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Interactions of Skeletal Muscles in the
Body (p. 321)
Naming Skeletal Muscles (pp. 321–322)
Muscle Mechanics: Importance of
Fascicle Arrangement and Leverage
(pp. 322–324)
Arrangement of Fascicles (pp. 322–323)
Lever Systems: Bone-Muscle Relationships (pp. 323–324)
Major Skeletal Muscles of the Body
(pp. 324–382)
T
he human body enjoys an incredibly wide range of movements. The gentle blinking of your eye, standing on tiptoe, and wielding a sledgehammer are just a small sample of the different activities promoted by your muscular system. Muscle tissue includes all contractile tissues (skeletal, cardiac, and smooth muscle), but when we study the muscular system, skeletal muscles take center stage. These muscular “ma- chines”that enable us to perform so many different activities are the focus of this chapter. Before describing the individual muscles in detail, we will describe the manner in which muscles “play”with or against each other to bring about movements, consider the criteria used for naming muscles, and explain the principles of leverage.
The Muscular
System
320
Interactions of Skeletal Muscles
in the Body
Describe the function of prime movers, antagonists,
synergists, and fixators.
The arrangement of body muscles permits them to work either
together or in opposition to achieve a wide variety of move-
ments. As you eat, for example, you alternately raise your fork to
your lips and lower it to your plate, and both sets of actions are
accomplished by your arm and hand muscles. But muscles can
only pull ; they never push. Generally as a muscle shortens, its
insertion (attachment on the movable bone) moves toward its
origin (its fixed or immovable point of attachment). Whatever
one muscle or muscle group can do, another muscle or group of
muscles can “undo.”
Muscles can be classified into four functional groups: prime
movers, antagonists, synergists, and fixators. A muscle that has
the major responsibility for producing a specific movement is
a prime mover , or agonist (ag o-nist; “leader”), of that move-
ment. The biceps brachii muscle, which fleshes out the ante-
rior arm (and inserts on the radius), is a prime mover of elbow
flexion.
Muscles that oppose, or reverse, a particular movement are
called antagonists (an-tag o-nists; “against the leaders”). When
a prime mover is active, the antagonist muscles may be
stretched or may remain relaxed. Usually however, antagonists
help to regulate the action of a prime mover by contracting
slightly to provide some resistance, thus helping to prevent
overshooting the mark or to slow or stop the movement. As you
might expect, a prime mover and its antagonist are located on
opposite sides of the joint across which they act. Antagonists
can also be prime movers in their own right. For example, flex-
ion of the forearm by the biceps brachii muscle of the arm is an-
tagonized by the triceps brachii, the prime mover for extending
the forearm. As noted in Chapter 9, it is important that the two
members of any agonist/antagonist pair be challenged and de-
veloped equally to prevent undue tension on the less developed
muscle and joint inflexibility.
In addition to agonists and antagonists, most movements in-
volve the action of one or more synergists (sin er-jists; syn
together, erg work). Synergists help prime movers by (1)
adding a little extra force to the same movement or (2) reducing
undesirable or unnecessary movements that might occur as the
prime mover contracts. This second function deserves more ex-
planation. When a muscle crosses two or more joints, its con-
traction causes movement at all of the spanned joints unless
other muscles act as joint stabilizers. For example, the finger
flexor muscles cross both the wrist and the interphalangeal joints,
but you can make a fist without bending your wrist because syn-
ergistic muscles stabilize the wrist. Additionally, as some flexors
act, they may cause several other (undesirable) movements at
the same joint. Synergists can prevent this, allowing all of the
prime mover’s force to be exerted in the desired direction.
When synergists immobilize a bone, or a muscle’s origin so
that the prime mover has a stable base on which to act, they are
more specifically called fixators (fik sa-terz). Recall from Chap-
ter 7 that the scapula is held to the axial skeleton only by mus- cles and is quite freely movable. The fixator muscles that run from the axial skeleton to the scapula can immobilize the scapula so that only the desired movements occur at the mobile shoulder joint. Additionally, muscles that help to maintain up- right posture are fixators. In summary, although prime movers seem to get all the credit for causing certain movements, antagonistic and syner- gistic muscles are also important in producing smooth, coordi- nated, and precise movements. Furthermore, a muscle may act as a prime mover in one movement, an antagonist for another movement, a synergist for a third movement, and so on.
Naming Skeletal Muscles
List the criteria used in naming muscles. Provide an
example to illustrate the use of each criterion.
Skeletal muscles are named according to a number of criteria, each of which describes the muscle in some way. Paying atten- tion to these cues can simplify the task of learning muscle names and actions.
