Skeletal System

The skeletal system is the structural framework that supports an animal's body. It also provides protection for an animal's soft tissues and internal organs and serves as an attachment for the body's muscles that push against it and apply force, resulting in movement. The skeletal system of invertebrate animals (animals without a backbone) is on the outside and is called an exoskeleton. Most vertebrates (animals with a backbone) have a skeleton on the inside of the body, called an endoskeleton.

Without some type of strong, rigid frame for support, an animal's body would be a soft mass of tissue without any real shape. Only animals have skeletons, and because of them, their muscles have an anchor or something to push against. Muscles and bones are really inseparable, since they were designed to always work together to allow an animal to move. The skeletal system of vertebrates has been described mechanically as bones that act as levers that, in turn, apply a force that muscles have generated.

HYDROSKELETON

The simplest and most primitive of all skeletal systems, the hydrostatic skeleton, does not resemble any type of rigid framework. Rather, it is what a soft-bodied animal, like an earthworm or a jellyfish, uses to keep its shape. Since these and other animals, like the hydra, have no rigid parts and are composed of fluid and soft tissue, they maintain their shape and get their support from the internal pressure of fluid pushing against the outer walls of their body cavity. Like air inside a balloon, the fluid that fills the animal's body cavity pushes against its sides and keeps it "inflated." These animals often live in water and move about by contracting their bodies and squeezing water out. Thus, when a jellyfish propels water from one opening, it darts quickly in the opposite direction. Earthworms make their internal fluid move inside their many segments in a particular, coordinated way and are able to lengthen and shorten their bodies and, as a result, crawl.

EXOSKELETON

Evolution transformed the primitive hydrostatic skeleton into an exoskeleton by simply hardening its outer wall. An exoskeleton covers all or part of an animal's body like a suit of armor, and gives it support from the outside. All arthropods (such as crabs, lobsters, spiders, and insects) have exoskeletons, and their muscles are attached to its inside and pull against it to create movement. The typical arthropod body is divided into several jointed regions all composed of a hard shell of dead tissue called chitin. This shell covers every part of the arthropod, and is especially thick and hard around its vital organs. However, it is thin and flexible at the joints so that the animal can move. Although a hard outer shell provides excellent protection, it also limits the size that an animal can reach since the larger it gets, the heavier its shell becomes. It also makes growing difficult and complicated, since the exoskeleton is made up of dead tissue and itself cannot grow. Arthropods achieve growth by periodic molts or splitting and shedding of an outer case. Molting is always a time of danger for arthropods, since they are vulnerable to attack and cannot move well. Mollusks (like clams and snails) have solved this problem since the space inside their extremely hard, outer shell gets bigger as the animal grows.

ENDOSKELETON

Unlike an exoskeleton, an endoskeleton cannot be seen since it is found inside the soft flesh of an animal. It is also very different from most exoskeletons because it grows in step with the rest of the body. Since endoskeletons are lighter than those carried on the outside, they can grow much larger in size. It is no surprise therefore that the largest animals on Earth have endoskeletons. An interior skeleton may be light, but it also provides little protection. While some vital organs may be surrounded by bones (like the ribs around the heart and lungs), other organs (like those below the ribs) are totally vulnerable to injury by force.

Surprisingly, there are some animals that, despite being invertebrates, nonetheless have an endoskeleton. Sponges do not have backbones, but they do have a form of endoskeleton. Other invertebrates, such as echinoderms like sea urchins and starfish, seem to have an exoskeleton, but their spines are really only extensions of their endoskeleton that lies just below their skin. Both squid and octopi are also invertebrates with an endoskeleton. There are also animals that are considered to be vertebrates but who do not have a single, real bone in their body. This is because their supporting framework is usually made up of cartilage. Cartilage is a tough, slippery substance that is both strong and flexible. The shark, ray, and lamprey are examples of vertebrates with an endoskeleton of cartilage, and they swim in a noticeably different way than true fish do.

PARTS OF AN ENDOSKELETON

All other vertebrates have an endoskeleton made of bone. As a living substance, bone is made up of cells surrounded by layers of hard mineral salts, such as calcium phosphate. Most bones have three structural parts: the periosteum or compact bone that is its dense outer layer and gives it strength; spongy bone, which is the light, softer inside layer; and marrow that fills its inside core and makes red and white blood cells. Before vertebrates are born, their skeleton exists in a cartilage form, which "ossifies" or hardens and turns into real bone at birth. After birth, some of the bones fuse together. For example, a human baby has 270 bones at birth. However, by the time the baby reaches adulthood many of the bones will have fused together, leaving only 206 separate bones in the human skeletal system.

For a skeleton to really work, however, it cannot be all bone. Bones must be connected to one another to make it all work together, and ligaments are the connective material that links bone to bone at the joints. A ligament is made up of many bands of tissue that, if torn, can heal; but if severed, must be surgically sewn back together. Muscles are connected to bones by connective tissue called tendons. Although tendons act like a very tough cord, they cannot be stretched. If severed, they must be surgically repaired or the muscle will not work the bone (since the connection is broken). Cartilage is also a form of connective tissue and serves to fill the spaces between bones and prevents them from scraping painfully against one another. Cartilage is slippery (called "gristle" in animals) and it allows bones to slide over each other at the joints.

Because vertebrate skeletons are jointed, they allow movement. There are six types of moving joints in vertebrate skeletons: ball-and-socket joints (like the hip and shoulder joint); gliding joints (wrist and ankle); pivotal joints (allow two kinds of movement—side-to-side and up-and-down); saddle joints (thumb joint); hinge joint (elbows, fingers, and knees); ellipsoid joints or condyloid joints (movement in two axes such as the tiny bones in people's fingers, toes, and jaw). Joints move easily because of their slippery cartilage but also because they are lubricated by a special fluid called synovial fluid. There are also some body joints that do not move, such as the bones of the skull and hip. In these joints, bones come together in joints that are really more like seams.

The basic form of all vertebrate skeletons is essentially the same for many different types of animals and is made up of an axial skeleton and

an appendicular skeleton. The standard axial model (for a typical mammal) has a vertebral column made up of individual small bones (vertebrae) that provide up-and-down or lengthwise support while also being flexible. The vertebrae (singular, vertebra) also connect the skull to the rest of the body. Ribs, also part of the axial skeleton, provide support and protection for the chest area.

The appendicular skeleton is made up of the forelimbs (arms and hands), the shoulders, and the pelvic bones to which the hindlimbs are attached. Finally, ligaments keep the bones securely attached at the joints. There are naturally many variations to this model. For instance, a snake is a vertebrate but it has no limbs at all. A manatee and a whale have only forelimbs (flippers). Humans have arms and birds have wings. The long, flexible human thumb allows us to perform many precise movements, whereas the chimpanzee, our close relative, has a shorter, much less useful thumb. While the number, size, and shape of skeletal elements may vary greatly within species during their development, and sometimes even between sexes, the basic vertebrate model is always the same.