Skeletal System
1. Number of bones
a. Usually a human skeleton has 206 bones, but the
number may vary.
b. Extra bones in sutures are called sutural bones.
2. Divisions of the skeleton
a. The skeleton can be divided into axial and
appendicular portions.
b. The axial skeleton consists of the skull, hyoid
bone, vertebral column, and thoracic cage.
c. The appendicular skeleton consists of the pectoral
girdle, upper limbs, pelvic girdle, and lower limbs.
The skull consists of twenty-two bones, which include
eight cranial bones and fourteen facial bones.
1. Cranium
a. The cranium encloses and protects the brain and
provides attachments for muscles.
b. Some cranial bones contain air-F
lled paranasal
sinuses that help reduce the weight of the skull.
c. Cranial bones include the frontal bone, parietal
bones, occipital bone, temporal bones, sphenoid
bone, and ethmoid bone.
2. ±acial skeleton
a. ±acial bones form the basic shape of the face and
provide attachments for muscles.
b. ±acial bones include the maxillary bones, palatine
bones, zygomatic bones, lacrimal bones, nasal
bones, vomer bone, inferior nasal conchae, and
3. Infantile skull
a. Incompletely developed bones, connected by
fontanels, enable the infantile skull to change
shape slightly during childbirth.
b. Infantile skull bones are thin, somewhat fl
and less easily fractured.
The vertebral column extends from the skull to the pelvis
and protects the spinal cord. It is composed of vertebrae
separated by intervertebral discs. An infant has thirty-
three vertebral bones and an adult has twenty-six. The
vertebral column has four curvatures—cervical, thoracic,
lumbar, and sacral.
1. A typical vertebra
a. A typical vertebra consists of a body, pedicles,
laminae, spinous process, transverse processes,
and superior and inferior articulating processes.
b. Notches on the upper and lower surfaces of the
pedicles on adjacent vertebrae form intervertebral
foramina through which spinal nerves pass.
2. Cervical vertebrae
a. Cervical vertebrae comprise the bones of the neck.
b. Transverse processes have transverse foramina.
c. The atlas (F
rst vertebra) supports the head.
d. The dens of the axis (second vertebra) provides a
pivot for the atlas when the head turns from side
to side.
2. Endochondral bones
a. Most of the bones of the skeleton are endochondral.
b. They develop as hyaline cartilage that bone tissue
later replaces.
c. The primary ossiF
cation center appears in the
diaphysis, whereas secondary ossiF
cation centers
appear in the epiphyses.
d. An epiphyseal plate remains between the primary
and secondary ossiF
cation centers.
3. Growth at the epiphyseal plate
a. An epiphyseal plate consists of layers of cells:
zone of resting cartilage, zone of proliferating
cartilage, zone of hypertrophic cartilage, and zone
of calciF
ed cartilage.
b. The epiphyseal plates are responsible for bone
c. Long bones continue to lengthen until the
epiphyseal plates are ossiF
d. Growth in bone thickness is due to ossiF
beneath the periosteum.
e. The action of osteoclasts forms the medullary
4. Homeostasis of bone tissue
a. Osteoclasts and osteoblasts continually remodel
b. The total mass of bone remains nearly constant.
5. ±actors affecting bone development, growth, and
a. DeF
ciencies of vitamin A, C, or D result in
abnormal bone development.
b. InsufF
cient secretion of pituitary growth hormone
may result in dwarF
sm; excessive secretion may
result in gigantism, or acromegaly.
c. DeF
ciency of thyroid hormone delays bone growth.
d. Male and female sex hormones promote bone
formation and stimulate ossiF
cation of the
epiphyseal plates.
1. Support, protection, and movement
a. Bones shape and form body structures.
b. Bones support and protect softer, underlying
c. Bones and muscles interact, producing movement.
2. Blood cell formation
a. At different ages, hematopoiesis occurs in the yolk
sac, the liver, the spleen, and the red bone marrow.
b. Red marrow houses developing red blood cells,
white blood cells, and blood platelets.
3. Inorganic salt storage
a. The extracellular matrix of bone tissue contains
abundant calcium phosphate in the form of
b. When blood calcium ion concentration is low,
osteoclasts resorb bone, releasing calcium salts.
c. When blood calcium ion concentration is high,
osteoblasts are stimulated to form bone tissue and
store calcium salts.
d. Bone stores small amounts of sodium, magnesium,
potassium, and carbonate ions.
e. Bone tissues may accumulate lead, radium, or
previous page 271 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 273 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off