1. Location of the muscle. Some muscle names indicate the bone or body region with which the muscle is associated. For example, the temporalis (tempor-a˘ lis) muscle over- lies the temporal bone, and intercostal ( costal rib) mus- cles run between the ribs. 2. Shape of the muscle. Some muscles are named for their distinctive shapes. For example, the deltoid (del toid) muscle is roughly triangular ( deltoid triangle), and to- gether the right and left trapezius (trah-pe ze-us) muscles form a trapezoid. 3. Relative size of the muscle. Terms such as maximus (largest), minimus (smallest), longus (long), and brevis (short) are often used in muscle names—as in gluteus maximus and gluteus minimus (the large and small glu- teus muscles, respectively). 4. Direction of muscle fibers. The names of some muscles re- veal the direction in which their fibers (and fascicles) run in reference to some imaginary line, usually the midline of the body or the longitudinal axis of a limb bone. In mus- cles with the term rectus (straight) in their names, the fibers run parallel to that imaginary line (axis).The terms transversus and oblique indicate that the muscle fibers run respectively at right angles and obliquely to that line. Spe- cific examples include the rectus femoris (straight muscle of the thigh, or femur) and transversus abdominis (trans- verse muscle of the abdomen). 5. Number of origins. When biceps , triceps , or quadriceps forms part of a muscle’s name, you can assume that the muscle has two, three, or four origins, respectively. For ex- ample, the biceps brachii (bra ke-i) muscle of the arm has two origins, or heads. 6. Location of the attachments. Some muscles are named according to their points of origin and insertion. The origin is always named first. For instance, the sternocleidomastoid
Chapter 10 The Muscular System 321
In a pennate (pen at) pattern, the fascicles (and thus the¯
muscle fibers) are short and they attach obliquely ( penna
feather) to a central tendon that runs the length of the muscle.
If, as in the extensor digitorum longus muscle of the leg, the fas-
cicles insert into only one side of the tendon, the muscle is
unipennate (Figure 10.1d). If the fascicles insert into the tendon
from opposite sides, so that the muscle’s “grain” resembles a
feather, the arrangement is bipennate (Figure 10.1e). The rectus
femoris of the thigh is bipennate. A multipennate arrangement
looks like many feathers situated side by side, with all their quills
inserted into one large tendon. The deltoid muscle, which forms
the roundness of the shoulder, is multipennate (Figure 10.1g).
The arrangement of a muscle’s fascicles determines its range
of motion (the amount of movement produced when a muscle
shortens) and its power. Because skeletal muscle fibers may
shorten to about 70% of their resting length when they con-
tract, the longer and the more nearly parallel the muscle fibers
are to a muscle’s long axis, the more the muscle can shorten.
Muscles with parallel fascicle arrangement shorten the most,
but they are not usually very powerful. Muscle power depends
more on the total number of muscle fibers in the muscle. The
greater the number of muscle fibers, the greater the power. The
stocky bipennate and multipennate muscles, which “pack in”
the most fibers, shorten very little but are very powerful.
C H E C K Y O U R U N D E R S TA N D I N G
3. Of the muscles illustrated in Figure 10.1, which could shorten
most? Which two would likely be most powerful? Why?
For answers, see Appendix G.
Lever Systems: Bone-Muscle Relationships
The operation of most skeletal muscles involves the use of lever- age and lever systems (partnerships between the muscular and skeletal systems). A lever is a rigid bar that moves on a fixed point called the fulcrum , when a force is applied to it. The ap- plied force, or effort , is used to move a resistance, or load. In your body, your joints are the fulcrums, and your bones act as levers. Muscle contraction provides the effort which is applied at the muscle’s insertion point on a bone. The load is the bone itself, along with overlying tissues and anything else you are try- ing to move with that lever. A lever allows a given effort to move a heavier load, or to move a load farther or faster, than it otherwise could. If, as shown in Figure 10.2a , the load is close to the fulcrum and the effort is applied far from the fulcrum, a small effort exerted over a relatively large distance can be used to move a large load over a small distance. Such a lever is said to operate at a mechanical advantage and is commonly called a power lever. For example, as shown to the right in Figure 10.2a, a person can lift a car with such a lever, in this case, a jack. The car moves up only a small distance with each downward “push” of the jack handle, but rel- atively little muscle effort is needed. If, on the other hand, the load is far from the fulcrum and the effort is applied near the fulcrum, the force exerted by the mus- cle must be greater than the load to be moved or supported, as Figure 10.2b shows. This lever system operates at a mechanical disadvantage and is a speed lever. These levers are useful be- cause they allow a load to be moved rapidly over a large distance (with a wide range of motion). Wielding a shovel is an example. As you can see, small differences in the site of a muscle’s inser- tion (relative to the fulcrum or joint) can translate into large
Chapter 10 The Muscular System 323
Figure 10.2 Lever systems operating at a mechanical advantage
and a mechanical disadvantage. The equation at the top expresses
the relationships among the forces and distances in any lever system.
(a) Mechanical advantage with a power lever. When using a jack, the
load lifted is greater than the applied muscular effort. Only 10 kg of
force (the effort) is used to lift a 1000-kg car (the load).
