Page 378 - Fundamentals of anatomy physiology

For example, the latissimus dorsi muscle is a large trunk
muscle that extends, adducts, and medially rotates the arm
at the shoulder joint. A much smaller muscle, the teres
(TER-e. z) major muscle, assists in starting such movements
when the shoulder joint is at full flexion.
A fixator is a synergist that assists an agonist by prevent-
ing movement at another joint, thereby stabilizing the
origin of the agonist. Recall that the biceps brachii muscle
is an agonist that produces flexion at the elbow. It has two
tendons that originate on the scapula and one that inserts
on the radius. During flexion, the trapezius and rhomboid
muscles act as fixators by stabilizing and preventing the
movement of the scapula.
Checkpoint
6. The gracilis muscle is attached to the anterior surface
of the tibia at one end, and to the pubis and ischium
of the pelvis at the other. When the muscle contracts,
flexion occurs at the hip. Which attachment point is
considered the muscle’s origin?
7. Muscle A abducts the humerus, and muscle B adducts
the humerus. What is the relationship between these
two muscles?
8. Define the term synergist as it relates to muscle action.
See the blue Answers tab at the back of the book.
11-4 Descriptive terms are used
to name skeletal muscles
Learning Outcome Explain how the name of a muscle can help identify
its location, appearance, or function.
Except for the platysma and the diaphragm, the complete names
of all skeletal muscles include the term muscle. The full name,
such as the biceps brachii muscle, will usually appear in the
text, but for simplicity, only the descriptive name (biceps bra-
chii) will appear in figures and tables.
You need not learn every one of the approximately
700 muscles in the human body, but you will have to become
familiar with the most important ones. Fortunately, the names
anatomists assigned to the muscles include descriptive terms
that can help you remember the names and identify the mus-
cles. When faced with a new muscle name, it is helpful to first
identify the descriptive portions of the name. The name of a
muscle may include descriptive information about its location
in the body, origin and insertion, fascicle organization, posi-
tion, structural characteristics, and action.
Location in the Body
Table 11–1 includes a useful summary of muscle terminology,
including terms that designate specific regions of the body.
Chapter 11 The Muscular System 365
Regional terms are most common as modifiers that help iden-
tify individual muscles. In a few cases, a muscle is such a promi-
nent feature of a body region that a name referring to the region
alone will identify it. Examples include the temporalis muscle of
the head and the brachialis (bra. -ke. -A-lis) muscle of the arm.
Origin and Insertion
Many muscle names include terms for body places that tell
you the specific origin and insertion of each muscle. In such
cases, the first part of the name indicates the origin, the
second part the insertion. The genioglossus muscle, for ex-
11ample, originates at the chin (geneion) and inserts in the
tongue (glossus). The names may be long and difficult to pro-
nounce, but Table 11–1 and the anatomical terms introduced
in Chapter 1 can help you identify and remember them.
pp. 40–44
Fascicle Organization
A muscle name may refer to the orientation of the muscle
fascicles within a particular skeletal muscle. Rectus means
“straight,” and rectus muscles are parallel muscles whose fibers
run along the long axis of the body. Because we have several
rectus muscles, the name typically includes a second term that
refers to a precise region of the body. For example, the rectus ab-
dominis muscle is located on the abdomen, and the rectus femo-
ris muscle on the thigh. Other common directional indicators
include transversus and oblique, for muscles whose fibers
run across (transversus) or at a slanting (oblique) angle to the
longitudinal axis of the body.
Position
Muscles visible at the body surface are often called externus
or superficialis. Deeper muscles are termed internus or pro-
fundus. Superficial muscles that position or stabilize an organ
are called extrinsic. Muscles located entirely within an organ
are intrinsic.
Structural Characteristics
Some muscles are named after distinctive structural features.
The biceps brachii muscle, for example, is named after its ori-
gin. It has two tendons of origin (bi-, two + caput, head). Simi-
larly, the triceps brachii muscle has three, and the quadriceps
group has four.
Shape is sometimes an important clue to the name of a
muscle. For example, the trapezius (tra-PE. -ze. -us), deltoid, rhom-
boid (ROM-boyd), and orbicularis (or-bik-u. -LA. -ris) muscles
look like a trapezoid, a triangle, a rhomboid, and a circle,
respectively.