Chapter 5 The Skeletal System
Although the word skeleton comes from the Greek word meaning “dried-up body,” our internal framework is so beautifully designed and engineered that it puts any modern skyscraper to shame. Strong, yet light, it is perfectly adapted for its functions of body protection and motion.
The skeleton is subdivided into two divisions: the axial skeleton, the bones that form the longitudinal axis of the body, and the appendicular skeleton, the bones of the limbs and girdles. In addition to bones, the skeletal system includes joints, cartilages, and ligaments (fibrous cords that bind the bones together at joints). The joints give the body flexibility and allow movement to occur.
Bones: An Overview
· Identify the subdivisions of the skeleton as axial or appendicular.
o Axial – bones that form the longitudinal axis of the body - skull, vertebral column, ribs, sternum
o Appendicular – bones of the limbs and girdles – all others
· List at least three functions of the skeletal system.
o Support – framework that supports and anchors soft organs
o Protection – protect soft organs
o Movement – place for skeletal muscles to attach and use bones as levers to move the body
o Storage – fat is stored in internal cavities, also stores minerals such as calcium and phosphorus
o Blood cell formation – aka hematopoiesis – occurs within the marrow of certain bones
· Name the four main kinds of bones.
o Long bones – longer than wide with shaft and heads at both ends made mostly of compact bone (dense and looks smooth and homogeneous) – all bones of limbs except wrist and ankle bones
o Short bones – cube-shaped and contain mostly spongy bone (small needlelike pieces of bone and lots of open space) – bones of the wrists and ankles, sesamoid bones (form within tendons), and patella
o Flat bones – thin, flattened, usually curved with two thin layers of compact bone sandwiching a layer of spongy bone between – most bones of the skull, ribs, sternum
o Irregular bones – all others that don’t fit above – vertebrae, hip bones
· Identify the major anatomical areas of a long bone.
o Diaphysis – shaft – bone’s length and mostly compact bone
o Periosteum – fibrous connective tissue membrane covering diaphysis
§ Sharpey’s fibers – hundreds of connective tissue fibers secure the periosteum to the underlying bone
o Epiphyses – ends of the long bones and made of a thin layer of compact bone enclosing an area filled with spongy bone
o Articular cartilage – covers the ends of long bones – glassy hyaline cartilage that provides a smooth, slippery surface that decreases friction at joint surfaces
o Epiphyseal line – remnant of plate that covers epiphysis in adults
o Epiphyseal plate – flat plate of hyaline cartilage seen in young, growing bones – causes lengthwise growth of a long bone – is replaced by bone when hormones stop bone growth by the end of puberty
o Yellow marrow – aka medullary cavity – cavity if the shaft that is primarily a storage area for adipose (fat) tissue – in infants it forms blood cells
o Red marrow – confined to cavities of spongy bone of flat bones and epiphyses of some long bones – makes blood cells
· Explain the role of bone salts and the organic matrix in making bone both hard and flexible.
o Calcium salts deposited in the matrix give bone its hardness, whereas the organic parts (collagen fibers) provide for bone’s flexibility and great tensile strength
· Describe briefly the process of bone formation in the fetus and summarize the events of bone remodeling throughout life.
o The skeleton of an early fetus is primarily hyaline cartilage while the skeleton of an young child, the cartilage is replaced by bone and cartilage only remains in isolated areas such as the bridge of the nose, parts of the ribs, and the joints
o Most bones develop using hyaline cartilage structures as models in a process called ossification that occurs in two steps
§ Hyaline cartilage model is completely covered with bone matrix by bone-forming cells called osteoblasts leaving the fetus with cartilage “bones” enclosed by “boney” bones
§ Then, the enclosed hyaline cartilage model is digested away, opening up a medullary cavity within the newly formed bone leaving only two areas by the time of birth – articular cartilage that covers bone ends and persists for life and the epiphyseal plates which allow longitudinal growth
o Osteoblasts in the periosteum add bone tissue to the external face of the diaphysis as osteoclasts in the endosteum remove bone from the inner face of the diaphysis wall – a process called appositional growth
o Bone growth is caused by the growth hormone which is most active during puberty and at the end of puberty the epiphyseal plates are completely converted to bone
o Bone is a dynamic and active tissue – bones are constantly remodeled in response to two factors
§ Calcium levels in blood – when calcium levels drop in the blood, the parathyroid releases parathyroid hormone (PTH) into the blood which activates osteoclasts to destroy bone matrix and add calcium ions to the blood – when blood calcium levels are too high, calcium is deposited in bone matrix as hard calcium salts
§ The pull of gravity and muscles on the skeleton – bones of the bedridden or physically inactive people tend to lose mass and to atrophy, because they are no longer subjected to stress
§ PTH tells when bone is broken down or formed in response to calcium levels while stress of muscle pull and gravity tell where bone matrix is broken down or formed
· Name and describe the various types of fractures.
o Closed (simple) – bone breaks cleanly but does not penetrate the skin
o Open (compound) – broken bone ends penetrate through the skin
o Comminuted – bone breaks into many fragments
o Compression – bone is crushed
o Depression – broken bone portion is pressed inward
o Impact – broken bone ends are forced into each other
o Spiral – ragged breaks occur when excessive twisting forces are applied to a bone
o Greenstick – bones break incompletely (only half way)
· Describe the process of how a bone fracture is repaired
o Hematoma is formed – blood-filled swelling that deprives bone cells of nutrients and causes them to die
o Break is splinted by a fibrocartilage callus that contains cartilage matrix, bony matrix, and collagen fibers to close the gap – and there is growth of new capillaries into the clotted blood and there is phagocytes disposing dead material
o The bony callus is formed – as more osteoblasts and osteoclasts migrate to the area and multiply, fibrocartilage is gradually replaced by a callus of spongy bone
o Over next few months, bony callus is remodeled in response to mechanical stress placed on it
Axial Skeleton
· On a skull or diagram, identify and name the bones of the skull.
