Sample Chapter 10 from the Textbook (35559.0K) - McGraw-Hill

312 PART 2 Support and Movement
less strength if the total number of fibers is low. However, if the
fibers are long, these muscles can have a large range of motion. One
example of convergent muscles with many long fibers is the pectoralis
muscles. Similarly, in parallel muscles, fasciculi are organized
parallel to the long axis of the muscle, but they terminate on
a flat tendon that spans the width of the entire muscle. As a consequence,
parallel muscles can shorten to a large degree because the
fasciculi are in a direct line with the tendon; however, they contract
with less force because fewer total fasciculi are attached to the tendon.
The hyoid muscles are an example of parallel muscles.
The fasciculi of some muscles emerge like the barbs on a feather
from a common tendon that runs the length of the entire muscle
and therefore are called pennate (pen′āt; pennatus, feather) muscles.
Muscles with all fasciculi on one side of the tendon are called unipennate,
muscles with fibers arranged on two sides of the tendon
are bipennate, and muscles with fasciculi arranged at many places
around the central tendon are multipennate. The long tendons of
pennate muscles can extend for some distance between a muscle
belly and its insertion. The pennate arrangement allows a large
number of fasciculi to attach to a single tendon, with the force of
contraction concentrated at the tendon. The muscles that extend
the knee are multipennate muscles.
Muscles whose fibers run the length of the entire muscle and
taper at each end to terminate at tendons, creating a wider belly
than the ends, are called fusiform. Because their fibers are long, but
are commonly numerous, these muscles generally tend to be stronger
than other muscles with parallel fascicle arrangements. The muscle
that flexes the forearm is an example of a fusiform muscle.
In summary, muscle strength is primarily related to the total
number of fibers in the muscle, whereas range of motion is more
correlated to fascicle arrangement, with parallel fibers having the
largest range of motion.
Nomenclature
Muscles are named according to several characteristics, including
location, size, shape, orientation of fasciculi, origin and insertion,
number of heads, and function. Recognizing the descriptive
nature of muscle names makes learning those names much easier.
1. Location. A pectoralis (chest) muscle is located in the chest, a
gluteus (buttock) muscle is in the buttock, and a brachial (arm)
muscle is in the arm.
2. Size. The gluteus maximus (large) is the largest muscle of the
buttock, and the gluteus minimus (small) is the smallest. A
longus (long) muscle is longer than a brevis (short) muscle. In
addition, a second part to the name immediately tells us there
is more than one related muscle. For example, if there is a
brevis muscle, most likely a longus muscle is present in the
same area.
3. Shape. The deltoid (triangular) muscle is triangular in shape,
a quadratus (quadrate) muscle is rectangular, and a teres (round)
muscle is round.
4. Orientation of fasciculi. A rectus (straight, parallel) muscle has
muscle fasciculi running straight with the axis of the structure
to which the muscle is associated, whereas the fasciculi of an
oblique muscle lie oblique to the longitudinal axis of the structure.
5. Origin and insertion. The sternocleidomastoid originates on
the sternum and clavicle and inserts onto the mastoid process
of the temporal bone. The brachioradialis originates in the
arm (brachium) and inserts onto the radius.
6. Number of heads. A biceps muscle has two heads, and a triceps
muscle has three heads. Each head has a separate origin.
7. Function. An abductor moves a structure away from the midline,
and an adductor moves a structure toward the midline.
The masseter (a chewer) is a chewing muscle.
Movements Accomplished by Muscles
Muscle movements can be explained in terms of the action of
levers. A lever is a rigid shaft capable of turning about a hinge, or
pivot point, called a fulcrum (F) and transferring a force applied
at one point along the lever to a weight (W), or resistance, placed
at another point along the lever. In the body, the joints function
as fulcrums, and the bones function as levers. When muscles contract,
the pull (P), or force, of muscle contraction is applied to the
levers (bones), causing them to move. Three classes of levers exist,
based on the relative positions of the levers, fulcrums, weights, and
forces (figure 10.2): classes I, II, and III.
Class I Lever
In a class I lever system, the fulcrum is located between the pull
and the weight (figure 10.2a). A child’s seesaw is this type of lever.
The children on the seesaw alternate between being the weight and
being the pull across a fulcrum in the center of the board. In the
body, the head is this type of lever; the atlantooccipital joint is the
fulcrum, the posterior neck muscles provide the pull depressing
the back of the head, and the face, which is elevated, is the weight.
With the weight balanced over the fulcrum, only a small amount
of pull is required to lift the weight. For example, only a very small
shift in weight is needed for one child to lift the other on a seesaw.
However, a class I lever is quite limited as to how much weight can
be lifted and how high it can be lifted. For example, consider what
happens when the child on one end of the seesaw is much larger
than the child on the other end.
Class II Lever
In a class II lever system, the weight is located between the fulcrum
and the pull (figure 10.2b). An example is a wheelbarrow; the wheel
is the fulcrum, and the person lifting on the handles provides the
pull. The weight, or load, carried in the wheelbarrow is placed
between the wheel and the operator. In the body, a class II lever
operates to depress the mandible, as in opening the mouth. (However,
to compare this movement to the wheelbarrow example, the human
head must be considered upside down.)
Class III Lever
In a class III lever system, the most common type in the body, the
pull is located between the fulcrum and the weight (figure 10.2c).
An example is a person using a shovel. The hand placed on the part
of the handle closest to the blade provides the pull to lift the
weight, such as a shovelful of dirt, and the hand placed near the end
of the handle acts as the fulcrum. In the body, the action of the