The Skeleton: an Ordered Assembly of Bones

The highlighted names of bones will take you to a new page with an illustration of each bone or region. Look at these as you read the terms. You will be responsible for all accessory terms. When you examine the human skeleton, you will see that bones vary in shape and size. It is helpful to distinguish four categories of bone shape: long bones, short bones, flat bones, and irregular bones.

Long bones are bones that are longer than they are wide; examples include the bones of the limbs, such as the femur and humerus, and the bones of the fingers. Short bones are approximately as wide as they are long; examples include the small bones of the wrist and ankle. Flat bones, not surprisingly, tend to be broad, thin, and flattened examples include the skull bones, sternum, scapula and ribs. Irregular bones are bones with rather complex shapes which do not easily fit into any of the other categories; examples include the vertebrae and several facial bones.

Let us begin our study of the skeleton by distinguishing its two major divisions: the axial skeleton and the appendicular skeleton.

The Axial Skeleton: Focus on the Skull

The axial skeleton includes the bones that form the "axis," or midline, of the body: the skull, vertebral column, and rib cage. We will describe each of these structures, beginning with the skull, which consists of both cranial bones and facial bones.

Cranial bones: Cranial bones are flat bones that surround and protect the brain. The frontal bone forms the entire front portion of the skull, including the forehead. Just posterior to this bone are the two parietal bones that constitute the upper left and right sides of the skull. Below the parietals are the two temporal bones that form the lower left and right sides of the skull. Each temporal bone has an external opening leading into the ear canal (auditory meatus)through which sound waves pass on their way to the eardrum.

Attached to the frontal, parietal, and temporal bones is the sphenoid bone. This sphenoid is centrally located within the skull and helps connect many of the cranial and facial bones. The occipital bone forms the back and base of the skull, and its lower portion has a large opening, the foramen magnum, which in Latin means "large opening." Through this opening the spinal cord enters the skull and connects with the brain. On either side of the foramen magnum are two small bony projections that rest on the first vertebra and support the entire skull while permitting a nodding motion of the head.

Facial bones: The facial bones make up the front of the skull. the two maxilla bones are situated on either side of the nose and help form the orbits of the eyes and upper part of the mouth (the hard palate). They also contain the sockets that secure the upper teeth. The hard palate is also formed by the two palatine bones. Between the maxillary bones, at the level of the eyes, are the two short, narrow nasal bones. These bones form only the upper bridge of the nose and are much smaller than the nose that is visible on the face. What we call the nose is mostly cartilage and is not considered part of the skeletal system. The maxilla bones and the nasal bones partially define a space called the nasal cavity.

One of the defining features of the face is provided by the two zygomatic bones. These bones join with narrow projections from the two temporal bones, and together they provide the bony structure that makes up the cheeks. Another defining feature of the face is provided by the lower jaw, the mandible. This is a single U-shaped bone that is hinged with the temporal bone of the cranium and contains sockets for the lower teeth.

Sinuses are spaces. Several of the cranial and facial bones have air spaces within them. These spaces, which are called sinuses, lighten the skull and help give the voice its distinctive resonant qualities. Each sinus is lined with mucus-secreting epithelium and is connected to the nasal cavity by small tubes through which mucus normally drains. During a sinus infection, the inflamed epithelium swells, preventing the tubes from draining properly. A sinus headache results from pressure caused by fluid accumulation in the sinuses.

The Axial Skeleton: Focus on the Vertebrae and Rib Cage

The vertebral column is also known as the backbone or spine. In an adult, it consiss of 26 irregular bones called vertebrae; a single spinal bone is called a vertebra. This series of the interacting bones extends from the skull to the pelvis. Together these bones have three major functions. They (1) provide flexible support for the head and trunk, (2) protect the delicate spinal cord, and (3) permit the passage of spinal nerves to and front he spinal cord.

The vertebral column is not straight like a series of spools on a string but instead is curved. The curvatures are due to the different structure of each vertebrae and the support given by the attached ligaments and muscles. Together these structures give the entire column strength and flexibility. This is an example of the integrated functioning of the skeletal and muscular systems.

Structurally, the vertebral column can be divided into five regions; the cervical, thoracic, lumbar, sacral, and coccygeal regions. These terms can also refer to specific vertebrae and to the curvatures associated with each region; thus, one can speak of the cervical vertebrae and the cervical curvature. The first two cervical vertebrae are unique in their shape. The are the Atlas, which holds the head at the top of the column, and the Axis, which allows the head to pivot in place atop the vertebral column.

