and not flexible. In the shafts of long bones such as the
femur, the osteocytes, matrix, and blood vessels are
in very precise arrangements called haversian sys-
temsor osteons(see Fig. 4–5). Bone has a good blood
supply, which enables it to serve as a storage site for
calcium and to repair itself relatively rapidly after a
simple fracture. Some bones, such as the sternum
(breastbone) and pelvic bone, contain red bone mar-
row, the primary hemopoietic tissue that produces
blood cells.
Other functions of bone tissue are related to the
strength of bone matrix. The skeleton supports the
body, and some bones protect internal organs from
mechanical injury. A more complete discussion of
bone is found in Chapter 6.
CARTILAGE
The protein–carbohydrate matrix of cartilagedoes
not contain calcium salts, and also differs from that of
bone in that it contains more water, which makes it
resilient. It is firm, yet smooth and flexible. Cartilage
is found on the joint surfaces of bones, where its
smooth surface helps prevent friction. The tip of the
nose and external ear are supported by flexible carti-
lage. The wall of the trachea, the airway to the lungs,
contains firm rings of cartilage to maintain an open air
passageway. Discs of cartilage are found between the
vertebrae of the spine. Here the cartilage is a firm
cushion; it absorbs shock and permits movement.
Within the cartilage matrix are the chondrocytes,
or cartilage cells (see Fig. 4–5). There are no capillar-
ies within the cartilage matrix, so these cells are nour-
ished by diffusion through the matrix, a slow process.
This becomes clinically important when cartilage is
damaged, for repair will take place very slowly or not
at all. Athletes sometimes damage cartilage within the
knee joint. Such damaged cartilage is usually surgically
removed in order to preserve as much joint mobility as
possible.
MUSCLE TISSUE
Muscle tissueis specialized for contraction. When
muscle cells contract, they shorten and bring about
some type of movement. There are three types of
muscle tissue: skeletal, smooth, and cardiac (Table
4–3). The movements each can produce have very dif-
ferent purposes.
SKELETAL MUSCLE
Skeletal musclemay also be called striatedmuscle or
voluntarymuscle. Each name describes a particular
aspect of this tissue, as you will see. The skeletal mus-
cle cells are cylindrical, have several nuclei each, and
appear striated, or striped (Fig. 4–6). The striations
are the result of the precise arrangement of the con-
tracting proteins within the cells.
Skeletal muscle tissue makes up the muscles that
are attached to bones. These muscles are supplied
with motor nerves, and thus move the skeleton. They
also produce a significant amount of heat, which is
important to help maintain the body’s constant tem-
perature. Each muscle cell has its own motor nerve
ending. The nerve impulses that can then travel to the
muscles are essential to cause contraction. Although
we do not have to consciously plan all our movements,
the nerve impulses for them originate in the cere-
brum, the “thinking” part of the brain.
Let us return to the three names for this tissue:
“skeletal” describes its location, “striated” describes its
appearance, and “voluntary” describes how it func-
tions. The skeletal muscles and their functioning are
the subject of Chapter 7.SMOOTH MUSCLE
Smooth musclemay also be called involuntarymus-
cle or visceralmuscle. The cells of smooth muscle
have tapered ends, a single nucleus, and no striations
(see Fig. 4–6). Although nerve impulses do bring
about contractions, this is not something most of us
can control, hence the name involuntary. The term vis-
ceralrefers to internal organs, many of which contain
smooth muscle. The functions of smooth muscle are
actually functions of the organs in which the muscle is
found.
In the stomach and intestines, smooth muscle con-
tracts in waves called peristalsis to propel food
through the digestive tract. In the walls of arteries and
veins, smooth muscle constricts or dilates the vessels
to maintain normal blood pressure. The iris of the eye
has two sets of smooth muscle fibers to constrict or
dilate the pupil, which regulates the amount of light
that strikes the retina.
Other functions of smooth muscle are mentioned
in later chapters. This is an important tissue that you
will come across again and again in our study of the
human body.Tissues and Membranes 79