Axial Muscle

MUSCUL AR 11 Axial Muscles SYSTEM O U T L I N E 11.1 Muscles of the Head and Neck 11.1a 11.1b 11.1c 11.1d 11.1e 11.1f 11.1g 11.2 11.3 11.4 11.5 5 323 Muscles of Facial Expression 323 Extrinsic Eye Muscles 328 Muscles of Mastication 332 Muscles That Move the Tongue 332 Muscles of the Pharynx 334 Muscles of the Anterior Neck 335 Muscles That Move the Head and Neck 337 Muscles of the Vertebral Column 340 Muscles of Respiration 343 Muscles of the Abdominal Wall 345 Muscles of the Pelvic Floor 348 MODULE 6: MUSCUL AR SYSTEM mck78097_ch11_322-353.indd 322 2/14/11 3:06 PM Chapter Eleven T he artificial partitioning of the skeletal system into axial and appendicular divisions provides a useful guideline for subdividing the muscular system as well. Axial muscles have both their origins and insertions on parts of the axial skeleton. Axial muscles support and move the head and spinal column, function in nonverbal communication by affecting facial features, move the lower jaw during chewing, assist in food processing and swallowing, aid breathing, and support and protect the abdominal and pelvic organs. The axial muscles are not responsible for stabilizing or moving the pectoral or pelvic girdles or their attached limbs; those are functions of the appendicular muscles (see chapter 12). Some muscles of both divisions are shown in figure 11.1. Study Tip! The following suggestions may help you learn the muscles, whether axial or appendicular: ■ ■ ■ ■ ■ ■ The axial muscles may be organized into groups based on their location or function. Learning the muscles in groups is easiest. When studying a particular muscle, try to palpate it on yourself. Contract the muscle to sense its action and identify its regional location. Repeat the name of a muscle aloud to become familiar with its name. It is easier to remember and spell terms that you can pronounce. Associate visual images from models, cadavers, a photographic atlas, or dissected animals with muscle names. Connecting a muscle name with direct observation will help you remember it. Locate the origins and insertions of muscles on an articulated skeleton. This helps you visualize the locations of muscles and understand how they produce particular actions. Learn the derivation of each muscle name because it usually describes the muscle’s action, location, number of heads, orientation of muscle fibers, shape, or size. Refer to figure 10.14 for examples of how some muscles are named. The axial muscles are organized into five groups based on their location: ■ ■ ■ ■ ■ Muscles of the head and neck Muscles of the vertebral column Muscles of respiration Muscles of the abdominal wall Muscles of the pelvic floor The discussion in this chapter has been organized according to these specific groups. For each group, tables provide descriptions of the muscles as well as information about their action, origin, insertion, and innervation. (Note: The word innervation refers to the nerve(s) that supplies a muscle and stimulates it to contract. For further information about the nerves listed in the tables, see chapters 15 and 16.) 11.1 Muscles of the Head and Neck Learning Objectives: 1. Identify the major muscles involved in facial expression. 2. List the extrinsic muscles of the eye, and describe how each affects eye movement. mck78097_ch11_322-353.indd 323 Axial Muscles 323 3. Name the muscles of mastication, and outline how each affects mandibular movement. 4. Describe the movements of the tongue, and compare its extrinsic and intrinsic muscles. 5. Identify the muscles of the pharynx, and explain their function in swallowing. 6. Explain the organization and distribution of the muscles of the anterior neck. 7. Detail the muscles involved in the major movements of the head and neck. The muscles of the head and neck are separated into several specific groups, based on their location or general functions. Almost all of these muscles (except for a few muscles of the anterior neck) originate on either the skull or the hyoid bone. 11.1a Muscles of Facial Expression The muscles of facial expression have their origin in the superficial fascia or on the skull bones (figure 11.2). These muscles insert into the superficial fascia of the skin, so when they contract, they contort the skin, causing it to move. Most of these muscles are innervated by the seventh cranial nerve (CN VII), the facial nerve. The epicranius is composed of the occipitofrontalis muscle and a