1-6 THE SKELETAL SYSTEM

LEARNING OBJECTIVE: Identify the parts of bone and their functions.

The skeleton, the bony framework of the body, is composed of 206 bones (fig. 1-13). It supports and gives shape to the body; protects vital organs; and provides sites of attachment for tendons, muscles, and ligaments. The skeletal bones are joined members that make muscle movement possible.

Figure 1-13.—Human skeleton.

ANATOMY OF BONES

Osteology is the study of the structure of bone. Bone is made up of inorganic mineral salts (calcium and phosphorus being the most prevalent) and an organic substance called ossein. If human bones were soaked in dilute acid until all inorganic mineral salts were washed out, all that would remain would be a flexible piece of tissue that could be easily bent and twisted. Inorganic mineral salts give bone its strength and hardness.

Bone consists of a hard outer shell, called compact bone, and an inner spongy, porous portion, called cancellous tissue (fig. 1-14). In the center of the bone is the medullary canal, which contains marrow. There are two types of marrow, red and yellow. Yellow marrow is ordinary bone marrow in which fat cells predominate. It is found in the medullary canals and cancellous tissue of long bones. Red marrow is one of the manufacturing centers of red blood cells and is found in the articular ends of long bones and in cancellous tissue.

Figure 1-14.—Anatomy of a long bone.

 

At the ends of the long bones is a smooth, glossy tissue that forms the joint surfaces. This tissue is called articular cartilage because it articulates (or joins) with, fits into, or moves in contact with similar surfaces of other bones. The thin outer membrane surrounding the bone is called the periosteum. An important function of the periosteum is to supply nourishment to the bone. Capillaries and blood vessels run through the periosteum and dip into the bone surface, supplying it with blood and nutrients. The periosteum is the pain center of the bone. When a bone fractures, the pain that is felt comes from the periosteum, not the bone proper. Periosteum also forms new bone. The diaphysis is the elongated, cylindrical portion (or “shaft”) of the bone that is between the epiphyses (sing. epiphysis) or ends of the bone.

 BONE CLASSIFICATIONS

Bones are classified according to their shape. The four bone classifications and examples of each are as follows:

DIVISIONS OF SKELETON

The human skeleton is divided into two main divisions, the axial skeleton and the appendicular skeleton.

Axial Skeleton

The axial skeleton consists of the skull, the vertebral column, and the thorax.

SKULL.—The skull consists of 28 bones (figs. 1-15 and 1-16), 22 of which form the framework of the head and provide protection for the brain, eyes, and ears; six are ear bones. With the exception of the lower jaw bone and the ear bones, all skull bones are joined together and fixed in one position. The seams where they join are known as sutures. The bones of the skull are classified as either cranial or facial bones.

Figure 1-15.—Lateral view of the skull.

 

Figure 1-16.—Facial bones.

Cranial Bones.—The cranium is formed by eight major bones, most of which are in pairs (fig. 1-15). The frontal bone forms the forehead and the roof of each orbit (or eye socket) and the nasal cavity. The parietal bones form the roof of the skull. The temporal bones help form the sides and base of the skull and also house the auditory and hearing organs. The occipital bone forms part of the base and back of the skull, and contains a large hole called the foramen magnum. This opening permits passage of the spinal cord from the cranium into the spinal column. The sphenoid bones are wedged between several other bones in the anterior portion of the skull. These bones help form the base of the cranium, the sides of the skull, and the floors and sides of the orbits. The ethmoid bones are located in front of the sphenoid bone. They form sections of the nasal cavity roof, the cranial floor, and the orbital wall.

Facial Bones.—The facial bones of the skull consists of 14 bones: 13 immovable bones and a movable lower jawbone (fig. 1-16). The facial bones give the face its basic shape and provide attachment sites for various muscles that move the jaw and control facial expressions.

The maxillary bones form the upper jaw, the anterior roof of the mouth, the floors of the orbits, and the sides and floor of the nasal cavity. The small holes on each side of the nasal opening are called the infraorbital foramina (sing. foramen). The maxillary bones contain large cavities called maxillary sinuses.

The palatine bones are L-shaped bones located behind the maxillary bones. They form the posterior section of the hard palate and the floor of the nasal cavity.

The zygomatic bones are responsible for the prominence of the cheeks. The zygomatic bones serve as part of the posterior section of the hard palate and the floor of the nasal cavity.

The lacrimal bones provide a pathway for a tube that carries tears from the eye to the nasal cavity. The lacrimal bone is a thin, scalelike structure located in the medial wall of each orbit.

The nasal bones have cartilaginous tissues attached to them. These tissues contribute significantly to the shape of the nose. The nasal bones are long, thin, and nearly rectangular in shape. They lie side by side and are fused together to form the bridge of the nose.

The vomer bone is connected to the ethmoid bone, and together they form the nasal septum (the wall separating the two nasal cavities).

The middle and inferior nasal conchae are fragile, scroll-shaped bones that are attached to the lateral wall of the nasal cavity. The inferior nasal concha provides support for mucous membranes within the nasal cavity.

