Graphics Reference
In-Depth Information
Bones
The structures of many animals, though by no means all, are supported by
a complex skeletal framework of rigid bones. Internal skeletal structures,
known as
endoskeletons
, enclose and protect the vital organs and soft tissue
while providing a network of connected bones to which the various muscles
are attached.
Exoskeletons
provide an external support for an animal and
are typical of invertebrates, forming a shell that almost entirely encloses the
animal's internal organs. By necessity such animals undergo the periodic
shedding and subsequent growing of their exoskeleton in order for them to
grow.
Bones are widely varied in their shape and size, depending on their role in the
skeleton, though all of them are generally strong and light. They are linked
and articulated in such a manner to allow for a range of movements that is
suited to serving the needs of various animals.
Muscle
Muscles can be classified as cardiac, smooth, or skeletal. Cardiac muscle, as
the name suggests, is located in the heart. Smooth muscle can be found
in internal organs such as the stomach. Skeletal muscles, at least for most
animals, provide the power for locomotion. For the purposes of this text we
will only be looking at skeletal muscle.
Skeletal muscles have the properties of
extensibility, elasticity
, and
contractility
.
The first two allow the muscle to be stretched, much in the same way as an
elastic band, returning to their rest position once the force creating the stretch
has been discontinued. Muscle's ability to contract allows that muscle to
produce tension. On average, muscle fibers can shorten and stretch to about
half as long (or short) again as their position at rest. The difference between
the maximum stretch and contraction of a muscle is called the
amplitude
of its
action.
Skeletal muscle may be classified as either fast-twitch muscle or slow-twitch
muscle.
Fast-twitch muscle
fibers are larger, are paler in color, and have a less
elaborate blood supply than slow-twitch muscle. They are capable of very fast
responses but are not able to sustain the activity over extended periods and
tire quickly. These properties make them highly suited to fast actions such as
sprint activities and lifting heavy weights. These muscle fibers are powered
through anaerobic metabolism, which involves the release of energy by the
conversion of glucose to lactic acid. Anaerobic metabolism releases far less
energy from a given mass of food than does aerobic metabolism.
Slow-twitch muscle
fibers are smaller and are red in appearance as a
consequence of their richer blood supply. They carry more oxygen than
the fast-twitch white muscle. Slow-twitch muscle fibers are much slower to
react than white muscle fibers; they are highly efficient and tire much less
quickly, making them ideal for endurance events and posturing actions. The
