CHAPTER 7: Anatomy of Bones and Joints

Anatomy of Bones and Joints

         I. General Considerations in Bones

Osteology is the branch of medicine concerned with the development and diseases of   bone tissue. The human skeleton is composed of 270 bones in the newborn, 222 bones in children and 206 bones in adults.

The skeletal system may be divided into two functional parts:

• The axial skeleton consists of the bones of the head (cranium or skull), neck (hyoid bone and cervical vertebrae), and trunk (ribs, sternum, vertebrae, and sacrum).
• The appendicular skeleton consists of the bones of the limbs, including those forming the pectoral and pelvic girdles.

Out of the 206 bones an adult has, 126 of them exist in the appendicular skeleton, whereas 80 of them exist in the axial skeleton.

Bone is a living tissue capable of changing its structure as the result of the stresses to which it is subjected. Like other connective tissues, bone consists of cells, fibers, and matrix. It is one of the hardest structures of the animal body, because of the calcification of its extracellular matrix. Living bones have some elasticity (results from the organic matter) and great rigidity (results from their lamellous structures and tubes of inorganic calcium phosphate). Its color, in a fresh state, is pinkish-white externally, and deep red within.

II. Axial Skeleton

The axial skeleton consists of the 80 bones along the central axis of the human body. It is composed of six parts; the human skull, the ossicles of the middle ear, the hyoid bone of the throat, the rib cage, sternum and the vertebral column. The axial skeleton and the appendicular skeleton together form the complete skeleton.

Skull

The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.

The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates. The skull is a part of the skeleton.

Functions of the skull include protection of the brain, fixing the distance between the eyes to allow stereoscopic vision, and fixing the position of the ears to help the brain use auditory cues to judge direction and distance of sounds. In some animals, the skull also has a defensive function (e.g. horned ungulates); the frontal bone is where horns are mounted.

The English word "skull" is probably derived from Old Norse "skalli" meaning bald, while the word cranium comes from the Greek root kranion.

Different Views of the Skull

Superior View

Posterior View

Lateral View

Anterior View

Inferior View

                    Vertebral Column

                 The vertebral column forms the central part of the skeleton. It supports the skull and protects the spinal cord. It also serves as attachment for the ribs, the pectoral and pelvic girdles. The vertebral column consists of separate bones, the vertebrae. The different vertebrae are arranged above each other. Because the separate vertebrae are attached to each other by means of fibrous cartilaginous discs they form a flexible column. Each vertebra has articular surfaces above and below, which allow articulation movement between them.

                The vertebral column of 33 vertebrae is divided into five regions according to their position and structure. The five regions consist of: Seven cervical (neck) vertebrae, Twelve thoracic (chest) vertebrae, Five lumbar vertebrae, Five fused sacral vertebrae, and Four fused vertebrae.

        The Cervical Vertebrae

• The neck region consists of 7 cervical vertebrae. These are the smallest vertebrae in the vertebral column. The first two cervical vertebrae are known as the atlas and axis. They are specially adapted to support the skull and to enable it to move. They differ from the structure of the typical vertebra in certain respects.

       The Atlas

• The atlas is the first neck vertebra and supports the skull. It is ring-shaped and has no centrum. A neural spine is absent. The atlas consists of posterior and anterior neural arches and 2 short transverse processes. The spinal foramen (neural canal) is very large. The 2 occipital condyles of the skull fit into the articulating facets on the upper surface of the atlas, on either side of the neural canal. On its lower surface (inferior) surface the atlas has 2 articular surfaces for articulation with the axis.

         

          The Axis

• The axis has a large, strong neural spine. The centrum is small and has become modified to bear the odontoid process (a tooth-like projection) on its upper surface. The odontoid process fits against the facet in the anterior arch of the atlas. This forms a pivot joint or axis, around which the atlas (together with the skull) can rotate, so allowing the head to turn from side to side.

           The Thoracic Vertebrae

• There are 12 thoracic vertebrae. The centrum is large and sturdy and the neural spines are long and directed downwards. The long neural spines form an anchorage for the muscles and ligaments that support the head and neck. The head (or capitulum) of each of the first 10 pairs of ribs fits into and articulates with the semi-circular facet which is situated between two successive centra, i.e. between the inferior surface of one and the superior surface of the next centrum. These facets occur on both sides of the centrum. The tubercle of the rib articulates with the facet at the tip of the transverse process.

