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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 (Figures 1A-C). 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. It is important to understand the relationship of the bones and muscles as they work together to provide support and movement for the human body.

Figure 1.—Skeleton. A, Anterior view. Skeleton. B, Posterior view. C, Lateral view

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. Inorganic mineral salts give bone its strength and hardness.

The bones of the human skeleton provide rigid support for muscles and skin, and serve to protect the easily injured organ systems of the body. Bone itself is a living, highly vascular tissue, which is made up of both inorganic (minerals) and organic (cells & connective tissue fiber) elements. The inorganic component of bone serves as a warehouse for calcium and phosphorous, two essential minerals for the body.

Bone consists of a hard outer shell, called compact bone, and an inner spongy, porous portion, called cancellous tissue (Fig. 2). In the center of the bone is the medullary canal, which contains marrow. There are two types of marrow, yellow and red. 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.

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 bone.

Figure 2.—Anatomy of a Long Bone

BONE CLASSIFICATIONS

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

Long bones: Femur and humerus

Short bones: Wrist and ankle bones

Flat bones: Skull, sternum, and scapula

Irregular bones: Vertebrae, mandible, and pelvic bones

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. 3 and 4), 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 bones of the face are a complex framework that helps to form facial features, the upper jaw (maxilla) and lower jaw (mandible). With the exception of the mandible and the bones of the inner ear, all skull bones are joined together firmly along seams. The seams where they join are known as sutures. The bones of the skull are classified as either cranial or facial bones.


Figure 3.—Anterior view of the skull. (Courtesy Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.)

    Cranial Bones. The cranium is formed by eight major bones, most of which are in pairs. Frontal Bone. The frontal bone forms the front part of the skull above the eyes, which includes the forehead and part of the nasal cavity. In children, the frontal bone develops as two parts. They are usually fused together by age 5 or 6. The two frontal sinuses (air spaces in the bone) are located above each eye socket. Parietal Bones. The two parietal bones are located behind the frontal bone. These bones form the greater part of the right and left sides and the roof of the skull. They each have four borders and are shaped like a curved plate.

    Temporal Bones. The temporal bones form the sides and part of the base of the skull in the area of the ear. One temporal bone is located on each side. It is readily recognized as “fan-shaped.” Each encloses the internal ear structures and has depressions called glenoid fossae that form the articulation with the mandible. The zygomatic process of the temporal bone projects out into the zygomatic bone of the face and forms the lateral part of the zygomatic arch. Both the glenoid fossae and zygomatic process can be seen in Figure 6-4.


Figure 4.—Skull viewed from the right side. (Courtesy Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.)

    Occipital Bone. The occipital bone forms the back part of the skull and the base of the cranium. It joins with the parietal and temporal bones. In the center, underside (inferior) portion of the cranium, there is a large opening called the foramen magnum (Fig. 4), through which nerve fibers from the brain pass and enter into the spinal cord.

    Ethmoid Bone. The ethmoid bone is situated in front of the sphenoid bone in the front part of the cranium (Fig. 3). Through small openings in this bone pass nerves to the roof of the mouth that are responsible for sense of smell.

    Sphenoid Bone. The sphenoid bone is posterior to the ethmoid bone providing for the front base of the cranium and forming the floor and sides of the orbits (Fig. 3)

Bones of the Face.—The facial skeleton consists of 14 stationary bones and a mobile lower jawbone (mandible). These 14 bones form the basic shape of the face, and are responsible for providing attachments for muscles that make the jaw move and control facial expressions. Figures 3 and 4 show the bones of the face. 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 inferior nasal conchae are curved, fragile, scroll-shaped bones that lie in the lateral walls of the nasal cavity. They provide support for mucous membranes within the nasal cavity.

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

The mandible is horseshoe-shaped, with an upward sloping portion at each end called the ramus. The rami are divided into two different processes:

    Condyloid process—Also called mandibular condyle, located posterior on the ramus and forms the head of the mandible. It is knuckleshaped, and articulates in the glenoid fossa of the temporal bone to form the temporal mandibular joint.

    Coronoid process—Located anterior of the condyle, and provides attachment for the temporal’s muscle, which helps lift the mandible to close the mouth.

Other important anatomical landmarks of the mandible that the HM should be able to recognize are as follows:

    Alveolar process—Supports the teeth of the mandibular arch.

    Mental protuberance—Also referred to as the chin and is located at the midline of the mandible.

