Skeletal System Health Article

Structure

The human skeletal system is divided into two main groups: the axial skeleton and the appendicular skeleton. The axial skeleton includes bones associated with the body's main axis including:

• the skull
• the spine or vertebral column
• the ribs

The appendicular skeleton consists of the bones that anchor the body's appendages to the axial skeleton including:

• the pectoral girdle (shoulder area)
• the pelvic girdle (hip area)
• the upper extremities (arms)
• the lower extremities (legs)

AXIAL SKELETON. There are 28 bones in the skull. Of these, eight 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 odd-shaped 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
• 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)
• 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 the hard palate of the mouth. 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.

Located behind these facial bones are other bones which shape the interior portions of the eyes, nose, and mouth. These include:

• lacrimal (2)
• palatine (2)
• conchae (2)
• 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 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 red blood cells as well as providing an attachment site for ribs.

APPENDICULAR SKELETON. The appendicular skeleton joins with the axial skeleton at the shoulders and hips. Forming a loose attachment with the sternum is the pectoral girdle, or shoulder. Two bones, the clavicle (collar bone) and scapula (shoulder blade), form one shoulder. The scapula rest on top of the ribs in the back of the body. It connects to the clavicle, the bone which attaches the entire shoulder structure to the skeleton at the sternum. The clavicle is a slender bone that is easily broken. Because the scapula is so loosely attached, it is easily dislocated from the clavicle, hence the dislocated shoulder injuries commonly suffered by persons playing sports. The major advantage to the loose attachment of the pectoral girdle is that it allows for a wide range of shoulder motions and greater overall freedom of movement.

Unlike the pectoral girdle, the pelvic girdle, or hips, is strong and dense. Each hip, left and right, consists of three fused bones, the ilium, ischium, and pubic. Collectively, these three bones are known as the innominate bone.

The innominates fuse with the sacrum to form the pelvic girdle. Specifically, the iliums shape the hips and the two ischial bones support the body when a person sits. The two pubic bones meet anteriorly at a cartilaginous joint. The pelvic girdle is bowl-shaped, with an opening at the bottom. In a pregnant woman, this bony opening is a passageway through which her baby must pass during birth. To facilitate the baby's passage, the body secretes a hormone called relaxin which loosens the joint between the pubic bones. In addition, the pelvic girdle of women is generally wider than that of men. This also helps to facilitate birth, but is a slight impediment for walking and running. Hence, men, with their narrower hips, are better adapted for such activities. The pelvic girdle protects the lower abdominal organs, such as the intestines, and helps supports the weight of the body above it.

The arms and legs, the upper and lower appendages of the body, are very similar in form. Each attaches to the girdle, pectoral or pelvic, via a ball and socket joint, a special type of synovial joint. In the shoulder, the socket, called the glenoid cavity, is shallow. The shallowness of the glenoid cavity allows for great freedom of movement. The hip socket, or acetabulum, is larger and deeper. This deep socket, combined with the rigid and massive structure of the hips, give the legs much less mobility and flexibility than the arms.

The humerus, or upper arm bone, is the long bone between the elbow and the shoulder. It connects the arm to the pectoral girdle. In the leg the femur, or thigh bone, is the long bone between the knee and hip which connects the leg to the pelvic girdle. The humerus and femur are sturdy bones, especially the femur, which is a weight bearing bone. Since the arms and legs are jointed, the humerus and femur are connected to other bones at the end opposite the ball and socket joint. In the elbow, this second joint is a type of synovial joint called a hinge joint. Two types of synovial joints occur in the knee region, a condylar joint (like the condylar joint in the first vertebra) which connects the leg bones, and a plane, or gliding joint, between the patella (knee cap) and femur.

At the elbow the humerus attaches to a set of parallel bones, the ulna and radius, bones of the forearm. The radius is the bone below the thumb that rotates when the hand is turned over and back. The ulna and radius then attach to the carpel bones of the wrist. Eight small carpel bones make up the wrist and connect to the hand. The hand is made up of five long, slender metacarpal bones (the palms) and 14 phalanges of the hand (fingers and thumb). Some phalanges form joints with each other, giving the human hand great dexterity.

Similarly, in the leg, the femur forms a joint with the patella and with the fibula and tibia bones of the lower leg. The tibia, or shin bone, is larger than the fibula and forms the joint behind the patella with the femur. Like the femur, the tibia is also a weight bearing bone. At the ankle joint, the fibula and tibia connect to the tarsals of the upper foot. There are seven tarsals of the upper foot, forming the ankle and the heel. The tarsals in turn connect to five long, slender metatarsals of the lower foot. The metatarsals form the foot's arch and sole and connect to the phalanges of the feet (toes). The 14 foot phalanges are shorter and less agile than the hand phalanges. Several types of synovial joints occur in the hands and feet, including plane, ellipsoid and saddle. Plane joints occur between toe bones, allowing limited movement. Ellipsoid joints between the finger and palm bones give the fingers circular mobility, unlike the toes. The saddle joint at the base of the thumb helps make the hands the most important part of the body in terms of dexterity and manipulation. A saddle joint also occurs at the ankles.