Several hormones make important contributions to
bone growth and maintenance. These include
growth hormone, thyroxine, parathyroid hormone,
and insulin, which help regulate cell division, pro-
tein synthesis, calcium metabolism, and energy
production. The sex hormones estrogen or testos-
terone help bring about the cessation of bone
growth. The hormones and their specific functions
are listed in Table 6–1.- Exercise or “stress”—for bones, exercise means
bearing weight, which is just what bones are spe-
cialized to do. Without this stress (which is nor-
mal), bones will lose calcium faster than it is
replaced. Exercise need not be strenuous; it can be
as simple as the walking involved in everyday activ-
ities. Bones that do not get this exercise, such as
those of patients confined to bed, will become thin-
ner and more fragile. This condition is discussed
further in Box 6–2: Osteoporosis.
THE SKELETON
The human skeleton has two divisions: the axial skele-
ton, which forms the axis of the body, and the appen-
dicular skeleton, which supports the appendages or
limbs. The axial skeleton consists of the skull, vertebral
column, and rib cage. The bones of the arms and
legs and the shoulder and pelvic girdles make up the
appendicular skeleton. Many bones are connected to
other bones across joints by ligaments, which are
strong cords or sheets of fibrous connective tissue. The
importance of ligaments becomes readily apparent
when a joint is sprained. A sprain is the stretching or
even tearing of the ligaments of a joint, and though the
bones are not broken, the joint is weak and unsteady.
We do not often think of our ligaments, but they are
necessary to keep our bones in the proper positions to
keep us upright or to bear weight.
There are 206 bones in total, and the complete
skeleton is shown in Fig. 6–4.SKULL
The skullconsists of 8 cranial bones and 14 facial
bones. Also in the head are three small bones in each
middle ear cavity and the hyoid bone that supports the
base of the tongue. The cranial bonesform the brain-
case (lined with the meninges) that encloses and pro-
tects the brain, eyes, and ears. The names of some of
these bones will be familiar to you; they are the same
as the terminology used (see Chapter 1) to describe
areas of the head. These are the frontal bone, parietal
bones (two), temporal bones (two), and occipital bone.
The sphenoid bone and ethmoid bone are part of the
floor of the braincase and the orbits (sockets) for
the eyes. The frontal boneforms the forehead and
the anterior part of the top of the skull. Parietalmeans
“wall,” and the two large parietal bonesform the pos-
terior top and much of the side walls of the skull. Each
temporal boneon the side of the skull contains an
external auditory meatus (ear canal), a middle ear cav-112 The Skeletal System
Table 6–1 HORMONES INVOLVED IN BONE GROWTH AND MAINTENANCEGrowth hormone (anterior pituitary gland)Thyroxine (thyroid gland)Insulin (pancreas)
Parathyroid hormone (parathyroid glands)Calcitonin (thyroid gland)Estrogen (ovaries) or
Testosterone (testes)- Increases the rate of mitosis of chondrocytes and osteoblasts
- Increases the rate of protein synthesis (collagen, cartilage matrix,
and enzymes for cartilage and bone formation) - Increases the rate of protein synthesis
- Increases energy production from all food types
- Increases energy production from glucose
- Increases the reabsorption of calcium from bones to the blood
(raises blood calcium level) - Increases the absorption of calcium by the small intestine and kid-
neys (to the blood) - Decreases the reabsorption of calcium from bones (lowers blood
calcium level) - Promotes closure of the epiphyses of long bones (growth stops)
- Helps retain calcium in bones to maintain a strong bone matrix