302
UNIT TWO
levers are seesaws and hemostats (devices used to clamp
blood vessels).
The parts of a second-class lever are in the sequence
fulcrum–resistance–force, as in a wheelbarrow. The parts of a
third-class lever are in the sequence resistance–force–fulcrum.
Eyebrow tweezers or forceps used to grasp an object illus-
trate this type of lever.
The actions of bending and straightening the upper limb
at the elbow illustrate bones and muscles functioning as
levers. When the upper limb bends, the forearm bones rep-
resent the rigid bar; the elbow joint is the fulcrum; the hand
is moved against the resistance provided by the weight; and
the force is supplied by muscles on the anterior side of the
arm
(f g. 9.21
a
)
. One of these muscles, the
biceps brachii,
is
A lever has four basic components: (1) a rigid bar or rod, (2) a
fulcrum or pivot on which the bar turns, (3) an object moved
against resistance, and (4) a force that supplies energy for the
movement of the bar.
A pair of scissors is a lever. The handle and blade form a
rigid bar that rocks on a fulcrum near the center (the screw).
The material to be cut by the blades represents the resis-
tance, while the person on the handle end supplies the force
needed for cutting the material.
Figure 9.20
shows the three types of levers, which dif-
fer in their arrangements. A F rst-class lever’s parts are like
those of a pair of scissors. Its fulcrum is located between the
resistance and the force, making the sequence of compo-
nents resistance–fulcrum–force. Other examples of F
rst-class
Intercalated disc
Cardiac muscle cells
TABLE
9.2
|
Characteristics of Muscle Tissues
Skeletal
Smooth
Cardiac
Dimensions
Length
Diameter
Up to 30 cm
10–100 µm
30–200 µm
3–6 µm
50–100 µm
14 µm
Major location
Skeletal muscles
Walls of hollow organs
Wall of the heart
Major function
Movement of bones at joints;
maintenance of posture
Movement of walls of hollow organs;
peristalsis; vasoconstriction
Pumping action of the heart
Cellular characteristics
Striations
Nucleus
Special features
Present
Multiple nuclei
Transverse tubule system is well
developed
Absent
Single nucleus
Lacks transverse tubules
Present
Single nucleus
Transverse tubule system is well developed;
intercalated discs separate cells
Mode of control
Voluntary
Involuntary
Involuntary
Contraction
characteristics
Contracts and relaxes relatively rapidly
Contracts and relaxes relatively slowly;
some types self-exciting; rhythmic
Network of F
bers contracts as a unit; self-exciting;
rhythmic; remains refractory until contraction ends
FIGURE 9.19
The intercalated discs of cardiac muscle, shown in this transmission electron micrograph, bind adjacent cells and allow ions to move
between cells (12,500×).
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