broad epicranial aponeurosis, also called the galea aponeurotica. The frontal belly of the occipitofrontalis is superficial to the frontal bone on the forehead. When this muscle contracts, it raises the eyebrows and wrinkles the skin of the forehead. The occipital belly of the occipitofrontalis covers the posterior side of the head. When this muscle contracts, it retracts the scalp slightly. Deep to the frontal belly is the corrugator supercilii. This muscle draws the eyebrows together and creates vertical wrinkle lines above the nose. The orbicularis oculi consists of circular muscle fibers that surround the orbit. When this muscle contracts, the eyelid closes, as when you wink, blink, or squint. The levator palpebrae superioris (shown in figure 19.10) elevates the upper eyelid when you open your eyes. Several muscles of facial expression are associated with the nose. The nasalis elevates the corners of the nostrils. When you “flare your nostrils,” you are using the nasalis muscles. If you wrinkle your nose in distaste after smelling a foul odor, you have used your procerus muscle. This muscle is continuous with the frontalis muscle, and it runs over the bridge of the nose, where it produces transverse wrinkles when it contracts. The mouth is the most expressive part of the face, and not surprisingly the muscles in that area are very diverse. The orbicularis oris consists of muscle fibers that encircle the opening of the mouth. When this muscle contracts, you close your mouth. When you “pucker up for a kiss,” you are using this muscle. The depressor labii inferioris does what its name suggests—it pulls the lower lip inferiorly. The depressor anguli oris is considered the “frown” muscle, because it pulls the corners of the mouth inferiorly. (Note, however, that it takes more muscles than this one to produce a frown!) In contrast, some muscles of the mouth elevate part or all of the upper lips. The levator labii superioris pulls the upper lip superiorly, as if a person is sneering or snarling. The levator anguli oris pulls the corners of the mouth superiorly and laterally. The zygomaticus major and zygomaticus minor work with the levator anguli oris muscles. You use these when you smile. The risorius pulls the corner of the lips laterally; you use this muscle if you make a closed-mouth smile. 2/14/11 3:06 PM 324 Chapter Eleven Axial Muscles Superficial Deep Frontal belly of occipitofrontalis Temporalis Orbicularis oculi Zygomaticus major Masseter Orbicularis oris Platysma Sternocleidomastoid Sternohyoid Trapezius Pectoralis minor Deltoid Pectoralis major Serratus anterior Triceps brachii External intercostal Internal intercostal Biceps brachii Brachialis External oblique Rectus abdominis Pronator teres Transversus abdominis Brachioradialis Internal oblique (cut) Flexor carpi radialis External oblique (cut) Palmaris longus Iliopsoas Tensor fasciae latae Pectineus Adductor longus Sartorius Gracilis Rectus femoris Quadriceps femoris Vastus lateralis Vastus medialis Vastus intermedius Figure 11.1 Selected Axial and Appendicular Muscles. (a) Anterior view shows superficial muscles on the right side of the body and some deeper muscles on the left side. (b) Posterior view shows superficial muscles on the left side of the body and some deeper muscles on the right side. Labels for the axial muscles are in bold; not all muscles shown in the figure are identified. Fibularis longus Tibialis anterior Extensor digitorum longus Extensor hallucis longus (a) Anterior view mck78097_ch11_322-353.indd 324 2/15/11 11:03 AM Chapter Eleven Superficial Axial Muscles 325 Deep Occipital belly of occipitofrontalis Semispinalis capitis Sternocleidomastoid Splenius capitis Splenius cervicis Levator scapulae Trapezius Teres minor Teres major Supraspinatus Rhomboid minor Rhomboid major Infraspinatus Teres minor Teres major Triceps brachii Serratus anterior Deltoid Latissimus dorsi Serratus posterior inferior External oblique Internal oblique Erector spinae Extensor digitorum Extensor carpi ulnaris Gluteus medius Gluteus minimus Gluteus maximus Gluteus medius (cut) Piriformis Quadratus femoris Adductor magnus Biceps femoris Hamstrings Gracilis Iliotibial tract Semitendinosus Semimembranosus Gastrocnemius Soleus Calcaneal tendon (b) Posterior view mck78097_ch11_322-353.indd 325 2/14/11 3:06 PM 326 Chapter Eleven Axial Muscles Deep Superficial Epicranial aponeurosis Figure 11.2 Epicranius Muscles of Facial