   

        The Lumbar Vertebrae

• These 5 vertebrae are the largest and strongest in the vertebral column. The transverse processes are very long for the attachment of the powerful back muscle that maintain the posture and flex the spine in movement.

          The Sacrum

• The sacrum is roughly triangular in shape and consists of 5 fused vertebrae. It lies between the hip bones, with which it articulates. Horizontal ridges indicate the divisions between the fused vertebrae. At the ends of these ridges are openings which allow nerves and blood vessels to pass through.

          The Coccyx

• The coccyx consists of 4 fused tail vertebrae which are small and have a relatively simple structure. They do not resemble the structure of a typical vertebra. The muscles of the buttocks are attached to the coccyx.

          The Ribs

• Twelve pairs of ribs articulate with the 12 vertebrae of the thoracic region. The ribs are flat, narrow bones with a distinctive bow-shaped curve. Each rib consists of a head or capitulum, a small tubercle (which is a short distance back from the head) and the shaft. The head of the rib articulates with the semi-circular articulating facets formed by the centra of two successive thoracic vertebrae. The tubercle fits into and articulates with the articulating facets on the transverse process. The first seven ribs on each side are joined to the breastbone by bars of hyaline cartilage (called costal cartilage in this region). The first seven pairs of ribs are referred to as true ribs. The cartilages of the 8th, 9th and 10th ribs are joined to the costal cartilage of the rib immediately above (i.e. to the costal cartilage of the 7th rib). These three pairs of ribs are known as vertebrochondral ribs. The last two pairs of ribs have free ends which are not attached to the sternum at all. They are floating ribs. The vertebrochondral ribs and the floating ribs are collectively known as false ribs. The ribs (together with their muscles) play an important role in the breathing mechanism of a mammal.

          The Sternum (Breastbone)

• The sternum is a long, flat, dagger-shaped bone. It is about 15 - 18 cm long and is found in the center of the chest region. The broad upper end supports the collar bones. The first seven pairs of ribs are attached to the articulating facets on the sides of the sternum. The 12 thoracic vertebrae, the 12 pair of ribs and the sternum forms the thorax which protects the delicate and vital organs of the thorax, viz. the heart and lungs.

               Thoracic Cage

            Thoracic Cage is made up of 24 bones arranged in 12 pairs. These bones are divided into three groups: true ribs, false ribs and floating ribs.

           The twelve pairs of ribs are often called the "rib cage." This is because they form a kind of cage that encloses the upper body. This cage gives the chest its familiar barrel-like shape.

           The ribs serve several important purposes. They protect the heart and lungs from injuries and shocks that might damage them. Ribs also protect parts of the stomach, spleen, and kidneys. The ribs help you to breathe. As you inhale, the muscles in between the ribs lift the rib cage up, allowing the lungs to expand. When you exhale, the rib cage moves down again, squeezing the air out of your lungs.

              III. Appendicular Skeleton
          The appendicular skeleton consists of the girdles and the skeleton of the limbs. The upper (anterior) limbs are attached to the pectoral (shoulder) girdle and the lower (posterior) limbs are attached to the pelvic (hip) girdle.

           The Pectoral Girdle

       The Pectoral girdle consists of two shoulder blades (scapulae) and two collar bones (clavicles). These bones articulate with one another, allowing some degree of movement.

          Shoulder Blades (Scapulae)

• The shoulder blade is a flat triangular bone which stretches from the shoulder to the vertebral column at the back. On the back side it has a bony ridge for the attachment of the muscles. The bony ridge forms a prominent projection, the acromion, above the shoulder joint. Beneath the collar bone and just on the inside of the shoulder joint, is another bony projection of the shoulder blade, the coracoid process, which also serves for the attachment of muscles. The upper outer corner of the shoulder blade ends in the glenoid cavity into which fits the head of the upper arm bone, forming a ball and socket joint.

         

         Collar Bones (Clavicles)

• Each collar bone is rod-shaped and roughly S-shaped. It lies horizontally and articulates with the upper end of the breastbone, right in the middle and front, just above the first rib. The lateral end articulates with the acromium. Collar bones serve as a support for the shoulder blades in front and keep the shoulder blades back so that the arms can hang freely at the sides of the body. They prevent the pectoral girdles from getting out of joint easily and ample movement of the shoulders.

