HUMAN BIOLOGY

the SKeLetaL SyStem 101

section 5.3 The skeleton has an

axial portion and an appendicular portion

(Table 5.2). The axial skeleton forms the

body’s vertical axis and is a central support

structure. In the spine, intervertebral disks of

fibrocartilage are shock pads and flex points.

Skull bones form the brain case, which

protects the brain. Sinuses in the frontal bone

reduce the skull’s weight.

patella

tibia

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zygomatic bone

mandible

frontal

bone

temporal

bone

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suMMary

section 5.1 Bones are organs that
contain bone tissue and other connective
tissues, nerves, and blood vessels.
A bone develops as osteoblasts secrete
collagen fibers and a matrix of protein and
carbohydrate. Calcium salts are deposited and harden the
matrix. Mature living bone cells, osteocytes, are located inside
spaces (lacunae) in the bone tissue.
Bones have both compact and spongy bone tissue. Denser
compact bone is organized as thin, circular layers called
osteons. In spongy bone, needlelike struts are fused in a lattice.
A cartilage model provides the pattern for a developing
bone. Long bones lengthen at their ends (epiphyses) until early
adulthood when bone growth ends.
Bones grow, gain strength, and are repaired by remodeling.
In this process, osteoblasts deposit bone and osteoclasts break
it down.

section 5.2 As the main elements of the skeleton,

bones interact with skeletal muscles to move body

parts. Bones also store minerals and help protect and

support other body parts. Ligaments connect bones

at joints; tendons attach muscles to bones or to other

muscles. Some bones, including the sternum, hip

bones, and femur, contain bone marrow. Blood cells

are produced in red bone marrow.

Appendicular skeleton

Pectoral girdle: clavicle and scapula

Arm: humerus, radius, ulna

Wrist and hand: carpals, metacarpals, phalanges (of fingers)

Pelvic girdle (6 fused bones at the hip)

Leg: femur (thighbone), patella, tibia, fibula

Ankle and foot: tarsals, metatarsals, phalanges (of toes)

Axial skeleton

Skull: cranial bones and facial bones

Rib cage: sternum (breastbone) and ribs (12 pairs)

Vertebral column: vertebrae (26)

Table 5.2 Review of the skeleton’s Parts

expLore on your oWn

When it comes to the skeleton and joints, your own body can be a great learning tool.

• Feel along the back of your neck beginning at your hairline. Can you feel any lumps made by the bony
  processes of your spinal vertebrae (Figure 5.6)? Locate the C7 vertebra, which in most people is the
  most prominent. Can you feel it at the base of your neck?


• While seated, feel your kneecap—the patella—move as you flex and extend your lower leg. Just below
  the patella you should also be able to feel a ligament that attaches it to your tibia. Can you find the
  upper protuberance of your tibia? Moving your fingers around to outside of the joint, can you feel the
  knobby upper part of the fibula?


• Using the diagram at right as a guide, see if you can locate the ridges of your frontal bone above your
  eyebrows; the arching part of your zygomatic bone, which forms your “cheekbones”; and the joint
  where your lower jaw articulates with the temporal bone.

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