Expression. (a) Illustration and cadaver photo, anterior view. (b) Illustration and cadaver photo, lateral view. Frontal belly of occipitofrontalis Procerus Orbicularis oculi Nasalis Levator labii superioris Zygomaticus minor Levator anguli oris (cut) Zygomaticus major Masseter Buccinator Risorius Depressor anguli oris Orbicularis oris Depressor labii inferioris Mentalis Platysma Sternocleidomastoid Epicranial aponeurosis Epicranius Frontal belly of occipitofrontalis Corrugator supercilii Procerus Orbicularis oculi Nasalis Levator labii superioris Zygomaticus minor Levator anguli oris Zygomaticus major Masseter Orbicularis oris Depressor anguli oris Mentalis Depressor labii inferioris Platysma Sternocleidomastoid (a) Anterior view mck78097_ch11_322-353.indd 326 2/14/11 3:06 PM Chapter Eleven Axial Muscles 327 Epicranial aponeurosis Epicranius Frontal belly Temporalis Occipital belly Orbicularis oculi Levator labii superioris Zygomaticus minor Levator anguli oris Zygomaticus major Masseter Buccinator Orbicularis oris Sternocleidomastoid Mentalis Depressor labii inferioris Depressor anguli oris Platysma Epicranial aponeurosis Epicranius Frontal belly of occipitofrontalis Temporalis Orbicularis oculi Levator labii superioris Zygomaticus minor Levator anguli oris Zygomaticus major Orbicularis oris Parotid gland Parotid duct Masseter Buccinator Sternocleidomastoid Depressor labii inferioris Depressor anguli oris Platysma (b) Lateral view mck78097_ch11_322-353.indd 327 2/14/11 3:06 PM 328 Chapter Eleven Axial Muscles CLINICAL VIEW Idiopathic Facial Nerve Paralysis (Bell Palsy) Unilateral paralysis of the muscles of facial expression is termed facial nerve paralysis. This condition results from either disease or injury to the facial nerve (CN VII). If the cause of the condition is unknown, doctors refer to it as idiopathic (id ḗ -ō -path ́ ik; idios = one’s own, pathos = suffering) facial nerve paralysis, or Bell palsy. Recent studies indicate a frequent link between CN VII paralysis and herpes simplex 1 viral infection. Facial nerve paralysis is also associated with exposure to cold temperatures, and is commonly seen in individuals who sleep with one side of their head facing an open window. Another possible cause of facial nerve paralysis is compression of the facial nerve by an adjacent blood vessel. Whatever the underlying cause, the nerve becomes inflamed and compressed within the narrow stylomastoid foramen. to combat the symptoms of dry eye, and sometimes it’s necessary to patch the affected eye to keep it closed while the patient is sleeping. Like its underlying cause, recovery from idiopathic facial nerve paralysis is mysterious. Over 50% of all patients experience a complete, spontaneous recovery within 30 days of their first symptoms. For others, recovery may take longer, while still others may never recover. Current statistics indicate that the recovery rate for idiopathic facial nerve paralysis averages about 80%, and does not appear to be related to its treatment. The facial nerve innervates all but one of the muscles of facial expression, so if its function becomes impaired, the muscles on the same side of the face are paralyzed. The patient may be unable to wrinkle the forehead (paralyzed occipitofrontalis muscle), pucker the lips (paralyzed orbicularis oris), or close the eyelid on the affected side (paralyzed orbicularis oculi). These symptoms lead to other problems as well. For example, if the affected individual cannot close his or her eye, the eye becomes dry, possibly damaging the cornea. If unable to close the mouth, the person drools, and the mucous membranes of the mouth become parched. Treatment of facial nerve paralysis usually means alleviating the symptoms. Doctors often use prednisone (a type of steroid) to reduce the inflammation and swelling of the nerve. If herpes simplex infection is suspected, an antiviral medication called acyclovir (Zovirax) is also given. Paralysis of the orbicularis oculi may require eyedrops The mentalis attaches to the lower lip, and when it contracts, it protrudes the lower lip (as when a person “pouts”). The platysma tenses the skin of the neck and pulls the lower lip inferiorly. If you stand in front of a mirror and tense the skin of your neck, you can see these thin muscles bulging out. The buccinator compresses the cheek against the teeth when we chew (and is the reason our cheeks don’t bulge like a squirrel’s cheeks when we eat). Infants use the buccinator when they suckle at the breast. Some trumpet players (such as Dizzy Gillespie) have stretched out their buccinator muscles, allowing their cheeks to be “puffy” with air when they play the trumpet. Table 11.1 summarizes the attachments and movements of the muscles of facial expression. Figure 11.3 illustrates how these muscles produce some of the more characteristic expressions. W H AT D O Y O U T H I N K ? 