The lower jawbone is called the mandible. The mandible is horseshoe-shaped with flat, bony projections on each end. The two small holes on the jawbone are called the mental foramina. The mandible's main function is mastication (chewing food).

VERTEBRAL (SPINAL) COLUMN.—The vertebral column consists of 24 movable or true vertebrae; the sacrum; and the coccyx, or tail bone (fig. 1-17). The vertebrae protect the spinal cord and the nerves that branch out from the spinal cord. Each vertebra has an anterior portion, called the body, which is the large solid segment of the bone (fig. 1-18). This vertebral body supports not only the spinal cord but other structures of the body as well. At the bottom of the spinal column is the sacrum and the coccyx. Many of the main muscles are attached to the vertebrae.

Figure 1-17.—Vertebral column:
A. Left lateral view of vertebral column; B. Posterior view of vertebral column.

Figure 1-18.—Typical vertebra:
A. Lateral view of a typical vertebra; B. Superior view of a typical thoracic vertebra.

The vertebral foramen is a hole directly behind the body of the vertebrae that forms the passage for the spinal cord. The vertebral projections are for the attachments of muscles and ligaments and for facilitating movement of one vertebra over another. The spinal column is divided into five regions in the following order: cervical (neck), thoracic (chest), A. B. lumbar (lower back), and sacral and coccygeal (pelvis).

Cervical.—There are seven cervical vertebrae in the neck. The first is called the atlas and resembles a bony ring. It supports the head. The second is the highly specialized axis. It has a bony prominence that fits into the ring of the atlas, thus permitting the head to rotate from side to side. The atlas and the axis are the only named vertebrae; all others are numbered. See figure 1-19. Each cervical vertebra has a transverse (orintervertebral) foramen (fig. 1-19) to allow passage of nerves, the vertebral artery, and a vein. The seventh cervical vertebra has a prominent projection that can easily be felt at the nape of the neck. This landmark makes it possible for physicians to count and identify the vertebrae above and below it.

Figure 1-19.—Cervical vertebrae.

Thoracic.—There are 12 vertebrae in the thoracic region. The thoracic vertebrae articulate with the posterior portion of the 12 ribs to form the posterior wall of the thoracic, or chest, cage.

Lumbar.—There are five lumbar vertebrae. Located in the small of the back, these vertebrae are the larger and stronger segments of the vertebral column.

Sacrum.—The sacrum is the triangular bone immediately below the lumbar vertebrae. It is composed of five separate vertebrae that gradually fuse together between 18 and 30 years of age. The sacrum is connected on each side with the hip bone and with the coccyx to form the posterior wall of the pelvis.

THORAX.—This cone-shaped bony cage is about as wide as it is deep (fig. 1-20). The thorax is formed by 12 ribs on each side and articulates posteriorly with the thoracic vertebrae. The first set of ribs are attached to the manubrium, a flat irregular bone atop the sternum. The first seven pairs of ribs are called true ribs. The remaining five pairs are called false ribs. They are called false ribs because their cartilages do not reach the sternum directly. The eighth, ninth, and tenth ribs are united by their cartilages and joined to the rib above. The last two rib pairs, also known as floating ribs, have no cartilaginous attachments to the sternum. The sternum is an elongated flat bone, forming the middle portion of the upper half of the chest wall in front. The xiphoid process, located at the inferior aspect of the sternum, serves as a landmark in the administration of cardiopulmonary resuscitation.

Figure 1-20.—Anterior view of thorax,

STERNAL ANGLE

Appendicular Skeleton

The appendicular skeleton consists of the bones of the upper and lower extremities.

UPPER EXTREMITY.—The upper extremity consists of the bones of the shoulder, the arm, the forearm, the wrist, and the hand (figs. 1-21 and 1-22). The bones that form the framework for the upper extremities are listed in table 1-1.

Figure 1-21.—Pectoral girdle.

 

BONE

COMMON
NAME

TOTAL
NUMBER
IN BODY

clavicle collar bone

2

scapula shoulder blade

2

humerus arm bone

2

radius and ulna forearm bones

4

carpals wrist bones

16

metacarpals bones of the palm

10

phalanges finger bones

28

Table 1-1.—Bones of the Upper Extremities

Clavicle.—The clavicle (commonly called the collar bone) lies nearly horizontally above the first rib and is shaped like a flat letter S. The clavicle is a thin brace bone that fractures easily. Its inner end is round and attached to the sternum; its outer end is flattened and fixed to the scapula. The clavicle forms the anterior portion of the pectoral girdle (fig. 1-21). The pectoral girdle is composed of the two clavicles and two scapulae (shoulder blades). It functions as a support for the arms and serves as an attachment for several muscles.

Scapula.—The scapula is a triangular bone that lies in the upper part of the back on both sides, between the second and seventh ribs, forming the posterior portion of the pectoral girdle. Its lateral corner forms part of the shoulder joint, articulating with the humerus.