               The Upper Limbs

               The skeleton of the upper limbs or arm may be divided into five main regions: an upper arm bone, the forearm (radius and ulna), the wrist, the palm of the hand and the fingers.

          The Upper Arm (Humerus)

• The upper arm is a single long bone. The upper end consists of a hemi-spherical ball which fits into the socket of the shoulder blade to form the shoulder joint. The lower end of the humerus forms a shallow ball and socket joint with the radius and a hinge joint with the ulna in the elbow.

          The Forearm (Radius and Ulna)

• The two long bones of the forearm are known as the radius and the ulna. The ulna is the larger of the two bones and is situated on the inner side (i.e. the little finger side) of the forearm. The upper end of the ulna articulates with the lower end of the humerus forming a strong hinge joint in the elbow region. The lower end of the ulna is slender and plays a minor role in the formation of the wrist joint. The radius is situated on the thumb side of the forearm. Its upper end articulates with both the humerus and the ulna. The broad, lower end of the radius forms a major part of the wrist joint, where it articulates with the wrist bones (carpals). The radius also allows the forearm to be rotated. The radio-ulnar joints are pivot joints in which the moving bone is the radius. As the head of the radius pivots at these joints, the lower end of the radius moves round the lower head of the ulna.

          The Wrist

• The wrist consists of eight carpal bones. These are small, short bones that are arranged in two rows of four. They have articulating facets which allow them to slide over one another.

          The Palm of the Hand

• The palm is supported by five long metacarpals. The metacarpals articulate with carpals at one end and with the phalanges at the other end.

          The Fingers

• The fingers are made up of fourteen phalanges. There are three phalanges in each finger but only two in the thumb.

                   The Pelvic Girdle

                 The pelvic girdle consists of two large, sturdy hip bones. Each hip bone consists of three fused bones namely the ilium, ischium and the pubis. The ilium is the largest of the three and forms the upper part of the hip bones. The sacrum fits like a wedge posteriorly between the two hip bones. The sacrum has a large, flat articular surface on each side for articulation with the ilia. The ischium forms the inferior part of the hip bone and the pubis the central in front. The two pubic bones are attached in the middle, on the front side by a symphysis which consists of fibrocartilage and ligaments, the pubic symphysis. The two hip bones and the sacrum form a complete bony ring, the pelvis . On the outer side of the point where the fused bones meet, there is a deep hip socket into which the head of the femur fits.

             The pelvic girdle forms a strong support for the attachment of the limbs. Strong muscles of the back, the legs and the buttocks are attached to it. It protects some of the internal organs. In females it forms a strong basin-like structure for supporting and protecting the developing fetus during child-bearing.

     

                  The Lower Limbs

                The skeleton of the lower limb may be divided into five main regions: the upper leg (thigh), the lower leg, the ankle, the arch of the foot and the toes.

          The Upper Leg or Thigh

• The upper leg has a single long bone, the femur and is the longest bone in the body. The head of the femur is turned slightly inwards and has a large, rounded portion which articulates in the acetubulum, forming a ball-and-socket joint. At its distal end, the femur widens to form two large knobs (condyles) which form the hinged knee joint with the main long bone (tibia) of the lower leg. On the anterior side of these two condyles, there is an articular surface against which the kneecap (patella) slides. The patella is a small, triangular, flat bone which develops on the tendon of the thigh muscle and is attached by ligaments to the tibia. This enables movement in the knee joint.

         The Lower Leg

• The two bones of the lower leg are the tibia (shinbone) in front and the fibula behind. The tibia is the larger of the two and extends from the knee to the ankle. The upper end of the tibia has two articulating facets into which the condyles of the femur fit to form the knee joint. The lower end of the tibia articulates with one of the tarsals to form the ankle joint. The fibula is smaller than the tibia and is situated on the outside and slightly behind it. The upper end articulates with the tibia but does not form part of the knee joint. The lower end forms part of the ankle joint.

          The Ankle

• There are seven short, thick tarsal bones, the largest of which is the heel bone (calcaneum), which presses firmly onto the ground when one stands, walks or runs. The calf muscles are attached to the calcenum, allowing the heel to be lifted during locomotion.

          The Arch of the Foot

• The arch is formed partly by some of the tarsals but mainly by the five long metatarsals, which extends from the tarsals to the toes. The arch is modified for receiving the weight of the body.

          The Toes

• There are fourteen short phalanges in the toes of each foot. The big toe has two phalanges and the other toes have three in each.