( Figure continues on p. 324 )
Load
Effort
Load
Effort
10 kg
1000 kg
Fulcrum
10 x 25 = 1000 x 0. 250 = 250
Effort x length of effort arm = load x length of load arm (force x distance) = (resistance x distance)
(a) Mechanical advantage with a power lever
Fulcrum
25 cm
0.25 cm
differences in the amount of force a muscle must generate to move a given load or resistance. Regardless of type, all levers follow the same basic principle:
Effort farther than lever operates at a load from fulcrum mechanical advantage
Effort nearer than lever operates at a load to fulcrum mechanical disadvantage
Depending on the relative position of the three elements— effort, fulcrum, and load—a lever belongs to one of three classes. In first-class levers , the effort is applied at one end of the lever and the load is at the other, with the fulcrum somewhere between. Seesaws and scissors are first-class levers. First-class leverage also occurs when you lift your head off your chest (Fig- ure 10.3a). Some first-class levers in the body operate at a me- chanical advantage (for strength), but others, such as the action of the triceps muscle in extending the forearm against resistance, operate at a mechanical disadvantage (for speed and distance). In a second-class lever , the effort is applied at one end of the lever and the fulcrum is located at the other, with the load be- tween them. A wheelbarrow demonstrates this type of lever sys- tem. Second-class levers are uncommon in the body, but the best example is the act of standing on your toes (Figure 10.3b). All second-class levers in the body work at a mechanical advan- tage because the muscle insertion is always farther from the ful- crum than is the load. Second-class levers are levers of strength, but speed and range of motion are sacrificed for that strength. In third-class levers , the effort is applied between the load and the fulcrum. These levers are speedy and always operate at a mechanical disadvantage. Tweezers or forceps provide this type of leverage. Most skeletal muscles of the body act in third-class lever systems. An example is the activity of the biceps muscle of the arm, lifting the distal forearm and anything carried in the
hand (Figure 10.3c). Third-class lever systems permit a muscle to be inserted very close to the joint across which movement occurs, which allows rapid, extensive movements (as in throw- ing) with relatively little shortening of the muscle. Muscles in- volved in third-class levers tend to be thicker and more powerful. In conclusion, differences in the positioning of the three ele- ments modify muscle activity with respect to (1) speed of con- traction, (2) range of movement, and (3) the weight of the load that can be lifted. In lever systems that operate at a mechanical disadvantage (speed levers), force is lost but speed and range of movement are gained. Systems that operate at a mechanical advantage (power levers) are slower, more stable, and used where strength is a priority.
C H E C K Y O U R U N D E R S TA N D I N G
4. Which of the three lever systems involved in muscle mechan-
ics would be the fastest lever—first-, second-, or third-class?
5. What benefit is provided by a lever that operates at a me-
chanical advantage?
For answers, see Appendix G.
Major Skeletal Muscles
of the Body
Name and identify the muscles described in Tables 10.1 to
10.17. State the origin, insertion, and action of each.
The grand plan of the muscular system is all the more impres- sive because of the sheer number of skeletal muscles in the body—more than 600 of them (many more than are shown in Figures 10.4 and 10.5 combined)! Trying to remember all the
324 UNIT 2 Covering, Support, and Movement of the Body
Figure 10.2 (continued) Lever systems operating at a mechanical
advantage and a mechanical disadvantage. (b) Mechanical disad-
vantage with a speed lever. When using a shovel to lift dirt, the muscular
force is greater than the load lifted. A muscular force (effort) of 100 kg
is used to lift 50 kg of dirt (the load). Levers operating at a mechanical
disadvantage are common in the body.
Load
Effort
Fulcrum
Load
Effort
100 kg
50 kg
Fulcrum
100 x 25 = 50 x 50 2500 = 2500
(b) Mechanical disadvantage with a speed lever
25 cm
50 cm
Trapezius
Shoulder
Deltoid
Arm Triceps brachii Biceps brachii Brachialis
Forearm Pronator teres Brachioradialis Flexor carpi radialis Palmaris longus
Pelvis/thigh Iliopsoas
Pectineus
Thigh Rectus femoris Vastus lateralis Vastus medialis
Leg Fibularis longus
Extensor digitorum longus
Tibialis anterior
Head
Facial
Temporalis
Epicranius, frontal belly
Orbicularis oculi Zygomaticus
Orbicularis oris Neck
Sternohyoid Sternocleidomastoid Thorax
Pectoralis major
Pectoralis minor
Serratus anterior
Intercostals
Abdomen
External oblique
Rectus abdominis
Internal oblique
Transversus abdominis
Thigh
Sartorius
Tensor fasciae latae
Adductor longus
Gracilis
Leg Gastrocnemius
Soleus
Masseter
Platysma
Figure 10.4 Anterior view of superficial
muscles of the body. The abdominal sur-
face has been partially dissected on the right
side to show somewhat deeper muscles. (See
A Brief Atlas of the Human Body , Figure 33.)