Humerus.—The humerus is the longest bone of the upper extremity and is often called the arm bone (fig. 1-22). It articulates with the pectoral girdle to form the shoulder joint, and with the bones of the forearm to form the elbow. Its anatomical portions include a head (a rounded portion that fits into a recess of the scapula) called the glenoid fossa; the shaft, which is the main part of the humerus; and the distal end, which includes the prominence (called an epicondyle) and the surfaces that articulate with the bones of the forearm.

Figure 1-22.—Left arm:
A. Frontal view of left arm with hand supinated; B. Frontal view of left arm with hand pronated.

Radius and Ulna.—When the arm is in the anatomical position with the palm turned forward, the radius is on the lateral (thumb) side and the ulna is on the medial (little finger) side of the forearm (fig. 1-22). When the hand is pronated (with the palm turned downward), the bones rotate on each other and cross in the middle. This pronation makes it possible to turn the wrist and hand (as when opening doors). The ulna and the radius articulate at their proximal ends with the humerus, at their distal ends with some of the carpal bones, and with each other at both ends.

Carpal.—There are eight carpal bones, arranged in two rows, forming the wrist.

Metacarpal.—The metacarpal bones are numbered one to five, corresponding with the five fingers, or digits, with which they articulate. The fingers are named as follows: 1st—thumb; 2nd—index; 3rd—middle; 4th—ring; and 5th—little.

Phalanges.—The small bones of the fingers are called phalanges, and each one of these bones is called a phalanx. Each finger has three phalanges, except the thumb (which has two). The phalanges are named for their anatomical position: The proximal phalanx is the bone closest to the hand; the distal phalanx is the bone at the end of the finger; and the middle phalanx is the bone located between the proximal and distal phalanges.

LOWER EXTREMITY.—The lower extremity includes the bones of the hip, thigh, leg, ankle, and foot. The bones that form the framework of the lower extremities are listed in table 1-2.

Table 1-2.—Bones of the Lower Extremities

BONE

COMMON
NAME

TOTAL
NUMBER
IN THE BODY

innominate hip bone

2

femur thigh bone

2

patella knee cap

2

tibia leg bone

2

fibula leg bone

2

tarsals ankle bones

14

metatarsals foot bones

10

phalanges toe bones

28

Innominate.—The innominate bone, commonly known as the hip, is a large, irregularly shaped bone composed of three parts: the ilium, ischium, and pubis (fig. 1-23). In children these three parts are separate bones, but in adults they are firmly united to form a cuplike structure, called the acetabulum, into which the head of the femur fits. The ilium forms the outer prominence of the hip bone (the crest of the ilium, referred to as the iliac crest, provides an anatomical landmark above the ilium); the ischium forms the hard lower part; and the pubis forms the front part of the pelvis.

Figure 1-23.—Pelvic girdle.

Symphysis Pubis.—The area where the two pubic bones meet is called the symphysis pubis and is often used in anatomical measurements. The largest foramen, or opening, is located in the hip bone, between the ischium and the pubis, and is called the obturator foramen (fig. 1-23). The crest of the ilium is used in making anatomical and surgical measurements (e.g., location of the appendix, which is approximately halfway between the crest of the ilium and the umbilicus).

Femur.—The femur, or thigh bone, is the longest bone in the body (fig. 1-24). The proximal end is rounded and has a head supported by a constricted neck that fits into the acetabulum. Two processes called the greater and lesser trochanters are at the proximal end for the attachment of muscles. The neck of the femur, located between the head and the trochanters, is the site on the femur most frequently fractured. At the distal end are two bony prominences, called the lateral and medial condyles, which articulate with the tibia and the patella.

Figure 1-24.—Anterior view of the right leg.

Patella.—The patella is a small oval-shaped bone overlying the knee joint. It is enclosed within the tendon of the quadriceps muscle of the thigh. Bones like the patella that develop within a tendon are known as sesamoid bones.

Tibia.—The tibia, or shinbone, is the larger of the two leg bones and lies at the medial side. The proximal end articulates with the femur and the fibula. Its distal end articulates with the talus (one of the foot bones) and the fibula (fig. 1-25). A prominence easily felt on the inner aspect of the ankle is called the medial malleolus.

Fibula.—The fibula, the smaller of the two leg bones, is located on the lateral side of the leg, parallel to the tibia. The prominence at the distal end forms the outer ankle and is known as the lateral malleolus.

Tarsus.—The tarsus, or ankle, is formed by seven tarsal bones: medial cuneiform, intermediate cuneiform, lateral cuneiform, cuboid, navicular, talus, and calcaneus. The strongest ofthese is the heel bone, or calcaneus.

Metatarsus.—The sole and instep of the foot is called the metatarsus and is made up of fivemetatarsal bones (fig. 1-25). They are similar in arrangement to the metacarpals of the hand.

Phalanges.—The phalanges are the bones of the toes and are similar in number, structure, and arrangement to the bones of the fingers.

Figure 1-25.—The foot: A. Lateral view of foot; B. Right foot viewed from above.