305
CHAPTER NINE
Muscular System
sternocleidomastoid
Attached to the sternum, clavicle,
and mastoid process.
external oblique
Located near the outside, with f
bers
that run obliquely or in a slanting direction.
Muscles of Facial Expression
A number oF small muscles beneath the skin oF the Face
and scalp enable us to communicate Feelings through Facial
expression. Many oF these muscles are located around the
eyes and mouth, and they make possible such expressions as
surprise, sadness, anger, Fear, disgust, and pain. As a group,
the muscles oF Facial expression connect the bones oF the
skull to connective tissue in regions oF the overlying skin.
movements depend upon the antagonists’ relaxing and giv-
ing way to the prime movers whenever the prime movers
contract. Once again, the nervous system coordinates these
complex actions, as described in chapter 11 (p. 410).
Sometimes the relationship between two muscles
changes. ±or example, the pectoralis major and latissimus
dorsi are antagonistic For fl
exion and extension oF the shoul-
der. However, they are synergistic For medial rotation oF the
shoulder. Thus, the role oF a muscle must be learned in the
context oF a particular movement.
The movements termed “f
exion” and “extension” describe changes
in the angle between bones that meet at a joint. For example, f
exion
o± the elbow joint re±ers to a movement o± the ±orearm that decreases
the angle at the elbow joint. Alternatively, one could say that f
ex-
ion at the elbow results ±rom the action o± the biceps brachii on the
radius o± the ±orearm.
Students ²
nd it help±ul to think o± movements in terms o± the
speci±ic actions o± the muscles involved, so we may also describe
f
exion and extension in these terms. Thus, the action o± the biceps
brachii may be described as “f
exion o± the ±orearm at the elbow” and
the action o± the quadriceps group as “extension o± the leg at the
knee.” We believe that this occasional departure ±rom strict anatomi-
cal terminology eases understanding and learning.
PRACTICE
27
Explain how parts o± the upper limb ±orm a ²
rst-class lever and a
third-class lever.
28
Distinguish between the origin and the insertion o± a muscle.
29
De²
ne prime mover.
30
What is the ±unction o± a synergist? An antagonist?
9.8
MAJOR SKELETAL MUSCLES
This section discusses the locations, actions, origins, and
insertions oF some oF the major skeletal muscles. The tables
that summarize the inFormation concerning groups oF these
muscles also include the names oF nerves that supply the
individual muscles in each group. Chapter 11 (pp. 414–423)
presents the origins and pathways oF these nerves.
Figures 9.23
and
9.24
show the locations oF superf cial
skeletal muscles—that is, those near the surFace. The names
oF muscles oFten describe them. A name may indicate a mus-
cle’s size, shape, location, action, number oF attachments, or
the direction oF its f
bers, as in the Following examples:
pectoralis major
A muscle oF large size (major) in the
pectoral region (chest).
deltoid
Shaped like a delta or triangle.
extensor digitorum
Extends the digits (f
ngers or toes).
biceps brachii
A muscle with two heads (biceps), or
points oF origin, in the brachium (arm).
Brachioradialis
Orbicularis oris
Sternocleido-
mastoid
Masseter
Zygomaticus
Orbicularis oculi
Frontalis
Deltoid
Pectoralis
major
Brachialis
Biceps brachii
Gracilis
Vastus medialis
Gastrocnemius
Soleus
Trapezius
Serratus
anterior
Tibialis anterior
External oblique
Rectus
abdominis
T
ensor
fasciae
latae
Sartorius
Rectus femoris
Adductor longus
Vastus lateralis
Fibularis longus
Extensor
digitorum longus
FIGURE 9.23
Anterior view o± super²
cial skeletal muscles.
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