skeleton The human skeleton has been somewhat nebulously defined as ‘a man with his insides out and his outsides off’. It is probably the most durable reminder of man's mortal existence and has intrigued, challenged, and stimulated morbid, mystical, and scientific minds from at least biblical times and probably before. To appreciate the public interest in this subject, one need only observe the media frenzy that ensues when a human skeleton is discovered. Paradoxically, the level of media interest seems to be almost inversely proportional to the academic value of a find. Remains that date from fossil times can have a monumental influence on our understanding of the evolution of our species but often attract less attention than more recent remains, especially if there is some hint of criminal involvement. It is the inseparable and inextricable nature of the bond between the skeleton and death which ensures that human bones are often perceived in a supernatural light that passes beyond common sense. Hopefully any sane anatomist will tell you that the bones in the dissecting room never rattle ominously and any sober gravedigger will assure you that they do not rise from their coffins and dance around the tombstones at midnight.

There are essentially five basic functions attributed to the skeleton. Each is arguably as important as the other, but given the evolutionary evidence for bone development, the primary function is probably to provide a stable framework that gives support and structure to the soft tissues. Various clinical conditions such as osteomalacia, osteoporosis, and osteogenesis imperfecta bear witness to the inadequacies of poorly formed bone in fulfilling the role of support to the human body. The skeleton also plays a protective role and this is most clearly seen in the region of the skull, which not only forms a box around the delicate tissues of the brain, but also serves to protect the special senses of sight, smell, and hearing. It is said that the thorax protects the heart and lungs, but this theory has little merit when one considers that equally delicate structures in the abdomen are not guarded in this way. It is more likely that the bones of the thorax are involved in the third function of the skeleton, which is to provide a rigid framework for the attachment of muscles and, in the case of the thorax, thereby facilitate breathing. For efficient movement to occur, each muscle must originate on the surface of one bone, pass across a joint and insert onto the surface of another bone. In this way one can accurately predict the movement produced by the contraction of each individual muscle or muscle group. The fourth function of the skeleton is to house sites of haemopoetic (blood-making) activity within the red marrow that occupies the cancellous spaces of many bones. In bone marrow transplantation, the blood-forming cells are aspirated from sites rich in red marrow, such as the iliac blade of the pelvic bone and the sternum. The final function of the skeleton is to provide a reservoir of minerals (calcium, phosphates, potassium, and many other trace elements), which the body can call upon to replenish depleted levels.

There is a myth that bone is an inactive, dry, and dusty material. This is reflected in the origin of the term ‘skeleton’, which is derived from the Greek word skeletos meaning ‘dried up’. However quite the opposite is true in life, as bone is unquestionably a dynamic tissue that will bleed if it is cut, hurt if it is damaged, and mend itself if it is broken. Furthermore, it will be resorbed if it is not needed and conversely will develop where it is required.

The official statement, although a virtually meaningless concept, concludes that there are 206 individual bones in the adult skeleton. However, when one considers that over forty inconstant accessory bones have been described in the foot alone, it is clear that, whilst of some value in a trivia quiz, the statement is essentially meaningless. Bones are classified according to either their location within the body or their shape. The latter should be avoided where possible, as the wide variety of bone shapes almost seems to defy useful classification. Whilst the skeleton is bilaterally symmetrical, those structures that lie on the midline do not have a corresponding partner and therefore form the axis, and hence the ‘axial’ skeleton (see Figure). This comprises the skull, the vertebral column (24 presacral, cervical, thoracic, and lumbar vertebrae; the sacrum; and the coccyx), and the sternum. The limbs and their attachments to the axial skeleton (girdles) belong to the ‘appendicular’ skeleton and are all paired. The pectoral girdle (scapula and clavicle) attaches the upper limb to the axial skeleton whilst the pelvic girdle (innominate bone) attaches the lower limb to the axial elements. In addition, the rib cage attaches the sternum in front to the vertebral column behind.

