cage protects the heart and lungs.

What are the main functions of the skeleton in the human body?

The human skeleton has three main functions: support, protection, and motion. The skeleton provides the body's framework, with the vertebral column offering primary trunk support. The skeleton protects vital organs, such as the brain, which is shielded by the cranium, and the spinal cord, which is protected by the vertebral column. The rib cage offers flexible protection for the heart and lungs, facilitating motion, expansion, and contraction. Motion is enabled through the skeletal muscles anchored to the bones, facilitating a range of movements from athletic actions to delicate artistic manipulations. Bones also store calcium, which is essential for maintaining constant calcium levels for neuromuscular function, blood clotting, muscle contractility, and cardiac function. Bone marrow within the skeleton produces red blood cells, white blood cells, and platelets.

What are the main parts of the human skeleton?

The human skeleton has two main divisions: the axial and the appendicular. The axial skeleton supports and protects the central nervous system and internal organs. It includes the skull , vertebral column, and rib cage. The appendicular skeleton facilitates movement and includes the pelvic and pectoral girdles, as well as the bones and cartilages of the limbs. The skeleton’s functions are support, protection, and motion. Bones are categorized based on their shapes and configurations, such as flat plates or hollow tubes. The human skeleton consists of 206 bones that vary in size and shape.

How do bones and joints work together in the skeleton?

Bones and joints are critical to the skeletal system, providing both structure and movement. Bones offer structural support, protect organs, and act as a mineral reservoir. Joints, the junctions between bones, allow the skeleton to move. There are two main categories of joints: synovial and nonsynovial. Synovial joints have a cleft between the bones that permits sliding during movement, while nonsynovial joints connect bones with nonosseous material that allows bending or twisting. A ball-and-socket joint , like the hip and shoulder, allows the most freedom of movement. Movement at joints is typically produced by voluntary muscular action.

What is the difference between the axial and appendicular skeleton?

The human skeleton has two main divisions: the axial skeleton and the appendicular skeleton. The axial skeleton includes the bones of the skull, vertebral column, and rib cage. It supports and protects the central nervous system, heart, and lungs. The appendicular skeleton includes the bones of the limbs, shoulder, and pelvic girdles. It enables movement and provides attachment points for muscles.

How does the human skeleton change as we grow and age?

The human skeleton undergoes significant changes throughout life. Bone formation begins in the embryo with cartilage or fibrous membranes gradually replaced by bone. Bones grow in length at the epiphyseal plate until skeletal maturity, typically by late adolescence, when the plate disappears. During puberty, a rapid increase in body size occurs, with boys experiencing greater muscle growth and strength gains. As we age, bones gradually lose calcium, becoming more fragile and prone to fractures. The incidence of osteoporosis increases, particularly in postmenopausal women, leading to decreased bone density and increased fracture risk. Maintaining good nutrition, adequate calcium and vitamin D intake, and regular weight-bearing exercise can help prevent bone loss and maintain skeletal health as we age.

skeleton

References & Edit History Related Topics

Images & Videos

Diagram comparing the forelimb bone structures of a human, horse, dolphin, box turtle, fruit bat, and chicken, with color-coded bones showing homologous parts to illustrate evolutionary relationships.

This diagram traces the evolution of the horse from the small, multi-toed Hyracotherium (dawn horse) of the Eocene epoch to the modern single-toed Equus, showing changes in size, limb structure, and forefoot bones, with illustrations of key ancestral species to scale.

Diagram of a dog's skeleton in side view, showing labeled bones including the skull, spine, ribs, pelvis, scapula, humerus, radius, ulna, femur, tibia, fibula, and various vertebrae.

The top half of the diagram shows a labeled illustration of a horse skeleton, identifying bones such as the skull, vertebrae, pelvis, ribs, and limbs. The bottom half displays the external anatomy of a horse (Equus caballus), with labels for key body points including muzzle, withers, shoulder, back, flank, hock, and hoof.

Side-by-side anatomical illustration comparing the right leg of a modern human and a gorilla, showing labeled skeletal structures and major muscles including pelvis, femur, tibia, fibula, and key thigh muscles.

Side-by-side skeletal diagrams compare a modern human standing upright and a gorilla walking on all fours, with labeled bones and close-up views of the female pelvis for each species.

Illustration of a full human skeleton with all major bones shown in beige and cartilage highlighted in blue, overlaid on a faint outline of a human body.

At a Glance

skeleton summary

skeleton

anatomy

Written by

William James HamiltonAll

Fact-checked by

Britannica Editors

Last updated

Oct. 30, 2026 •History

News •

75,000-Year-Old Neanderthal Skeleton Reveals a Mysterious Spear Wound Aimed at the Heart • Oct. 20, 2026, 3:00 AM ET (SciTechDaily)

skeleton, the supportive framework of an animal body. The skeleton of invertebrates, which may be either external or internal, is composed of a variety of hard nonbony substances. The more complex skeletal system of vertebrates is internal and is composed of several different types of tissues that are known collectively as connective tissues. This designation includes bone and the various fibrous substances that form the joints, connect bone to bone and bone to muscle, enclose muscle bundles, and attach the internal organs to the supporting structure. For a more detailed discussion of the human skeleton, see skeletal system, human.