326 UNIT 2 Covering, Support, and Movement of the Body
Chapter 10 The Muscular System 327
Arm
Triceps brachii
Brachialis
Forearm
Brachioradialis
Extensor carpi ulnaris Extensor digitorum
Iliotibial tract
Leg Gastrocnemius
Soleus
Fibularis longus
Neck Epicranius, occipital belly
Sternocleidomastoid
Trapezius
Shoulder
Hip Gluteus medius
Gluteus maximus
Thigh
Biceps femoris
Adductor magnus
Semitendinosus
Semimembranosus
Hamstrings:
Latissimus dorsi
Rhomboid major
Infraspinatus
Deltoid
Teres major
Flexor carpi ulnaris
Extensor carpi radialis longus
Calcaneal (Achilles) tendon
Figure 10.5 Posterior view of super-
ficial muscles of the body. (See A Brief
Atlas of the Human Body , Figures 32, 35,
and 36.)
ORIGIN (O) AND NERVE
MUSCLE DESCRIPTION INSERTION (I) ACTION SUPPLY
The muscles that promote facial expression lie in the scalp and face just deep to the skin. They are thin and variable in shape and strength, and adjacent muscles tend to be fused. They are unusual muscles in that they insert into skin (or other muscles), not bones. In the scalp, the main muscle is the epicranius , which has distinct anterior and posterior parts. The lateral scalp muscles are vestigial in humans. Muscles clothing the facial bones lift the eyebrows, flare the nostrils, open and close the eyes and mouth, and provide one of the
best tools for influencing others—the smile. The tremendous impor- tance of facial muscles in nonverbal communication becomes espe- cially clear when they are paralyzed, as in some stroke victims and in the expressionless “mask” of patients with Parkinson’s disease. All mus- cles listed in this table are innervated by cranial nerve VII , the facial nerve (see Table 13.2). The external muscles of the eyes, which act to direct the eyeballs, and the levator palpebrae superioris muscles that raise the eyelids are described in Chapter 15.
Epicranius (occipitofrontalis) (epı˘-kra ne-us; ok-sipı˘-to-fron-ta lis) ( epi over; cran skull)
Covers forehead and dome of skull; no bony attachments
O—galea aponeurotica I—skin of eyebrows and root of nose
With aponeurosis fixed, raises the eyebrows (as in surprise); wrinkles fore- head skin horizontally
■ (^) Frontal belly (fron tal) ( front forehead)
TABLE 10.1 Muscles of the Head, Part I: Facial Expression (Figure 10.6)
Facial nerve (cranial VII)
MUSCLE GALLERY
MUSCLES OF THE SCALP
Bipartite muscle consisting of the frontal and occipital bellies connected by a cranial aponeurosis, the galea aponeurotica; the alternate actions of these two muscles pull scalp forward and backward
Overlies posterior occiput; by pulling on the galea, fixes origin of frontalis
O—occipital and tempo- ral bones I—galea aponeurotica
Fixes aponeurosis and pulls scalp posteriorly
■ (^) Occipital belly Facial nerve (ok-sipı˘-tal ) ( occipito base of skull)
Small muscle; activity associated with that of orbicularis oculi
O—arch of frontal bone above nasal bone I—skin of eyebrow
Draws eyebrows together and inferiorly; wrinkles skin of forehead vertically (as in frowning)
Facial nerve
MUSCLES OF THE FACE
Corrugator supercilii (kor ah-ga-ter sooper-sı˘ le-i) ( corrugo wrinkle; supercilium eyebrow)
Chapter 10 The Muscular System 329
TABLE 10.1 Muscles of the Head, Part I: Facial Expression (Figure 10.6) (continued)
MUSCLE GALLERY
Complicated, multilay- ered muscle of the lips with fibers that run in many different directions; most run circularly
O—arises indirectly from maxilla and mandible; fibers blended with fibers of other facial muscles as- sociated with the lips I—encircles mouth; inserts into muscle and skin at angles of mouth
Closes lips; purses and protrudes lips; kissing and whistling muscle
Facial nerve
One of the muscle pair forming a V-shaped mus- cle mass on chin
O—mandible below inci- sors I—skin of chin
Wrinkles chin; protrudes lower lip
Facial nerve
Thin, horizontal cheek muscle; principal muscle of cheek; deep to mas- seter (see also Figure 10.7)
O—molar region of max- illa and mandible I—orbicularis oris