    Mental foramen—Located on the facial surfaces of the mandible on both the right and left sides, just below the second premolars. This opening contains the mental nerve and blood vessels.

    Body—The heavy, horizontal portion of the mandible below the mental foramen extending to the angle.

    Angle—Juncture where the body of the mandible meets with the ramus.

    Mandibular foramen—Located near the center of each ramus on the medial side (inside), through this opening passes blood vessels and the interior alveolus nerve, which supply the roots of the mandibular teeth. This is a common area where the dental officer will inject anesthetic to block the nerve impulses and make the teeth on that side insensitive (numb).

BONES OF THE EAR

In each middle ear (Fig. 5) and located in the auditory ossicles are three small bones named the malleus (hammer), incus (anvil), and stapes (stirrup). Their function is to transmit and amplify vibrations to the ear drum and inner ear.

Figure 5.—The ear. External, middle, and inner ear. (Anatomical structures are not drawn to scale.)

VERTEBRAL (SPINAL) COLUMN

The vertebral column consists of 24 movable or true vertebrae, the sacrum, and the coccyx or tail bone (Fig. 6). 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. This vertebral body supports not only the spinal cord but other structures of the body as well. At the bottom of the spinal column are the sacrum and the coccyx. Many of the main muscles are attached to the vertebrae.

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), lumbar (lower back), and sacral and coccygeal (pelvis).

Figure 6.—The vertebral column. A, Right lateral view. B, Anterior view. C, Posterior view. The photo inset shows a midline sagittal magnetic resonance image (MRI) of the vertebral column.

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 (Fig. 6). Each cervical vertebra has a transverse (or intervertebral) foramen (Fig. 6A) 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 bottom of the neck. This landmark makes it possible for physicians to count and identify the vertebrae above and below it.

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 region (chest) or rib 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. 7). 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.

Figure 7.—Thoracic cage. Note the costal cartilages and their articulations with the body of the sternum.

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, arm, forearm, wrist, and hand.

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. 8). 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 (Fig.8). Its lateral corner forms part of the shoulder joint, articulating with the humerus.

Figure 8.— Right scapula. A, Anterior view. B, Posterior view. C, Lateral view. D, Posterior view showing articulation of the right scapula with the clavicle. (The inset shows the relative position of the right scapula within the entire skeleton.)

Humerus. The humerus is the longest bone of the upper extremity and is often called the arm bone (Fig. 6-19). 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 theses surfaces articulate with the bones of the forearm.

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. 9). 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.

Figure 9.—Radius and Ulna

Carpal. There are eight carpal bones, arranged in two rows, forming the wrist (Figure 10).

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 (Fig.10).

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 (Fig. 10)

Figure 10.— Bones of the hand and wrist. A, Dorsal view of the right hand and wrist. B, Palmar view of the right hand and wrist. (Courtesy Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.)

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.

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. 11). 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.

Table 1.—Bones of the Lower Extremity

Figure 11.— The female pelvis. A, Pelvis viewed from above. Note that the brim of the true pelvis (dotted line) marks the boundary between the superior false pelvis (pelvis major) and the inferior true pelvis (pelvis minor). B and C, Pelvis viewed from below. (C: Courtesy Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.)

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.

    Symphysi 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. 11).

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. 12). 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.

    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 (Fig. 12D).

Figure 12.— Bones of the thigh and leg. A, Right femur, anterior surface. B, Right femur, posterior view. C, Right tibia and fibula, anterior surface. D, Anterior aspect of the right knee skeleton. E, Right tibia and fibula, posterior aspect. (The inset shows the relative position of the bones of the thigh and leg within the entire skeleton.) (D-E: Courtesy Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.)

    Tibia. The tibia, or shin bone, 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. 12C). 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 (Fig. 12C). 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 of these is the heel bone, or the calcaneus (Fig. 13).

    Metatarsus. The sole and instep of the foot is called the metatarsus and is made up of five metatarsal bones (Fig. 13). 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 (Fig. 13).

Figure 13.— The foot. A, Bones of the right foot viewed from above. The tarsal bones consist of the cuneiforms, navicular, talus, cuboid, and calcaneus. B, Posterior aspect of the right ankle skeleton and inferior aspect of the right foot skeleton. C, X-ray film of the left foot showing prominent sesamoid bones (arrows) near the distal end (head) of the first metatarsal bone of the great toe. (B: Courtesy Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.)


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David L. Heiserman, Editor

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Revised: June 06, 2015