Each bone displays an intimate correlation between form and function. This relationship is fundamentally governed by a variety of factors including genetics, mechanics, and metabolism. It is clear however that the human skeleton is unlike that of any other animal and this uniqueness is exploited in the science of osteology, where recognition of ‘human’ plays a vital role. The human skeleton is different for many reasons, including the fact that we are the only habitual biped with upper limbs that are solely dedicated to manipulation and not involved in locomotion. Relatively speaking, we also have the largest brain and give birth to babies with relatively large heads. All of these factors, plus many others, lead to levels of specialization in our skeleton that allow anthropologists (both archaeological and forensic) to persuade our bones to give up many secrets regarding our identity and way of life. One of the first steps in the analysis of human skeletal remains is to establish whether or not they are human, since a murder investigation initiated on the misidentification of some sheep bones is unlikely to be successful. The second question is often an attempt to establish how long the person has been deceased. If more than 70 years have elapsed since death then the remains are classified as archaeological, but if they are more recent then it is a forensic problem. Biological identity is one of the first things to be established and this includes sex, age at death, stature, and race. Beyond that, information regarding individual identity may be established through recognition of personal idiosyncrasies (previous fractures, dental treatment, previous diseases, congenital anomalies, etc.), all of which might lead to a positive identification of the deceased in a forensic situation. Given an intact skeleton, sex can be determined with up to 95% accuracy, and whilst this is relies heavily on differences in the pelvis, every bone displays some degree of sexual dimorphism. The determination of age at death is accurate if the individual was younger than 25 years of age but becomes more difficult with advancing age as there are degenerative changes which occur at different rates in different individuals. Stature is relatively easy to determine as it involves measuring the lengths of the limb bones and inserting the values into previously computed regression equations. The ethnic affinity of skeletal remains is very difficult to assess and normally requires the skull to be intact and to show characteristic racial traits. In cases of trauma-related deaths, evidence of the cause of death may remain on the skeleton, such as bullet entry and exit wounds, fractures caused by implements such as hammers or crow bars and also, in cases of stabbing, blades may penetrate and leave marks on the underlying bone. It is probably true that the most important evidence left behind at the scene of a homicide is the body, and this holds true even if it is not discovered for a very long time and only the skeleton remains.

Sue M. Black

Bibliography

Brothwell, D. R. (1981). Digging up bones. The excavation, treatment and study of human skeletal remains. Oxford University Press, Oxford.
Reichs, K. J. (1998). Forensic osteology: advances in the identification of human remains, (2nd edn). CC Thomas, Illinois.

Skeletons

Skeletons provide the framework for the bodies of most multicellular animals. They lend structural support to soft tissues and give muscles something to attach to and pull against. Without skeletons, most animal bodies would resemble a limp bag of gelatin.

Skeletons come in a number of forms, each suited for a particular set of lifestyles and environments. Skeletons can be rigid, semirigid, or soft. They can also be external or internal. Vertebrates have internal skeletons, called bony skeletons, which consist mainly of calcified bone tissue . Most invertebrates, such as insects, spiders, and crustaceans, have outer skeletons called exoskeletons. Some aquatic animals, such as octopuses, sea anemones, and tunicates, and a number of small, land-dwelling invertebrates such as earthworms and velvetworms, have soft supporting structures called hydrostatic skeletons .

Bony Skeleton

Vertebrata (vertebrates) is an animal group that includes fishes, amphibians, reptiles, and mammals. The vertebrate skeleton is an internal collection of relatively rigid structures joined by more flexible regions. The hard components of the skeleton are made up of bone, cartilage , or a combination of these two connective tissues .

Vertebrates are closely related to a number of less-familiar aquatic organisms, such as tunicates, sea squirts, and lancelets (Amphioxus). These animals have a skeleton composed entirely of a cartilaginous rod called a notochord. The notochord is somewhat flexible and runs along the back of the animal.

In all vertebrates, the framework first laid down during development is cartilaginous. As development proceeds, most of the cartilage is replaced by calcified bone through the action of bone precursor cells called osteoblasts . This process is called ossification . During ossification, some bones fuse together, reducing the total number of bony elements. At birth, human infants have over 300 bones. As adults, they have 206.

The bony skeleton of vertebrates consists of an axial skeleton and an appendicular skeleton . The axial skeleton is made up of the skull, spinal column, and ribs. This skeleton provides the general framework from which the appendicular skeleton hangs. The appendicular skeleton consists of the pelvic girdle, pectoral girdle, and the appendages (arms and legs).

Bony skeletons have a number of advantages over other types of skeletons. Because bony skeletons are living tissue, they can grow along with the rest of the body as an individual ages. As a result, animals with bony skeletons do not replace their skeletons as they grow older. Bone itself is a dynamic tissue that adjusts to the demands imposed by its environment and by its owner. Bone not only repairs itself when broken, but thickens in response to external stresses.