1 ● What muscles of facial expression must contract for you to smile? mck78097_ch11_322-353.indd 328 Facial nerve (CN VII) paralysis on the left side of the face. Note the drooping left side of the mouth (arrow) and the lack of contraction by the left orbicularis oculi when the woman tries to smile. 11.1b Extrinsic Eye Muscles The extrinsic eye muscles, often called extraocular muscles, move the eyes. They are termed extrinsic because they originate within the orbit and insert onto the white outer surface of the eye, called the sclera. There are six extrinsic eye muscles: the rectus muscles (medial, lateral, inferior, and superior) and the oblique muscles (inferior and superior) (figure 11.4). The rectus eye muscles have their origin from a common tendinous ring in the orbit. These muscles insert on the anterior part of the sclera of the eye, and are named according to which side of the eye they are located at (medial, lateral, inferior, or superior). The medial rectus attaches to the anteromedial surface of the eye and pulls the eye medially (adducts the eye). It is innervated by CN III (oculomotor nerve). The lateral rectus attaches to the anterolateral surface of the eye and pulls the eye laterally (abducts the eye). This muscle is innervated by CN VI (abducens). (Note that this nerve’s name tells you what muscle it innervates—the eye muscle that abducts the eye.) The inferior 2/14/11 3:06 PM Chapter Eleven Table 11.1 Muscles of Facial Expression Region/Muscle Action Origin/Insertion 329 Axial Muscles Innervation SCALP Epicranius (ep ́ı̆-krā ́nē-us) epi = over cran = skull Composed of an epicranial aponeurosis and the occipitofrontalis muscle Frontal belly of occipitofrontalis (ok-sip ́i-tō-fron-tă ́lis) front = forehead Moves scalp, eyebrows; wrinkles skin of forehead O: Frontal bone I: Epicranial aponeurosis CN VII (facial nerve) Occipital belly of occipitofrontalis occipito = base of skull Retracts scalp O: Superior nuchal line I: Epicranial aponeurosis CN VII (facial nerve) Nasalis (nā ́ză-lis) nasus = nose Compresses bridge and depresses tip of nose; elevates corners of nostrils O: Maxillae and alar cartilage of nose I: Dorsum of nose CN VII (facial nerve) Procerus (prō-sē ́rŭs) procerus = long Moves and wrinkles nose O: Nasal bone and lateral nasal cartilage I: Aponeurosis at bridge of nose and skin of forehead CN VII (facial nerve) Buccinator (buk ́sı̆-nā ́tōr) bucco = cheek Compresses cheek; holds food between teeth during chewing O: Alveolar processes of mandible and maxillae CN VII (facial nerve) Depressor anguli oris (dē-pres ́ōr ang ́ū-lı̄ ōr ́ŭs) depressor = depresses angul = angle or = mouth Draws corners of mouth inferiorly and laterally (“frown” muscle) O: Body of mandible I: Skin at inferior corner (angle) of mouth CN VII (facial nerve) Depressor labii inferioris (dē-pres ́ōr lā ́bē-ı̄ in-fēr ́ē-ōr-is) labi = lip infer = below Draws lower lip inferiorly O: Body of mandible lateral to midline I: Skin at inferior lip CN VII (facial nerve) Levator anguli oris (lē-vā ́tor, le-vā ́ter ang ́ū-lı̄ ōr ́ūs) leva = raise Draws corners of mouth superiorly and laterally (“smile” muscle) O: Lateral maxilla I: Skin at superior corner of mouth CN VII (facial nerve) Levator labii superioris (lē-vā ́tor, le-vā ́ter lā ́bē-ı̄ sū-pēr ḗ -ōr-is) Opens lips; raises and furrows the upper lip (“Elvis” lip snarl) O: Zygomatic bone; maxilla I: Skin and muscle of superior lip CN VII (facial nerve) Mentalis (men-tā ́lis) ment = chin Protrudes lower lip (“pout”); wrinkles chin O: Central mandible I: Skin of chin CN VII (facial nerve) Orbicularis oris (ōr-bik ū ́ -lā ŕ is ōr í s) orb = circular or = mouth Compresses and purses lips (“kiss” muscle) O: Maxilla and mandible; blend with fibers from other facial muscles I: Encircling mouth; skin and muscles at angles to mouth CN VII (facial nerve) Risorius (ri-sōr ́ē-ŭs) risor = laughter Draws corner of lip laterally; tenses lips; synergist of zygomaticus O: Deep fascia associated with masseter muscle I: Skin at angle of mouth