303
CHAPTER NINE
Muscular System
muscles attached to the chin area of the mandible provide
the force that opens the mouth.
Levers provide a range of movements. Levers that move
limbs, for example, produce rapid motions, whereas others,
such as those that move the head, help maintain posture
with minimal effort.
Origin and Insertion
Recall from chapter 8 (p. 267) that one end of a skeletal mus-
cle is usually fastened to a relatively immovable or F xed part,
and the other end is connected to a movable part on the other
side of a joint. The immovable end is called the
origin
of the
muscle, and the movable end is called its
insertion.
When
a muscle contracts, its insertion is pulled toward its origin
(f g. 9.22)
. The head of a muscle is the part nearest its origin.
Some muscles have more than one origin or insertion.
The
biceps brachii
in the arm, for example, has two origins.
attached by a tendon to a projection (radial tuberosity) on
the
radius
bone in the forearm, a short distance below the
elbow. The parts of this lever are arranged in the sequence
resistance–force–fulcrum, so it is a third-class lever.
When the upper limb straightens at the elbow, the fore-
arm bones again serve as the rigid bar, the hand moves against
the resistance by pulling on the rope to raise the weight (F g.
9.21
b
), and the elbow joint serves as the fulcrum. However,
this time the
triceps brachii,
a muscle located on the posterior
side of the arm, supplies the force. A tendon of this muscle
attaches to a projection (olecranon process) of the ulna bone
at the point of the elbow. The parts of the lever are arranged
resistance–fulcrum–force, so it is a F rst-class lever.
A second-class lever (fulcrum–resistance–force) is
also demonstrated in the human body. The fulcrum is the
temporomandibular joint; muscles supply the resistance,
attaching to a projection (coronoid process) and body of
the mandible, that resist or oppose opening the mouth. The
Fulcrum
Force
Force
Force
Resistance
Force
Force
Fulcrum
Resistance
Fulcrum
(a)
First-class lever
Resistance
Force
Fulcrum
(c)
Third-class lever
Resistance
Resistance
Resistance
Fulcrum
(b)
Second-class lever
Fulcrum
FIGURE 9.20
Three types of levers. (
a
) A F
rst-class lever is used in a pair of scissors, (
b
) a second-class lever is used in a wheelbarrow, and (
c
) a
third-class lever is used in a pair of forceps.
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