Comparative study of skeletal systems

In addition to its supportive function, the animal skeleton may provide protection, facilitate movement, and aid in certain sensory functions. Support of the body is achieved in many protozoans by a simple stiff, translucent, nonliving envelope called a pellicle. In nonmoving (sessile) coelenterates, such as coral, whose colonies attain great size, it is achieved by dead structures, both internal and external, which form supporting axes. In the many groups of animals that can move, it is achieved either by external structures known as exoskeletons or by internal structures known as endoskeletons. Many animals remain erect or in their normal resting positions by means of a hydrostatic skeleton—i.e., fluid pressure in a confined space.

The skeleton’s protective function alone may be provided by structures situated on the body surface—e.g., the lateral sclerites of centipedes and the shell (carapace) of crabs. These structures carry no muscle and form part of a protective surface armour. The scales of fish, the projecting spines of echinoderms (e.g., sea urchins), the minute needlelike structures (spicules) of sponges, and the tubes of hydroids, all raised from the body surface, are similary protective. The bones of the vertebrate skull protect the brain. In the more advanced vertebrates and invertebrates, many skeletal structures provide a rigid base for the insertion of muscles as well as providing protection.

The skeleton facilitates movement in a variety of ways, depending on the nature of the animal. The bones of vertebrates and the exoskeletal and endoskeletal units of the cuticle of arthropods (e.g., insects, spiders, crabs) support opposing sets of muscles (i.e., extensors and flexors). In other animal groups the hydrostatic skeleton provides such support.

In a limited number of animals, the hard skeleton transmits vibrations that are sensed by the hearing mechanism. In some forms—e.g., bony fishes and fast-swimming squids—it aids in the formation of buoyancy mechanisms that enable the animal to adjust its specific gravity for traveling at different depths in the sea.

Male muscle, man flexing arm, bicep curl.

Britannica Quiz

Facts You Should Know: The Human Body Quiz

Principal types of skeletal elements

Certain types of skeletons usually characterize particular animal phyla, but there are a limited number of ways in which an animal can form its skeleton. Similar modes of skeleton formation have evolved independently in different groups to fulfill similar needs. The cartilaginous braincase of the octopus and the squid, which are invertebrates, has a microscopic structure similar to the cartilage of vertebrates. The calcareous (i.e., calcium-containing) internal skeleton of the echinoderms is simply constructed but is essentially not far different from the much more elaborate bones of vertebrates. Skeletal fibres of similar chemical composition occur in unrelated animal groups; for example, coiled shells of roughly similar chemical composition are present in gastropods (e.g., snails), brachiopods (e.g., lamp shells), and cephalopods (e.g., chambered nautilus). The mechanical properties of different skeletal types vary considerably according to the needs of animals of particular size ranges or habits (e.g., aquatic, terrestrial).

Skeletal elements are of six principal types: hard structures, semirigid structures, connective tissue, hydrostatic structures, elastic structures, and buoyancy devices.

Explore Britannica Premium!

The trusted destination for professionals, college students, and lifelong learners.

Cuticular structures

Hard structures may be either internal or external. They may be composed of bone (calcareous or membranous structures that are rigid), crystals, cuticle, or ossicles (i.e., minute plates, rods, or spicules).

The scales of some fishes (e.g., sturgeon) may be heavy, forming a complete external jointed armour; calcareous deposits make them stiff. They grow at their margins, and their outer surfaces become exposed by disintegration of the covering cell layer, epithelium. Other fish scales—i.e., those of most modern bony fishes—are thin, membranous, and flexible.

Calcareous structures

The external shells of gastropods and bivalve mollusks (e.g., clams, scallops) are calcareous, stiff, and almost detached from the body. The laminated, or layered, shell grows by marginal and surface additions on the inner side. Muscles are inserted on part of the shell, and the body of the animal can be withdrawn into the protection of the shell. Chambered calcareous shells formed by cephalopods and by protozoans of the order Foraminifera become so large and so numerous that the broken remains of the shells may constitute a type of sand covering large areas of tropical beaches; the pieces may also consolidate into rock. Protozoans of the order Radiolaria form skeletons of silica in the form of very complicated bars. The body of the animal flows partly inside and partly outside among the bars.

Coral skeletons are also partly inside and partly outside the animal. Calcareous depositions below a young coral polyp (i.e., an individual member of the animal colony) are secreted by the ectoderm (generally, the outermost of three basic tissue layers), fixed to the surface to which the animal is attached, and thrown up into ridges, which form a cup into which the polyp can contract. A spreading of the base and the formation of more polyps on the base are followed by a central humping up of the soft tissue and further secretion of skeleton. An upright branch is thus formed, and, in time, large branching corals many feet high may arise from the seafloor. Most of the soft tissue is then external to an axial calcareous skeleton, but in rapidly growing corals the skeleton is perforate, and soft tissue lies both inside and outside it. Protection of the animal is provided by the skeletal cups into which each polyp can contract, but usually neither the whole colony nor a single animal has mobility.

The starfishes, brittlestars, and crinoids (Echinodermata) have many types of calcareous ossicles in the mesoderm (generally, the tissue layer between the gut and the outermost layer). These form units that articulate with each other along the arms, spines that project from the body covering and articulate with ossicles, and calcareous jaws (in sea urchins). Less well organized calcareous deposits stiffen the body wall between the arms of the starfish.

skeleton

News •

Comparative study of skeletal systems

Thank you for signing up!

Principal types of skeletal elements

Cuticular structures

Calcareous structures