CN VII (facial nerve) Zygomaticus major (zı̄ g ́ ō-mat í -kŭs) zygomatic = cheekbone major = greater Elevates corner of the mouth (“smile” muscle) O: Zygomatic bone I: Skin at superolateral edge of mouth CN VII (facial nerve) Zygomaticus minor (zı̄ g ́ ō-mat í -kŭs) minor = lesser Elevates corner of the mouth (“smile” muscle) O: Zygomatic bone I: Skin of superior lip CN VII (facial nerve) NOSE MOUTH I: Orbicularis oris (continued on next page) mck78097_ch11_322-353.indd 329 2/14/11 3:06 PM 330 Chapter Eleven Axial Muscles Table 11.1 Muscles of Facial Expression (continued) Region/Muscle Action Origin/Insertion Innervation Corrugator supercilii (kōr ́ŭ-gā-ter soo ́per-sil ́ē-ı̄) corrugo = to wrinkle cilium = eyelid Pulls eyebrows inferiorly and medially; creates vertical wrinkles above nose O: Medial end of superciliary arch I: Skin superior to supraorbital margin and superciliary arch CN VII (facial nerve) Levator palpebrae superioris (see figure 19.10) (le-vā ́ter pal-pē ́brā soo-pēr ́ē-ōr-ı̆s) levo = to lift palpebra = eyelid Elevates superior eyelid O: Lesser wing of sphenoid bone I: Superior tarsal plate and skin of superior eyelid CN III (oculomotor nerve) Orbicularis oculi (ōr-bik ́ū-lā ́ris ok ́ū-lı̄) orb = circular ocul = eye Closes eye; produces winking, blinking, squinting (“blink” muscle) O: Medial wall or margin of orbit I: Skin surrounding eyelids CN VII (facial nerve) Pulls lower lip inferiorly; tenses skin of neck O: Fascia of deltoid and pectoralis major muscles and acromion of scapula I: Skin of cheek and mandible CN VII (facial nerve) EYE NECK Platysma (plă-tiz ́mă) platy = flat rectus attaches to the anteroinferior part of the sclera. The inferior rectus pulls the eye inferiorly (as when you look down) and medially (as when you look at your nose). The superior rectus is located superiorly and attaches to the anterosuperior part of the sclera. The superior rectus pulls the eye superiorly (as when you look up) and medially (as when you look at your nose). The inferior and superior rectus muscles are innervated by CN III. Figure 11.4d illustrates that the superior and inferior rectus muscles do not pull directly parallel to the long axis of the eye; that is why both muscles also move the eye slightly in the medial direction. Figure 11.3 Depressor anguli oris (frown) The oblique eye muscles originate from within the orbit and insert on the posterolateral part of the sclera of the eye. The inferior oblique elevates the eye and turns the eye laterally. Since this muscle attaches to the inferior posterior part of the eye, contracting this muscle pulls the posterior part of the eye inferiorly (but elevates the anterior part of the eye). This muscle is innervated by CN III. The superior oblique depresses the eye and turns the eye laterally. This muscle passes through a pulleylike loop, called the trochlea, in the anteromedial orbit. This muscle attaches to the superior posterior part of the eye, so contracting this muscle pulls Orbicularis oculi (blink/close eyes) Zygomaticus major (smile) Surface Anatomy of Some Muscles of Facial Expression. These muscles permit complex expressions that are often used as a means of communication. Orbicularis oris (close mouth/kiss) mck78097_ch11_322-353.indd 330 Frontal belly of occipitofrontalis (wrinkle forehead, raise eyebrows) Platysma (tense skin of neck) 2/14/11 3:06 PM Chapter Eleven Study Tip! Remembering the innervation of the eye muscles can be difficult. Use the following “chemical formula” to help you learn the eye muscle innervation: 331 Axial Muscles the posterior part of the eye superiorly (but depresses the anterior surface of the eye). This muscle is innervated by CN IV (trochlear). (Note that this nerve’s name is derived from the trochlea that holds the superior oblique in place.) Table 11.2 compares the extrinsic muscles of the eye. [(S04)(LR6)]3 W H AT D I D Y O U L E A R N? In other words, the superior oblique (SO) is innervated by cranial nerve IV (4), the lateral rectus (LR) is innervated by cranial nerve VI (6), and the rest of the eye muscles are innervated by cranial nerve III (3). 1 ● What are the origins for all the muscles of facial expression? 2 ● List the extrinsic eye muscles, and describe the function of each muscle. 