Compresses cheek (as in whistling and sucking); trampoline-like action holds food between teeth during chewing; draws corner of mouth laterally; well developed in nursing infants
Facial nerve
Orbicularis oris
Mentalis (men-ta lis) ( ment chin)
Buccinator (bu sı˘-nater) ( bucc cheek or “trumpeter”)
Unpaired, thin, sheetlike superficial neck muscle; not strictly a head muscle, but plays a role in facial expression
O—fascia of chest (over pectoral muscles and deltoid) I—lower margin of mandible, and skin and muscle at corner of mouth
Tenses skin of neck (e.g., during shaving); helps de- press mandible; pulls lower lip back and down, i.e., produces downward sag of mouth
Platysma (plah-tiz mah) Facial nerve ( platy broad, flat)
ORIGIN (O) AND NERVE
MUSCLE DESCRIPTION INSERTION (I) ACTION SUPPLY
Slender muscle inferior and lateral to zygomaticus
O—lateral fascia associ- ated with masseter muscle I—skin at angle of mouth
Draws corner of lip later- ally; tenses lips; synergist of zygomaticus
Facial nerve
Thin muscle between orbicularis oris and inferior eye margin
O—zygomatic bone and infraorbital margin of maxilla I—skin and muscle of upper lip
Opens lips; raises and fur- rows the upper lip
Facial nerve
Risorius (ri-zor e-us) ( risor laughter)
Small muscle running from mandible to lower lip
O—body of mandible lat- eral to its midline I—skin and muscle of lower lip
Draws lower lip inferiorly (as in a pout)
Facial nerve
Small muscle lateral to depressor labii inferioris
O—body of mandible be- low incisors I—skin and muscle at an- gle of mouth below inser- tion of zygomaticus
Draws corners of mouth downward and laterally (as in a “tragedy mask” grimace); zygomaticus an- tagonist
Facial nerve
Levator labii superioris (le˘ -va tor la be-i soo-pere-or is) ( leva raise; labi lip; superior above, over) Depressor labii inferioris (de-pres or la be-i in-fere-or is) ( depressor depresses; infer below) Depressor anguli oris (ang gu-li or-is) ( angul angle, corner; or mouth)
330 UNIT 2 Covering, Support, and Movement of the Body
Thin, tripartite sphincter muscle of eyelid; sur- rounds rim of the orbit
O—frontal and maxillary bones and ligaments around orbit I—tissue of eyelid
Closes eye; various parts can be activated individu- ally; produces blinking, squinting, and draws eye- brows inferiorly
Orbicularis oculi Facial nerve (or-bik u-lar-is ok u-li) ( orb circular; ocul eye)
Muscle pair extending di- agonally from cheekbone to corner of mouth
O—zygomatic bone I—skin and muscle at cor- ner of mouth
Raises lateral corners of mouth upward (smiling muscle)
Zygomaticus —major and Facial nerve minor (zi-go-mat ı˘-kus) ( zygomatic cheekbone)
ORIGIN (O) AND NERVE
MUSCLE DESCRIPTION INSERTION (I) ACTION SUPPLY
Four pairs of muscles are involved in mastication (chewing and biting) activities, and all are innervated by the mandibular branch of cranial nerve V (the trigeminal nerve ). The prime movers of jaw closure (and biting) are the powerful masseter and temporalis muscles, which can be palpated easily when the teeth are clenched (Figure 10.7a). Side-to- side grinding movements are brought about by the pterygoid muscles (Figure 10.7b). The buccinator muscles (see Table 10.1) also play a role in chewing. Normally, gravity is sufficient to depress the mandible, but if there is resistance to jaw opening, neck muscles such as the digastric and mylohyoid muscles (see Table 10.3) are activated.
The tongue is composed of muscle fibers that curl, squeeze, and fold the tongue during speaking and chewing. These intrinsic tongue muscles , arranged in several planes, change the shape of the tongue and contribute to its exceptional nimbleness, but they do not really move the tongue. They are considered in Chapter 23 with the digestive system. In this table, we consider only the extrinsic tongue muscles , which anchor and move the tongue (Figure 10.7c). All extrinsic tongue muscles are innervated by cranial nerve XII , the hypoglossal nerve (see Table 13.2).
