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Skeletal System

Axial Skeleton

There are 28 bones in the skull. Of these, 8 bones comprise the cranium and provide protection for the brain. In adults, these bones are flat and interlocking at their joints, making the cranium immobile. Fibrous joints, or sutures occur where the bony plates of the cranium meet and interlock. Cartilage-filled spaces between the cranial bones of infants, known as soft spots or fontanelles, allow their skull bones to move slightly during birth. This makes birth easier and helps prevent skull fractures, but may leave the infant with an oddshaped head temporarily while the skull regains its shape. Eventually, the fontanelles in an infant's head are replaced by bone and fibrous joints develop. In addition to protecting the brain, skull bones also support and protect the sensory organs responsible for sight, hearing, smell and taste.

The eight bones of the cranium are: frontal, parietal (2), temporal (2), ethmoid, sphenoid and occipital. The frontal bone forms the forehead and eyebrows. Behind the frontal bone are the two parietal bones. Parietal bones form the roof of the cranium and curve down to form the sides of the cranium. Also forming the sides of the cranium are the two temporal bones, located behind the eyes. Each temporal bone encloses the cochlea and labyrinth of the inner ear, and the ossicles, three tiny bones of the middle ear which are not part of the cranium. The ossicles are the malleus (hammer), incus (anvil), and stapes (stirrups). The temporal bones also attach to the lower jaw, and this is the only moveable joint in the skull. Between the temporal bones is the irregular shaped sphenoid bone, which provides protection for the pituitary gland. The small ethmoid bone forms part of the eye socket next to the nose. Olfactory nerves, or sense of smell nerves, pass through the ethmoid bone on their way to the brain. Forming the base and rear of the cranium is the occipital bone. The occipital bone has a hole, called the foramen magnum, through which the spinal cord passes and connects to the brain.

Fourteen bones shape the cheeks, eyes, nose and mouth. These include the nasal (2), zygomatic (2), maxillae (2), and the mandible. The upper, bony bridge of the nose is formed by the nasal bones and provides an attachment site for the cartilage making up the softer part of the nose. The zygomatic bones form the cheeks and part of the eye sockets. Two bones fuse to form the maxillae, the upper jaw of the mouth. These bones also form hard palate of the mouth. Failure of the maxillary bones to completely fuse in the fetus results in the condition known as cleft palate. The mandible forms the lower jaw of the mouth and is moveable, enabling chewing of food and speech. The mandible is the bone which connects to the temporal bones. The joint between these bones, the temporomandibular joint, is the source of the painful condition known as temporomandibular joint dysfunction, or TMJ dysfunction. Sufferers of TMJ dysfunction experience a variety of symptoms including headaches, a sore jaw and a snapping sensation when moving the jaw. There a several causes of the dysfunction. The cartilage disk between the bones may shift, or the connective tissue between the bones may be situated in a manner that causes misalignment of the jaw. Sometimes braces on the teeth can aggravate TMJ dysfunction. The condition may be corrected with exercise, or in severe cases, surgery.

Located behind these facial bones are other bones which shape the interior portions of the eyes, nose and mouth. These are the lacrimal (2), palatine (2), conchae (2), and vomer bones. In addition to these 28 skull bones is the hyoid bone, located at the base of the tongue. Technically, the hyoid bone is not part of the skull but it is often included with the skull bones. It provides an attachment site for the tongue and some neck muscles.

Several of the facial and cranial bones contain sinuses, or cavities, that connect to the nasal cavity and drain into it. These are the frontal, ethmoid, sphenoid and maxillae bones, all located near the nose. Painful sinus headaches result from the build up of pressure in these cavities. Membranes that line these cavities may secrete mucous or become infected, causing additional aggravation for humans.

The skull rests atop of the spine, which encases and protects the spinal cord. The spine, also called the vertebral column or backbone, consists of 33 stacked vertebrae, the lower ones fused. Vertebra are flat with two main features. The main oval shaped, bony mass of the vertebra is called the centrum. From the centrum arises a bony ring called the neural arch which forms the neural canal (also called a vertebral foramen), a hole for the spinal cord to pass through. Short, bony projections (neural spines) arise from the neural arch and provide attachment points for muscles. Some of these projections (called transverse processes) also provide attachment points for the ribs. There are also small openings in the neural arch for the spinal nerves, which extend from the spinal cord throughout the body. Injury to the column of vertebrae may cause serious damage to the spinal cord and the spinal nerves, and could result in paralysis if the spinal cord or nerves are severed.

There are seven cervical, or neck, vertebrae. The first one, the atlas, supports the skull and allows the head to nod up and down. The atlas forms a condylar joint (a type of synovial joint) with the occipital bone of the skull. The second vertebra, the axis, allows the head to rotate from side to side. This rotating synovial joint is called a pivot joint. Together, these two vertebrae make possible a wide range of head motions.

Below the cervical vertebrae are the 12 thoracic, or upper back, vertebrae. The ribs are attached to these vertebrae. Thoracic vertebrae are followed by five lumbar, or lower back, vertebrae. Last is the sacrum, composed of five fused vertebrae, and the coccyx, or tail bone, composed of four fused bones.

The vertebral column helps to support the weight of the body and protects the spinal cord. Cartilaginous joints rather than synovial joints occur in the spine. Disks of cartilage lie between the bony vertebrae of the back and provide cushioning, like shock absorbers. The A frontal view of the human skeleton. Photograph by Martin Dohrn. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
vertebrae of the spine are capable of only limited movement, such bending and some twisting.

A pair of ribs extends forward from each of the 12 thoracic vertebrae, for a total of 24 ribs. Occasionally, a person is born with an extra set of ribs. The joint between the ribs and vertebrae is a gliding (or plane) joint, a type of synovial joint, as ribs do move, expanding and contracting with breathing. Most of the ribs (the first seven pair) attach in the front of the body via cartilage to the long, flat breastbone, or sternum. These ribs are called true ribs. The next three pair of ribs are false ribs. False ribs attach to another rib in front instead of the sternum, and are connected by cartilage. The lower two pair of ribs which do not attach anteriorly are called floating ribs. Ribs give shape to the chest and support and protect the body's major organs, such as the heart and lungs. The rib cage also provides attachment points for connective tissue, to help hold organs in place. In adult humans, the sternum also produces A scanning electron micrograph (SEM) of normal human cancellous (spongy) bone. The shafts of long bones such as the femur are comprised of two types of bone of differing densities: compact bone forms the outer region, and cancellous bone forms the core. In living cancellous bone, the cavities of the open structure contain bone marrow. Photograph by Prof. P. Motta/Department of Anatomy/University "La Sapienza", Rome/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
red blood cells as well as providing an attachment site for ribs.

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Science EncyclopediaScience & Philosophy: Semiotics to SmeltingSkeletal System - Structure, Axial Skeleton, Appendicular Skeleton, Types Of Bone, Bone Development And Growth, Bones And Medicine