Arm
arm, upper limb in humans. Three long bones form the framework of the arm: the humerus of the upper arm, and the radius (outer bone) and ulna (inner bone) of the forearm. The radius and ulna run parallel but meet at their ends in such a manner that the radius can rotate around the ulna. This arrangement permits turning the forearm to bring the hand palm up (supination) or palm down (pronation). The radius and ulna hinge with the bones of the hand at the wrist, and with the humerus at the elbow. The biceps brachii, a muscle of the upper arm, bends the arm at the elbow; the triceps brachii straightens the arm. Movement of the arm across the chest and above the head is accomplished by the pectoral muscles of the chest and deltoid muscles of the shoulder, respectively. In an adult the arm is normally five sixths as long as the leg.
biceps
any muscle having two heads, or fixed ends of attachment, notably the biceps brachii at the front of the upper arm and the biceps femoris in the thigh. Originating in the shoulder area, the heads of the biceps merge partway down the arm to form a rounded mass of tissue linked by a tendon to the radius, the smaller of the two forearm bones. When the biceps contracts, the tendon is pulled toward the heads, thus bending the arm at the elbow. For this reason the biceps is called a flexor. It works in coordination with the triceps brachii, an extensor. The biceps also controls rotation of the forearm to a palm-up position, as in turning a doorknob. The size and solidity of the contracted biceps are a traditional measure of physical strength.
triceps
triceps, any muscle having three heads, or points of attachment, but especially the triceps brachii at the back of the upper arm. One head originates on the shoulder blade and two on the upper-arm bone, or humerus. Uniting part of the way down the arm, the heads swell into the belly, or muscle proper. This tapers to a tendon that rounds the elbow and attaches to the ulna, the larger of the two forearm bones. Since contraction of the triceps straightens the arm, the muscle is called an extensor. It also helps lock the elbow when the forearm pushes forward against resistance. The triceps works in coordination with a flexor muscle, the biceps brachii of the upper arm.
skeleton
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.
skull
skull, the skeletal structure of the head, composed of the facial and cranial bones. The skull houses and protects the brain and most of the chief sense organs; i.e., the eyes, ears, nose, and tongue. Among humans, some 14 bones shape the face, most occurring in symmetrical pairs. They are the lacrimals at the inner sides of the eyes, the nasals and nasal conchae of the nose, the palatines (palate), the zygomatics, or malars at the cheeks, the vomer (nasal septum), and the maxillae, or upper jaw. The mandible, or lower jaw, is not technically part of the skull. The adult human cranium, or braincase, is formed of fused skull bones: the parietals, temporals, ethmoid, sphenoid, frontal, and occipital. These are separate plates of bone in the fetus, but by birth they have generally grown sufficiently for most of their edges to meet. The remaining separations are known as fontanels, the most prominent being the soft spot atop a newborn's head. By the age of two years, all of these fontanels have been closed over by the growing cranial bones. However, the seams, or sutures, between the bones do not completely knit until the age of 20. The occipital bone at the base of the skull forms a complex joint with the first vertebra of the neck, known as the atlas, permitting rotation and bending of the head (see spinal column). Study of the fossil skulls of humans and their precursors has made important contributions to evolutionary theory, and to the science of physical anthropology. Earlier skulls of human ancestors, for instance, have been shown to have markedly smaller cranial capacities, as well as more powerful jaws, than do the Homo sapiens species which exist today.
rib
rib, one of the slender, elongated, curved bones that compose the chest cage in higher vertebrates. Ribs occur in pairs, and are found in most vertebrates; however, in some lower vertebrates, including fishes, they run along the entire length of the backbone. The ribs of the snake are used in locomotion. In the human there are 12 pairs of ribs. Each rib is connected to the vertebral column by strong ligaments. In the front, a flexible section of cartilage connects the rib to the sternum, or breastbone. Below the 7th rib, the 8th, 9th, and 10th ribs are not attached directly to the sternum, but to the cartilage of the 7th rib. The 11th and 12th pairs of ribs are not attached in front at all, and hence are known as floating ribs. Technically, these ribs do not “float,” however, but are attached to the vertebral column in the rear and extend only part of the way around the chest. In birds and mammals, ribs enclose the lungs and heart and assist in the process of breathing. During inhalation the ribs move upward and farther apart, expanding the chest cavity. During exhalation their downward motion aids in expelling air from the lungs.
Backbone (spinal column)
spinal column, bony column forming the main structural support of the skeleton of humans and other vertebrates, also known as the vertebral column or backbone. It consists of segments known as vertebrae linked by intervertebral disks and held together by ligaments. In human beings, the spinal column of the child contains more vertebrae than the adult, in whom a number become fused into two immovable bones, the sacrum and the coccyx, forming the back of the pelvis. The 24 movable vertebrae are the 7 cervical (neck), 12 thoracic (back of chest), and 5 lumbar (loin). The remaining vertebrae include 5 fused sacral, and between 3 and 5 fused caudal. Each vertebra has a somewhat cylindrical bony body (centrum), a number of winglike projections, and a bony arch. The bodies of the vertebrae form the strong but pliable supporting column of the skeleton. The arches are positioned so that the space they enclose is in effect a tube, the vertebral canal. It houses and protects the spinal cord, and within it the spinal fluid circulates. Ligaments and muscles are attached to various projections of the vertebrae. The 12 pairs of ribs that make up the front of the chest are linked to the thoracic vertebrae. The spine is subject to abnormal curvature, injury, infections, tumor formation, arthritic disorders, and puncture or slippage of the cartilage disks. Scoliosis is one relatively common disease which affects the spinal column. It involves moderate to severe lateral curvature of the spine, and, if not treated, may lead to serious deformities later in life.
pelvis
pelvis, bony, basin-shaped structure that supports the organs of the lower abdomen. It receives the weight of the upper body and distributes it to the legs; it also forms the base for numerous muscle attachments. In the human pelvis there are two large hip bones, each consisting of three fused bones, the illium, ischium, and pubis. The hip bones form a ring around a central cavity. The fused terminal segments of the spine, known as the sacrum and coccyx, connect the hip bones at the back of the central cavity; a fibrous band connects them at the front. In women the pelvis is wider and has a larger capacity than in men, a condition that reflects the child-bearing function in women. See skeleton.
spinal cord
spinal cord, the part of the nervous system occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal column, technically known as the vertebral column. Extending from the first lumbar vertebra to the medulla at the base of the brain, the spinal cord of a human adult is about 18 in. (45 cm) long. Structurally, the cord is a double-layered tube, roughly cylindrical in cross section. The outer layer consists of white matter, i.e., myelin-sheathed nerve fibers. These are bundled into specialized tracts that conduct impulses triggered by pressure, pain, heat, and other sensory stimuli or conduct motor impulses activating muscles and glands. The inner layer, or gray matter, is primarily composed of nerve cell bodies. Within the gray matter, running the length of the cord and extending into the brain, lies the central canal through which circulates the cerebrospinal fluid. Three protective membranes, the meninges, wrap the spinal cord and cover the brain—the pia mater is the innermost layer, the arachnoid lies in the middle, and the dura mater is the outside layer, to which the spinal nerves are attached. Connecting with the cord are 31 pairs of these spinal nerves, which feed sensory impulses into the spinal cord, which in turn relays them to the brain. Conversely, motor impulses generated in the brain are relayed by the spinal cord to the spinal nerves, which pass the impulses to muscles and glands. The spinal cord mediates the reflex responses to some sensory impulses directly, i.e., without recourse to the brain, as when a person's leg is tapped producing the knee jerk reflex. Nerve fibers in the spinal cord usually do not regenerate if injured by accident or disease.