skeleton(redirected from visceral skeleton)
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skeleton, in anatomy
skeleton, in anatomy, the stiff supportive framework of the body. The two basic types of skeleton found among animals are the exoskeleton and the endoskeleton. The shell of the clam is an exoskeleton composed primarily of calcium carbonate. It provides formidable protection, but it is bulky and severely restrictive of movement. The smallest exoskeletons are found on microscopic animals such as diatoms and certain protozoans. Coral reefs are made up of the accumulated exoskeletons of the coral polyp. The firm, flexible, chitinous (horny) insect skeleton is a combination of protective armor and a framework for attachment of the muscles used in rapid movement. The disadvantage of an exoskeleton is that it is nonliving, and must be shed periodically to allow for growth—a process limiting the maximum size of the organism.
The endoskeleton, a framework of living material enclosed within the body, permits larger size coupled with freedom of movement and is characteristic of vertebrate animals. In certain fish, it is made up entirely of cartilage, but in most vertebrates it is a mixture of bone and cartilage. The general arrangement of skeletal parts into skull, spinal column, ribs, and appendages is the same in all vertebrates. In addition to its supportive function, the skeleton provides sites for the attachment of the muscles used in movement and shields vital organs such as the brain and lungs. The skeleton of birds is especially adapted for flight; the bones are modified into light, hollow tubes penetrated by air sacs.
The human skeleton consists of 206 bones held together by flexible tissue consisting of cartilage and ligaments. It is composed of two basic parts, the axial and the appendicular skeletons. The axial skeleton includes the cranium, jawbone, ribs, sternum, and spinal column. The appendicular skeleton is made up of the upper (shoulder or pectoral) and lower (pelvic) girdles (see pelvis) and the bones of the arms and legs. Many diseases associated with the skeleton occur at the joints, notably the various types of arthritis, although such diseases as bone cancer may directly affect the skeleton. Skeletal remains are vital to physical anthropologists, who use them to trace human evolution.
See P. Shipman, A. Walker, and D. Bichell, The Human Skeleton (1985).
skeleton, in winter sports
in animals and humans, the collective hard tissue that supports the body and protects it from mechanical injury.
There are external and internal skeletons. Most invertebrates have an external skeleton, usually in the form of a shell or cuticle. In many protozoans the shell is secreted by the outer layer of cytoplasm and impregnated with mineral salts or encrusted with grains of sand. The most common shells are those of mol-lusks (spirally coiled, bivalve, multichambered) and brachio-pods (bivalve). Many worms and especially arthropods are characterized by a cuticular exoskeleton. Among arthropods the cuticle forms an external shell made of chitin. The chitinous shell does not grow but is periodically shed and replaced by a new shell. The growth of the animal coincides with the molting periods. Individual chitinous layers may be joined by flexible membranes; muscles are attached to the layers from within. Hydroid colonies are covered by a common skeletal integument called a perisarc. The calcareous skeleton of madreporari-ans (stony corals) is also an exoskeleton. However, the ectoderm that secretes the skeleton forms folds extending deeply into the body.
The simplest endoskeleton among invertebrates is found in sponges and is formed by calcareous and siliceous needles called spicules. The internal horny or calcareous skeleton of horny corals is derived from an exoskeleton (ectodermal skeleton). The calcareous skeleton of echinoderms, which is deeply embedded in the skin, is formed by the mesoderm. Cephalo-pods have an internal cartilaginous skeleton that protects the brain and eyes. The gills of marine polychaetes also have a cartilaginous skeleton.
Among the lower chordates (acraniates) the endoskeleton is a long axial structure called a cord. In vertebrates the endoskeleton is divided into the axial skeleton, the skull (the bony framework of the head), and the skeleton of the extremities. In embryos the endoskeleton, which is initially cartilaginous, is gradually and usually partially replaced by bone. In cyclo-stomes and chondrichthians the skeleton remains cartilaginous for the entire life of the animal. Bones and cartilage may be joined by flexible joints or may lack mobility. The axial skeleton is usually represented by a spine, which consists of individual vertebrae that replace the embryonic cord. The upper processes, or arches, of the vertebrae join to form the cerebrospinal canal, which contains the spinal cord. The ribs, which are attached to the centrums (vertebral bodies), articulate with the sternum to form the thorax in reptiles, birds, and mammals. In chondrichthians the centrums are cartilaginous, whereas among higher fishes and terrestrial vertebrates they are bony. Among lower aquatic vertebrates, including cyclostomes, chi-maeras, Acipenseridae, and dipnoans, centrums are absent and the axial skeleton is represented by a cord, along which are found cartilaginous or bony arches.
In fish the extremities are represented by unpaired fins (dorsal, anal, caudal) and paired fins (pectoral and pelvic). The skeleton of the fins is formed by cartilaginous or bony fin rays. Cyclostomes do not have paired fins. The rays of paired fins are supported by girdles, which are cartilaginous or bony layers located deep in the muscles. In terrestrial vertebrates unpaired fins disappear and paired fins become pentadactyl lever-type extremities. The girdle of the posterior extremities, which is called the pelvic girdle, is attached to the sacral vertebrae. In birds and bats the anterior extremities have become wings. The skull is divided into a brain section, which contains the brain and the organs of smell, hearing, and (partially) sight, and the visceral skeleton, which is represented by the jaws and gill arches. The jaws develop from anterior gill arches. (Cyclostomes do not have jaws.)