Bony skeletons are denser and stronger than exoskeletons and hydrostatic skeletons, and are able to support animals of a large size. By assuming a more upright posture, large animals can support a tremendous amount of weight on their skeletons. Internal skeletons are also less cumbersome in large animals than external skeletons would be. As an animal increases in size, its surface expands to an area that would be too large to be reasonably accommodated by an exoskeleton. All large, land-dwelling animals have bony skeletons.

Bony skeletons can respond to increasing weight-bearing demands by adjusting bone density and by distributing the weight through changes in posture. However, the bones of some animals have actually become lighter to accomodate other functions. Bird bones, which are hollow structures, constitute a mere 4 percent of the animal's body weight, compared to 6 percent in the mammals.

A major disadvantage to the bony skeleton, relative to an exoskeleton, stems from its internal location. Although certain elements of the bony skeleton, such as the skull and rib cage, provide protection to the soft organs they encase, the skeleton offers no protection to the other soft tissues of the body. External protection is therefore left to other structures, such as the skin and its associated hair, fur, and nails.

Exoskeleton

Exoskeletons are found in most invertebrates and assume a variety of forms. Some exoskeletons are made of calcium or silica, as seen in protozoa called foraminiferans. Exoskeletons can also be elastic, such as those worn by sponges, or hard and stony, like those secreted by coral. In contrast, mollusks (clams and snails) house themselves in hard shells comprised mainly of calcium carbonate.

When compared to bony skeletons, exoskeletons have two advantages. First, they provide a hard, protective layer against the environment and potential predators. And second, they protect their wearer against drying out, which is a great threat to land-dwelling species. It is important to avoid dessication because water molecules play an important role in many of life's critical physiological processes, including those related to digestion and circulation.

Insect, spider, and shellfish (lobster and shrimp) exoskeletons contain a compound called chitin , a white horny substance. These arthropods have segmented exoskeletons that bend only at the joints. The exoskeleton covers the entire surface of an arthropod's body, including the eyes. The thickness of the exoskeleton varies depending on the nature and function of the body part it covers. For example, the exoskeleton is thinner at the joints, which require a degree of flexibility in order to bend. The chemical composition of the exoskeleton also differs depending on location and function.

Insects have three principle body segments—the head, thorax, and abodomen—and six segmented legs. Each segment is curved and hinged to its neighbor. Spiders have a thorax and abdomen and eight legs. Their head is fused to their thorax. Other arthropods, including scorpions, centipedes, and shrimp, may have more body segments than an insect and more legs than a spider.

Exoskeletons have two major disadvantages when compared to bony skeletons. Because they are composed of a blend of rigid, inorganic substances, they cannot expand as their owner grows. Arthropods must therefore shed their exoskeletons periodically through a process called molting. A newly molted animal is vulnerable to attack by predators before its new shell hardens. The shell hardens through a process similar to the tanning of leather.

The second disadvantage of exoskeletons is that physical contraints limit the size attainable by animals that have them. As the animal gets larger, the size of the exoskeleton required to cover its surface area would render the covering heavy and cumbersome.

Hydrostatic Skeletons

Hydrostatic skeletons are found mostly in aquatic organisms such as octopusses, jellyfishes, sea anemones, and tunicates. Although earthworms and velvetworms have hydrostatic skeletons as well. Bony skeletons and exoskeletons are made of relatively rigid substances, but hydrostatic skeletons contain no hard parts at all.

These soft, supporting structures have two components, a fluid-filled body cavity and a muscular body wall. Animals with hydrostatic skeletons move using the combined actions of these two features. They use the muscles of the body wall to squeeze fluid into other regions of the body cavity, allowing them to change shape. These shape changes allow the animal to extend parts of its body in the direction it wants to move and withdraw other parts from areas it is leaving.

One benefit that hydrostatic skeletons give to some soft-bodied organisms is an ability to take in important materials such as oxygen, water, and waste products through the skin. This eliminates the need for a separate transport system. It is beneficial for these animals not to have a separate transport system because transfer of these materials is a passive process, which means that energy does not need to be expended to take in oxygen, rid the body of waste products, and maintain the balance of water between the body and the environment. In addition, these skeletons are relatively light compared to bony skeletons and exoskeletons. This is beneficial because not as much muscle mass is required to move it. Hydrostatic skeletons work well in aquatic environments, but they would not be useful on land. They give little protection against drying out, and larger animals would be too flimsy to stand up on their own.

see also Bones; Cartilage; Chitin; Keratin.