Masseter (mah-se ter) ( maseter chewer)
Fan-shaped muscle that covers parts of the temporal, frontal, and parietal bones
O—temporal fossa I—coronoid process of mandible via a tendon that passes deep to zygomatic arch
Closes jaw; elevates and retracts mandible; maintains position of the mandible at rest; deep anterior part may help protract mandible
Temporalis (tempor-a˘ lis) ( tempora time; pertaining to the temporal bone)
TABLE 10.2 Muscles of the Head, Part II: Mastication and Tongue Movement (Figure 10.7)
Trigeminal nerve
Powerful muscle that covers lateral aspect of mandibular ramus
O—zygomatic arch and zygomatic bone I—angle and ramus of mandible
Prime mover of jaw clo- sure; elevates mandible
Trigeminal nerve (cranial V)
MUSCLE GALLERY
MUSCLES OF MASTICATION
Deep two-headed muscle that runs along internal surface of mandible and is largely concealed by that bone
O—medial surface of lateral pterygoid plate of sphenoid bone, maxilla, and palatine bone I—medial surface of mandible near its angle
Acts with the lateral pterygoid muscle to pro- trude (protract) mandible and to promote side- to-side side (grinding) movements; synergist of temporalis and masseter muscles in elevation of the mandible
Medial pterygoid Trigeminal nerve (me de-ul ter ı˘-goid) ( medial toward median plane; pterygoid winglike)
Deep two-headed muscle; lies superior to medial pterygoid muscle
O—greater wing and lateral pterygoid plate of sphenoid bone I—condyle of mandible and capsule of temporo- mandibular joint
Provides forward sliding and side-to-side grinding movements of the lower teeth; protrudes man- dible (pulls it anteriorly)
Lateral pterygoid Trigeminal nerve ( lateral away from median plane)
See Table 10.1 See Table 10.1 Compresses the cheek; helps keep food between grinding surfaces of teeth during chewing
Buccinator Facial nerve (cranial VII)
Slender muscle running superiorly to and at right angles to hyoglossus
O—styloid process of tem- poral bone I—inferolateral tongue
Retracts (and elevates) tongue
Styloglossus Hypoglossal nerve (sti-lo-glah sus) ( stylo pertaining to styloid process)
Fan-shaped muscle; forms bulk of inferior part of tongue; its attachment to mandible prevents tongue from falling back- ward and obstructing res- piration
O—internal surface of mandible near symphysis I—inferior aspect of the tongue and body of hyoid bone
Protracts tongue; can de- press or act in concert with other extrinsic mus- cles to retract tongue
Hypoglossal nerve (cranial XII)
Genioglossus (jene-o-glah sus) ( geni chin; glossus tongue)
Flat, quadrilateral muscle O—body and greater horn of hyoid bone I—inferolateral tongue
Depresses tongue and draws its sides downward
Hyoglossus (hi o-glosus) Hypoglossal nerve ( hyo pertaining to hyoid bone)
MUSCLES PROMOTING TONGUE MOVEMENTS (EXTRINSIC MUSCLES)
TABLE 10.2 (continued)
MUSCLE GALLERY
Orbicularis oris
Temporalis
Masseter Buccinator
(a)
Lateral pterygoid
Medial pterygoid
Masseter pulled away
(b)
Tongue (^) Styloid process
Styloglossus
Hyoglossus Stylohyoid Hyoid bone
Thyrohyoid
Genioglossus
Mandibular symphysis
Geniohyoid Thyroid cartilage
(c)
Figure 10.7 Muscles promoting mastication and tongue movements. (a) Lateral view of
the temporalis, masseter, and buccinator muscles. (b) Lateral view of the deep chewing muscles,
the medial and lateral pterygoid muscles. (c) Extrinsic muscles of the tongue. Some suprahyoid
muscles of the throat are also illustrated.
Chapter 10 The Muscular System 333
ORIGIN (O) AND NERVE
MUSCLE DESCRIPTION INSERTION (I) ACTION SUPPLY
TABLE 10.3 (continued)
MUSCLE GALLERY
Composite of three paired muscles whose fibers run circularly in pharynx wall; superior muscle is innermost and inferior one is outermost; substantial overlap
O—attached anteriorly to mandible and medial pterygoid plate (supe- rior), hyoid bone (mid- dle), and laryngeal cartilages (inferior) I—posterior median raphe of pharynx
Constrict pharynx dur- ing swallowing, which propels a food bolus to esophagus (via a mas- sagelike action called peristalsis)
Pharyngeal plexus [branches of vagus nerve (X)]
Pharyngeal constrictor muscles —superior, middle, and inferior (far-rin je-al)
Mylohyoid
Anterior belly Posterior belly
Stylohyoid (cut)
Thyrohyoid
Thyroid cartilage of the larynx
Median raphe
Sternothyroid
Stylohyoid Hyoid bone
Omohyoid (superior belly) Sternohyoid
Sternocleido- mastoid
Omohyoid (inferior belly)
Platysma (cut)
Digastric
Thyroid gland
(a)
Buccinator
Tensor veli palatini Levator veli palatini
Superior pharyngeal constrictor
Middle pharyngeal constrictor
Hyoid bone Thyrohyoid membrane Inferior pharyngeal constrictor Esophagus
Mandible
Mylohyoid (cut) Geniohyoid
Hyoglossus Thyroid cartilage of larynx
Trachea
Styloid process
(c)
(b)
Figure 10.8 Muscles of the anterior neck and throat that
promote swallowing. (a) Anterior view of the suprahyoid and infra-
hyoid muscles. The sternocleidomastoid muscle (not involved in
swallowing) is shown on the right side of the illustration to provide
an anatomical landmark. Deeper neck muscles are illustrated on the
left side. (b) Cadaver photo of suprahyoid and infrahyoid muscles.
(See A Brief Atlas of the Human Body, Figures 44 and 45.) (c) Lateral
view of the constrictor muscles of the pharynx shown in their proper
anatomical relationship to the buccinator (a chewing muscle) and
the hyoglossus muscle (which promotes tongue movements).
Chapter 10 The Muscular System 335
336 UNIT 2 Covering, Support, and Movement of the Body
*The ligamentum nuchae (ligah-men tum noo ke) is a strong, elastic ligament extending from the occipital bone of the skull along the tips of the spinous processes of the cervical vertebrae. It binds the cervical vertebrae together and inhibits excessive head and neck flexion, thus preventing damage to the spinal cord in the vertebral canal.
