HUMAN BIOLOGY

Tendons connect skeletal muscle to bones. The origin end

of a muscle attaches to the bone that moves least during a

movement. The insertion end attaches to the bone that moves

most. Some muscles work antagonistically—the action of one

opposes or reverses the action of the other. Synergist muscles

assist each other’s movements.

section 6.3 Bones move when they are

pulled by the shortening, or contraction, of

skeletal muscles. This shortening occurs because

individual muscle fibers are shortening.

Skeletal muscle fibers contain threadlike myofibrils, which are

divided lengthwise into sarcomeres, the basic units of

ExpLorEon your oWn

summary

section 6.1 The body’s muscle tissue

includes skeletal, smooth, and cardiac

muscle. Despite having different functions,

cells in all three types of muscle generate

force by contracting.

section 6.2 The muscular system

consists of more than 600 skeletal muscles,

which transmit force to bones and move

body limbs or other parts (Table 6.1). Skeletal

muscles also help to stabilize joints and generate body heat.

Each one contains bundles of muscle fibers (muscle cells)

wrapped in connective tissue.

A good way to improve your understanding of your muscular system is to explore the movements of your own

muscles. Try the following quick exercises.

Human hands don’t contain many of the muscles that control hand movements. Instead, as you can see in Figure 6.20A, most of

those muscles are in the forearm. Tendons extending from one muscle, the flexor digi torum superficialis (the superficial finger flexer),

bend your fingers. Place one hand on the top of the opposite forearm, and then wiggle your fingers on that side or make a fist several

times. Can you feel the “finger flexer” in action?

Place your fingers on the skin above your nose, between your eyebrows. Now frown. The muscle you feel pulling your eyebrows together

is the corrugator supercilii (Figure 6.20B). One effect of its contraction is to “corrugate” the skin of your forehead into vertical wrinkles.

A grin calls into action other facial muscles, including the zygomaticus major (Figure 6.20C). On either side of the skull, this muscle origi-

nates on the cheekbones and inserts at the corners of the mouth. To feel it contract, place the tips of your index fingers at the corners of

your mouth, and then smile.

Figure 6.20 explore these muscles! (A, C: © Cengage

Learning; B: © Nickolay Stanev/Shutterstock.com)

118 Chapter 6

b Corrugator supercilii

muscles form a frown.

zygomaticus major

c The zygomaticus major,

which helps you smile.

flexor digitorum

superficialis

A The flexor digitorum superficialis, a forearm muscle that

helps move the fingers.

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