Judy P. Sheen

Skeletons

Everyone is familiar with the human skeleton and its role in supporting the body. Less familiar is the variety of skeletons in other animals and the additional functions they provide. Zoologists generally recognize three types of skeletons: a hydroskeleton, an exoskeleton, and an endoskeleton.

A hydroskeleton, also called hydrostatic skeleton, occurs in many soft-bodied animals, such as earthworms. A hydroskeleton is not bony, but rather is a cavity filled by pressurized fluid. Like air in a truck's tires, the pressurized fluid keeps the body from collapsing from the forces of gravity or movement. By manipulating the pressure in different parts of the cavity, many soft-bodied animals can change shape and produce considerable force. Earthworms (annelids), for example, can burrow through soil using pressure in the hydroskeleton.

An exoskeleton is a hard, nonliving structure that encloses the rest of the body. The exoskeleton may consist of a single hard piece, like the shell of a snail, or it may have two or more hard pieces linked together by flexible tissue, as in a clam. In crustaceans, insects, spiders and other arthropods (Arthropoda), and also in some other groups of animals, the exoskeleton is called a cuticle. Animals with exoskeletons of two or more pieces can generally move the parts by means of muscles that attach to the inner surface. Exoskeletons have the advantage of providing protection from predators. (Consider the work it takes to eat a lobster.) One disadvantage, however, is that it restricts the growth of the animal inside it. Snails and many other mollusks solve that problem by continually enlarging their shells as they grow. An arthropod sheds (molts) the old cuticle as it grows, then it secretes a new and larger one.

Endoskeletons are enclosed in other tissues. The human endoskeleton does not offer much protection from predators, but it does a good job of keeping the body from collapsing into a helpless pile. It also provides sites for attachment of muscles. Most muscles connect two different bones, and almost all movements result when muscle contraction moves some bones relative to others.

The skeletons of humans and other vertebrates consist of differing proportions of cartilage and bone. Cartilage, being flexible yet resilient, is well suited to cushioning joints and changing size and shape easily. Cartilage serves as a temporary skeleton in the embryos of vertebrates. In sharks and a few other vertebrates, cartilage persists as the skeleton throughout life. In humans and most other vertebrates, most cartilage is gradually replaced by bone, but some remains as cushions for joints and flexible supports in the nose, ears, and trachea. Bone is, of course, harder and more rigid than cartilage, but it is still living tissue that can slowly adapt to strains imposed upon it.

The 206 bones of the adult human skeleton occur in several distinctive parts of the skeleton. The skull, vertebrae, and ribs belong to the axial skeleton. The bones of the arms and legs and the pectoral and pelvic girdles are parts of the appendicular skeleton, which attaches to the axial skeleton.

see also Annelid; Arthropod; Bone; Connective Tissue; Insect; Musculoskeletal System

C. Leon Harris

Bibliography

Hickman, Cleveland P., Jr., Larry S. Roberts, and Allan Larson. Integrated Principles of Zoology, 11th ed. Boston: McGraw-Hill Higher Education, 2001.

Saladin, Kenneth S. Anatomy and Physiology, 2nd ed. Dubuque, IA: McGraw-Hill Higher Education, 2001.

skel·e·ton / ˈskelitn/ • n. an internal or external framework of bone, cartilage, or other rigid material supporting or containing the body of an animal or plant. ∎  used in exaggerated reference to a very thin or emaciated person or animal: she was no more than a skeleton at the end. ∎  the remaining part of something after its life or usefulness is gone: the chapel was stripped to a skeleton of its former self. ∎  the supporting framework, basic structure, or essential part of something: the concrete skeleton of an unfinished building the skeleton of a report. ∎  [as adj.] denoting the essential or minimum number of people, things, or parts necessary for something: there was only a skeleton staff on duty.PHRASES: skeleton in the closet a discreditable or embarrassing fact that someone wishes to keep secret.DERIVATIVES: skel·e·ton·ize / -ˌīz/ v.

skeleton The structure in an animal that provides mechanical support for the body, protection for internal organs, and a framework for anchoring the muscles. The skeleton may be external (see exoskeleton) or internal (see endoskeleton). Both types require joints to allow locomotion. The skeleton of higher vertebrates consists of a system of bones (see appendicular skeleton; axial skeleton). Soft-bodied animals have a hydrostatic skeleton.