o See diagrams on pages 126 and 127 and 129
· Describe how the skull of a newborn infant (or fetus) differs from that of an adult, and explain the function of fontanels.
o The infants face is very small compared to the size of its cranium but the skull as a whole is large compared to the total body length equaling 1/4th of the total length
o The adult skull represents only 1/8th of the total body length
o The skull of the newborn still has some areas of hyaline cartilage that has not converted to bone and these fibrous membranes connecting the cranial bones are called fontanels (soft spots) which allow the fetal skull to be compressed slightly during birth and they allow the infant’s brain to grow during the later part of pregnancy and early infancy, which would not be possible if the skull was fused in sutures already
· Name the parts of a typical vertebra and explain in general how the cervical, thoracic, and lumbar vertebrae differ from one another.
o See diagrams on page 131 and 133
o Cervical
§ The typical cervical vertebrae are the smallest, lightest and most often their spinous processes are short and divided into two branches
§ atlas (C1) has no body and its superior surface of the transverse process contains large depression that receive the occipital condyles of the skull, allows you to nod yes
§ axis (C2) acts as pivot for the rotation of the atlas, it has a large upright process, the odontoid process (dens), which acts as the pivot point, allows you to indicate no
o Thoracic (T1-T12) – are all typical, they are larger than the cervical vertebrae, the body is somewhat heart-shaped and has two costal demifacets on each side, which receive the heads of the ribs, the spinous process is long and hooks sharply down
o Lumbar (L1-L5) – have massive, block-like bodies with short, hatchet-shaped spinous processes, these are the sturdiest of the vertebrae due to the stress they are under
o Sacrum – formed by fusion of five vertebrae
o Coccyx – formed from the fusion of three to five tiny, irregularly shaped vertebrae – tailbone
· Discuss the importance of the intervertebral discs and spinal curvatures.
o The single vertebrae are separated by pads of flexible fibrocartilage – intervertebral discs – which cushion the vertebrae and absorb shocks
§ In youth, the discs have a high water content and are spongy and compressible
§ In old age, the water content is less and the disks become harder and less compressible
o The discs and the S-shaped structure of the spine work together to prevent shock to the head and help make the body trunk flexible and allow you to walk upright
§ Primary curvatures – thoracic and sacral regions present at birth
§ Secondary curvatures – develop later – cervical appears when baby raises its head and lumbar curvature develops when the baby begins to walk
· Explain how the abnormal spinal curvatures (scoliosis, lordosis, and kyphosis) differ from one another.
o See diagram on page 132
o Scoliosis – spine curves to left or right
o Kyphosis – spine curves outward at shoulders (upper thoracic)
o Lordosis - spine curves inward in lumbar region
Appendicular Skeleton
· Identify on a skeleton or diagram the bones of the shoulder and pelvic girdles and their attached limbs.
o See diagrams on pages 139 – 145
· Describe important differences between a male and female pelvis.
o The female inlet is larger and more circular
o The female pelvis as a whole is shallower, and the bones are lighter and thinner
o The female ilia flare more laterally
o The female sacrum is shorter and less curved
o The female ischial spines are shorter and farther apart, thus the outlet is larger
o The female pubic arch is more rounded because the angle of pubic arch is greater
Joints
· Name the three major categories of joints and compare the amount of movement allowed by each.
o Fibrous joints – where bones are united by fibrous tissue – sutures of the skull – allow essentially no movement
o Cartilaginous joints – where the bone ends are connected by cartilage – pubic symphysis of the pelvis and intervertebral joints of the spine are slightly moveable joints (amphiarthrotic) – hyaline cartilage epiphyseal plates of growing bones and cartilaginous joints between first ribs and sternum are immovable cartilaginous joints (synarthrotic)
o Synovial joints – where articulating bone ends are separated by a joint cavity containing synovial fluid
§ All have four things in common
§ Articular cartilage – covers ends of the bones of the joint
§ Fibrous articular capsule – encloses the joint surfaces and is lines with a smooth synovial membrane
§ Joint cavity – contains the synovial fluid
§ Reinforcing ligaments usually present
§ Types based on shape
§ Plane joint – intercarpal joints of wrist – articular surface essentially flat and only short slipping or gliding movements allowed – nonaxial joints
§ Hinge joint – elbow, ankle, joints between phalanges of fingers – cylindrical end of one bone fits into a trough-shaped surface of another bone – allow movement around one axis only – uniaxial joints
§ Pivot joint – proximal radioulnar joint and the joint between the atlas and the dens of the axis – rounded end of one bone fits into a sleeve or ring of bone – rotating bone can turn only around its long axis, these are also called uniaxial joints
§ Condyloid joint – knuckles – egg-shaped articular surface of one bone fits into an oval concavity in another, both of these articular surfaces are oval and allow the moving bone to travel from side to side and back and forth but it cannot rotate around its long axis – movement occurs around two axes – biaxial joint
§ Saddle joint – carpometacarpal joints in the thumb – each articular surface has both convex and concave areas – allow same movements as condyloid such as twiddling your thumbs – biaxial joints
§ Ball-and-socket – shoulder and hips – spherical head of one bone fits into a round socket in another – allow movement in all axes including rotation – multiaxial joints
Developmental Aspects of the Skeleton
· Identify some of the causes of bone and joint problems throughout life.
o Arthritis – inflammatory or degenerative disease that damages the joints – all forms begin with pain, stiffness, and swelling of the joint then depending on the type, certain changes in the joint structure occur