Although the vertebrae from different regions of the column differ from one another, they all share two features: (1) A bony canal for the spinal cord, and (2) a notch on each side that forms an opening which permits nerves to pass through the vertebral column to and from the spinal cord.

Adjacent vertebrae are separated from each other by a pad of cartilage known as the intervertebral disk. These disks act as cushions, protecting the vertebrae from the shocks associated with activities such as walking, running, and jumping. They also provide a certain degree of flexibility, which we experience when we swing a golf club or touch our toes.

A common cause of severe back pain occurs when an intervertebral disk ruptures and presses against the spinal cord or against nerves that enter and leave the spinal cord through the notch in the vertebra. This condition is referred to as a slipped disk.

The third component of the axial skeleton is the rib cage, which in both men and women consists of 12 paired ribs that form a protective cage for some internal organs. The interior of this cage is known as the thoracic cavity, and it houses the heart and lungs. One end of each pair of ribs joins with one of the 12 thoracic vertebrae. The other ends of the upper 10 pairs of ribs are attached to the sternum, or breastbone, by cartilage. The lower two pairs of ribs do not attach to the sternum and are called floating ribs.

The Appendicular Skeleton: The Limbs and Their Support

The appendicular skeleton is the second division o the human skeleton. It includes the two supportive frames called the pectoral and pelvic girdles and their attached limbs, the arms and legs. Each girdle is supportive bony structures that is secured to the axial skeleton by ligaments and muscles.

Pectoral girdle. The pectoral girdle forms the bony portion of each shoulder and consists of right and left scapulas and clavicles. You know the scapula as the shoulder blade. It is a triangular flat bone that forms a movable joint with the upper arm bone. The clavicle is also known as the collarbone. It is a curved bone that joins with the scapula and acts as a brace helping to hold the scapula in position.

The scapula and clavicle are not well secured to each other or to the rib cage. The pectoral girdle is actually stabilized and secured to the rib cage by a group of muscles. This arrangement allows a considerable range of movement in the entire upper appendicular skeleton. When you move your upper arm in a 36-degree circle, you can appreciate the wide range of motion associated with the pectoral girdle. This is greater than the range of movement possible with any other joint.

The uppermost bone of the arm is called the humerus. Its upper end fits into a saucer-shaped socket on the scapula. This attachment (joint) is stabilized by muscles and tendons and permits a wide range of movement. The lower end of the humerus joins with the two bones of the lower arm, the ulna and radius, forming what is called the elbow. When you accidentally bump your elbow at the "crazy bone," you may feel a tingling sensation. The tingling sensation results from hitting the ulnar nerve, which passes along the back of the elbow.

The lower end of the ulna and radius is attached to a group of bones called the carpal bones that form the wrist. The palm of the hand contains the metacarpal bones. The fingers, or phalanges, consist of bones that join with the metacarpal bones.

Pelvic Girdle. The pelvic girdle consists of two large bones called the coxal bones, commonly known as the hipbones. Linked together by cartilage in the front (the pubic symphysis), the two hipbones also join with the vertebral column in back, forming a ring like structure called the pelvis. The pelvises of adult males and females differ somewhat in shape and size; this accounts for the visible differences in the shape and size of the hips in men and women.

The female pelvis is wider, shallower, and rounder than the male pelvis and provides a larger opening for the passage of the infant's head during birth. Differences between the male and female pelvises begin to develop during puberty when the production of sex hormones initiates bone remodeling that specializes the female pelvis for pregnancy and birth.

The upper leg bone is called the femur or thighbone. Its upper end is formed into a ball-shaped unit that fits into a well-defined socket of a coxal. This ball-and-socket arrangement results in a stable load-bearing joint that is capable of a great range of movement. At the knee, the lower end of the femur meets two bones of the lower leg: the larger tibia and the smaller fibula. The knee joint forms where the femur joins directly with the tibia. In front of each knee joint is a small triangular bone, the patella, or kneecap, which protects the knee joint and acts as a pulley for the upper thigh muscles.

The tibia and fibula join with the number of small tarsal bones to form the ankle. The tarsal and metatarsal bones, along with the phalanges of the toes, form the foot.