ORIGIN (O) AND NERVE
MUSCLE DESCRIPTION INSERTION (I) ACTION SUPPLY
Head movements
The head is moved by muscles originating from the axial skeleton. The major head flexors are the sternocleidomastoid muscles (Fig- ure 10.9 a, c), with some help from the suprahyoid and infrahyoid muscles described in Table 10.3. Lateral head movements (rotating or tilting the head) result when the muscles on only one side of the neck contract. These actions are effected by the sternocleidomastoids and a number of deeper neck muscles, considered in this table. Head ex- tension is aided by the trapezius muscles of the back, but the main extensors of the head are the splenius muscles deep to the trapezius muscles (Figure 10.9b).
Trunk extension
Trunk extension is effected by the deep or intrinsic back muscles asso- ciated with the bony vertebral column. These deep muscles of the back also maintain the normal curvatures of the spine, acting as pos- tural muscles. As you consider these back muscles, keep in mind that they are deep. The superficial back muscles that cover them are con- cerned primarily with movements of the shoulder girdle and upper limbs (see Tables 10.8 and 10.9). The deep muscles of the back form a broad, thick column extend- ing from the sacrum to the skull. Many muscles of varying length
contribute to this mass. It helps to regard each of these individual muscles as a string that when pulled causes one or several vertebrae to extend or to rotate on the vertebrae below. The largest of the deep back muscle groups is the erector spinae group (Figure 10.9d). Because the origins and insertions of the different muscle groups overlap extensively, and many of these muscles are long, large regions of the vertebral column can be moved simultaneously and smoothly. Acting in concert, the deep back muscles extend (or hyperextend) the spine, but contraction of the muscles on only one side causes lateral bending (flexion) of the spine. Lateral flexion is automatically accom- panied by some degree of rotation of the vertebral column. During vertebral movements, the articular facets of the vertebrae glide on each other. In addition to the long back muscles, a number of short muscles ex- tend from one vertebra to the next. These small muscles act primarily as synergists in extension and rotation of the spine and as spine stabi- lizers. They are not described in the table but you can deduce their actions by examining their origins and insertions in Figure 10.9e. As noted, it is the trunk extensors that we consider in this table. The more superficial muscles, which have other functions, are consid- ered in subsequent tables. For example, the anterior muscles of the abdominal wall that cause trunk flexion are described in Table 10.6.
ANTEROLATERAL NECK MUSCLES (FIGURE 10.9a AND c)
Two-headed muscle lo- cated deep to platysma on anterolateral surface of neck; fleshy parts on either side of neck delin- eate limits of anterior and posterior triangles; key muscular landmark in neck; spasms of one of these muscles may cause torticollis (wryneck)
O—manubrium of ster- num and medial portion of clavicle I—mastoid process of temporal bone and supe- rior nuchal line of occipi- tal bone
Flexes and laterally ro- tates the head; simultane- ous contraction of both muscles causes neck flex- ion, generally against re- sistance as when one raises head when lying on back; acting alone, each muscle rotates head toward shoulder on opposite side and tilts or laterally flexes head to its own side
Sternocleidomastoid (sterno-klido-mas toid) ( sterno breastbone; cleido clavicle; mastoid mastoid process)
TABLE 10.4 Muscles of the Neck and Vertebral Column: Head Movements and Trunk Extension (Figure 10.9)
Accessory nerve (cranial nerve XI) and branches of cervical spinal nerves C 2 and C 3 (ventral rami)
MUSCLE GALLERY
Located more laterally than anteriorly on neck; deep to platysma and sternocleidomastoid
O—transverse processes of cervical vertebrae I—anterolaterally on first two ribs
Elevate first two ribs (aid in inspiration); flex and rotate neck
Scalenes (ska le¯ nz)— Cervical spinal nerves anterior, middle, and posterior ( scalene uneven)
Broad bipartite superficial muscle (capitis and cervi- cis parts) extending from upper thoracic vertebrae to skull; capitis portion known as “bandage mus- cle” because it covers and holds down deeper neck muscles
O—ligamentum nuchae,* spinous processes of ver- tebrae C 7 –T 6 I—mastoid process of temporal bone and occip- ital bone (capitis); trans- verse processes of C 2 –C 4 vertebrae (cervicis)
Extend or hyperextend head; when splenius mus- cles on one side are acti- vated, head is rotated and bent laterally toward same side
Cervical spinal nerves (dorsal rami)
Splenius (sple ne-us)—capitis and cervicis portions (ka˘ pı˘-tis; ser-vis us) ( splenion bandage; caput head; cervi neck) (Figures 10.9b and 10.6)
INTRINSIC MUSCLES OF THE BACK (FIGURE 10.9b, d, e)
338 UNIT 2 Covering, Support, and Movement of the Body
ORIGIN (O) AND NERVE
MUSCLE DESCRIPTION INSERTION (I) ACTION SUPPLY
Erector spinae (e-rek tor spi ne) Also called sacrospinalis (Figure 10.9d, left side)
Most lateral muscle group of erector spinae muscles; extend from pelvis to neck
O—iliac crests (lumbor- um); inferior 6 ribs (tho- racis); ribs 3 to 6 (cervicis) I—angles of ribs (lumbor- um and thoracis); trans- verse processes of cervical vertebrae C 6 –C 4 (cervicis)
Extend and laterally flex the vertebral column; maintain erect posture; acting on one side, bend vertebral column to same side
■ (^) Iliocostalis (ile-o-kos- ta˘ lis)—lumborum, thoracis, and cervicis portions (lum bor-um; tho-ra sis) ( ilio ilium; cost rib; thorac thorax)
Muscles of the Neck and Vertebral Column: Head Movements and Trunk Extension (Figure 10.9) (continued)
TABLE 10.