In terrestrial vertebrates gill arches have evolved into the auditory ossicle (stapes), the skeleton of the hypoglossal apparatus, and the skeleton of the larynx.
The exoskeleton, or dermal skeleton, which is absent in cyclostomes, is found in fish in the form of scales. Teeth are derivatives of scales and in microscopic structure are very similar to the placoid scales of chondrichthians. The membrane, or dermal, bones of the head and humeral girdle of higher fish and terrestrial vertebrates are also derivatives of thickened scale formations. On the head, membrane bones join tightly with and partially replace the endoskeleton. Bony scales derived from fish are found among terrestrial vertebrates in stegocephalians and among modern amphibians in caecilians.
Transformed fish scales have been preserved in many reptiles in the form of abdominal ribs. Bony scales or layers may be found on the skin of terrestrial vertebrates even secondarily; they are well developed among crocodiles and some saurians, whereas among testudinates and dasypodids (mammals) they form an external bony shell. Among testudinates the shell joins with the vertebrae and ribs.
L. P. TATARINOV
The human skeleton consists of the skull, the extremities, and the trunk. There are more than 200 bones in the body. The human skeleton has the same general structure as the skeleton of higher vertebrate animals and is most similar to the skeleton of anthropoid apes. It is distinguished from the skeleton of anthropoid apes by the structure and greater capacity of the skull, the form of the bones of the extremities (resulting in functional changes among humans), and the shape of the spine and pelvis (caused by walking upright).
Toward the end of the second month of intrauterine development the fibrous skeleton turns into the cartilaginous base of future bones, whose growth and development is complete at 25 or 26 years of age. The stages of ossification as shown by X-ray photographs can be used to determine an individual’s age. The aging process affects skeletal bones in middle-aged and elderly individuals. The amount of calcium in bone cells decreases, a process accompanied by osteoporosis. The bone surfaces become more pronounced where ligaments and muscles join. The aging process is also reflected in the corrugation of intervertebral disks, which often causes the thoracic spine to curve in the elderly (kyphosis).
Sexual differences exist in the structure of the skeleton. Compared to women, men have relatively more massive bones of the extremities, a narrower pelvis, and a broader thorax. Diseases of the skeleton are associated with metabolic disorders (particularly in bony tissue) and with disorders of some endocrine glands. Various developmental disturbances of skeletal bones during the prenatal period are manifested as congenital bone deformations.
V. F. POZHARISKII
In paleoanthropology the skeleton is the main source used in studying human morphological evolution and in reconstructing the physical appearance of man’s ancestors. The earliest and most important transformations of the skeleton in the process of anthropogenesis are associated with the development of erect posture. Adaptation of the skeleton of the lower extremities for walking on two legs is evident in such early primates as Australopithecines and related forms. All later hominids, including archanthropines and Paleoanthropus, were fully able to walk upright because of the structure of the axial skeleton and the lower extremities.
Paleoanthropology has less information on the evolution of the arms, but based on existing data it can be assumed that the modern type of human hand developed in the late stages of anthropogenesis among the progressive forms of ancient peoples (Palaeoanthropus). The same situation is probably true of the human skull. Some species of Palaeoanthropus, including Neanderthal man, exhibited unique morphological skeletal traits resulting from functional changes, especially in the structure of the hand and foot.
Skeletal studies also clarify some aspects of the vital activities of modern man’s fossil precursors. Thus, on the basis of osteo-logic data it is possible to indirectly assess the state of some functional systems, including hormone balance (paleoendocrinology), diet (the manifestation of protein deficiencies and bone pigmentation), growth characteristics (the rate of skeletal growth, teething periods, premature and physiological aging), sexual dimorphism, and diseased states (studied by paleo-pathology).
E. N. KHRISANFOVA
REFERENCESDogel’, V. A. Sravnitel’naia anatomiia bespozvonochnykh, part 1. Leningrad, 1938.
Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Ivanov, G. F. Osnovy normal’noi anatomii cheloveka, vol. 1. Moscow, 1949.
Bystrov, A. P. Proshloe, nastoiashchee, budushchee cheloveka. Leningrad, 1957.
Roginskii, Ia. Ia., and M. G. Levin. Antropologiia, 2nd ed. Moscow, 1963.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vols. 1-2. Moscow, 1964.
Sinel’nikov, R. D. Atlas anatomii cheloveka, 4th ed., vol. 1. Moscow, 1972. [23–1494–]
What does it mean when you dream about a skeleton?
numerals in the number 16 add up to seven, a lucky number.
What does it mean when you dream about a skeleton?
A dream about a skeleton is often about death or something within us that has died. Like ancient ruins, a skeleton can symbolize the past, too. Also note the familiar idiom about the “skeleton in the closet.”