Spinal nerves (dorsal rami)
Prime mover of back extension. Each side consists of three columns—the iliocostalis, longissimus, and spinalis muscles—forming intermediate layer of intrinsic back muscles. Erector spinae provide resistance that helps control action of bending forward at the waist and act as powerful extensors to promote return to erect position. During full flexion (i.e., when touching fingertips to floor), erector spinae are relaxed and strain is borne entirely by liga- ments of back; on reversal of the movement, these muscles are initially inactive, and extension is initiated by ham- string muscles of thighs and gluteus maximus muscles of buttocks. As a result of this peculiarity, lifting a load or moving suddenly from a bent-over position is potentially dangerous (in terms of possible injury) to muscles and ligaments of back and intervertebral discs; erector spinae muscles readily go into painful spasms following injury to back structures.
MUSCLE GALLERY
Intermediate tripartite muscle group of erector spinae; extend by many muscle slips from lumbar region to skull; mainly pass between transverse processes of the vertebrae
O—transverse processes of lumbar through cervi- cal vertebrae I—transverse processes of thoracic or cervical verte- brae and to ribs superior to origin as indicated by name; capitis inserts into mastoid process of tem- poral bone
Thoracis and cervicis act together to extend and laterally flex vertebral col- umn; capitis extends head and turns the face toward same side
Spinal nerves (dorsal rami)
■ (^) Longissimus (lon-jis ı˘- mus)— thoracis, cervicis, and capitis parts ( longissimus longest)
Most medial muscle col- umn of erector spinae; cervicis usually rudimen- tary and poorly defined
O—spinous process of upper lumbar and lower thoracic vertebrae I—spinous process of up- per thoracic and cervical vertebrae
Extends vertebral column Spinal nerves (dorsal rami)
■ (^) Spinalis (spi-na˘ lis)— thoracis and cervicis parts ( spin vertebral column, spine)
Composite muscle form- ing part of deep layer of intrinsic back muscles; ex- tends from thoracic re- gion to head
O—transverse processes of C 7 –T 12 I—occipital bone (capitis) and spinous processes of cervical (cervicis) and thoracic vertebrae T 1 –T 4 (thoracis)
Extends vertebral column and head and rotates them to opposite side; acts synergistically with sternocleidomastoid mus- cles of opposite side
Spinal nerves (dorsal rami)
Semispinalis (sem e-spı˘-na˘ lis)—thoracis, cervicis, and capitis regions ( semi half) (Figure 10.9d, right side)
Fleshy muscle forming part of posterior abdomi- nal wall
O—iliac crest and lumbar fascia I—transverse processes of lumbar vertebrae L 1 –L 4 and lower margin of 12th rib
Flexes vertebral column laterally when acting separately; when pair acts jointly, lumbar spine is extended and 12th rib is fixed; maintains upright posture; assists in forced inspiration
T 12 and upper lumbar spinal nerves (ventral rami)
Quadratus lumborum (kwod-ra tus lum-bor um) ( quad four-sided; lumb lumbar region) (See also Figure 10.19a)
Chapter 10 The Muscular System 339
TABLE 10.4 (continued)
MUSCLE GALLERY
Mastoid process of temporal bone
Semispinalis capitis
Semispinalis cervicis
Multifidus
Quadratus lumborum
Semispinalis thoracis
Longissimus capitis
Iliocostalis cervicis Longissimus cervicis
Iliocostalis thoracis
Longissimus thoracis
Spinalis thoracis
Iliocostalis lumborum
External oblique
Iliocostalis
Erector Longissimus spinae Spinalis
Ligamentum nuchae
(d)
Intertransversarius
Interspinales
Multifidus
Rotatores
(e)
O
I
= origin = insertion
I
I
I
I
I
O
O
O
O
Figure 10.9 (continued) Muscles of the neck and vertebral
column causing movements of the head and trunk. (d) Deep
muscles of the back; superficial, intermediate, and splenius muscles
removed. The erector spinae is shown on the left; deeper semi-
spinalis muscles are shown on the right. (e) The deepest muscles
of the back associated with the vertebral column.