3 ● The corners of the mouth are pulled inferiorly into a frown position by the contraction of what muscle? Trochlea Trochlea Superior rectus Superior oblique Frontal bone Superior rectus Superior oblique Orbicularis oculi Medial rectus Common tendinous ring Optic nerve Optic nerve Lateral rectus Inferior rectus Inferior oblique Common tendinous ring Inferior rectus Maxilla Inferior oblique (a) Lateral view, right eye (b) Medial view, right eye Trochlea Sagittal axis Superior rectus Coronal axis Trochlea Center of eye Inferior oblique Superior oblique Lateral rectus Optic canal Common tendinous ring Inferior oblique Inferior rectus Medial rectus (c) Anterior view of right orbit, eye removed Figure 11.4 Inferior rectus Lateral rectus Medial rectus Superior rectus Superior oblique (d) Superior view Extrinsic Muscles of the Eye. The extrinsic eye muscles control movements of the eye. (a) The insertions for the lateral, superior, and inferior rectus, as well as both the inferior and superior oblique muscles, appear prominently in a lateral view of the right eye. (b) The medial rectus muscle appears prominently in a medial view of the right eye. (c) Most of the eye muscles originate from a common tendinous ring, shown here in an anterior view of the right orbit. (d) A superior view of the left and right orbits illustrates the insertion differences between the rectus and oblique muscles, and how these differences affect their movement of the eye. mck78097_ch11_322-353.indd 331 2/14/11 3:06 PM 332 Chapter Eleven Axial Muscles Table 11.2 Extrinsic Eye Muscles Group/Muscle Action Origin/Insertion Innervation Medial rectus (mē´dē-ăl rek t́ us) rectus = straight Moves eye medially (adducts eye) O: Common tendinous ring I: Anteromedial surface of eye CN III (oculomotor nerve) Lateral rectus (lat é r-ăl rek t́ us) Moves eye laterally (abducts eye) O: Common tendinous ring I: Anterolateral surface of eye CN VI (abducens nerve) Inferior rectus (in-fē´rē-ōr rek ́tus) Moves eye inferiorly (depresses eye) and medially (adducts eye) O: Common tendinous ring I: Anteroinferior surface of eye CN III (oculomotor nerve) Superior rectus (soo-pēr ́ē-ōr rek ́tus) Moves eye superiorly (elevates eye) and medially (adducts eye) O: Common tendinous ring I: Anterosuperior surface of eye CN III (oculomotor nerve) Inferior oblique (in-fē ́rē-ōr ob-lēk ́) obliquus = slanting Moves eye superiorly (elevates eye) and laterally (abducts eye) O: Anterior orbital surface of maxilla I: Posteroinferior, lateral surface of eye CN III (oculomotor nerve) Superior oblique (soo-pēr ́ē-ōr ob-lēk ́) Moves eye inferiorly (depresses eye) and laterally (abducts eye) O: Sphenoid bone I: Posterosuperior, lateral surface of eye CN IV (trochlear nerve) RECTUS MUSCLES OBLIQUE MUSCLES CLINICAL VIEW Strabismus When the eyes are improperly aligned, the condition is called strabismus (stra-biz m ́ ŭs; strabismos = a squinting). The misalignment means the eyes are not working synchronously to transmit a stereoscopic view to the brain. With each eye sending a slightly different image, the brain becomes confused and ignores one of the images. The ignored eye becomes weaker and weaker over time, resulting in a condition termed “lazy eye.” If uncorrected, the lazy eye loses visual acuity, a condition termed strabismic amblyopia (amblē-ō ṕ ē-ă; amblys = dull, ops = eye). Causes of strabismus include birth injuries, diseases localized to the eye or its bony orbit, improper attachment of the extrinsic eye muscles, and heredity. Two forms of strabismus are recognized. External strabismus occurs when the oculomotor nerve (CN III) is injured, so that the affected eye moves laterally while at rest but cannot move medially and inferiorly. Conversely, internal strabismus occurs when the abducens nerve (CN VI) is injured. The affected eye moves medially but cannot move laterally. 11.1c Muscles of Mastication The term mastication (mas-ti-kā ś hŭn; masticatus = to chew) refers to the process of chewing. These muscles move the mandible at the temporomandibular joint. There are four paired muscles of mastication: the temporalis, the masseter, and the lateral and medial pterygoids (figure 11.5). The muscles of mck78097_ch11_322-353.indd 332 mastication are innervated by the mandibular division of CN V (trigeminal nerve). The temporalis (or temporal muscle) is a broad, fan-shaped muscle that extends from the temporal lines of the skull and inserts on the coronoid process of the mandible. It elevates and retracts (pulls posteriorly) the mandible. You can palpate the temporalis by placing your fingers along your temple (lateral skull at same level of orbits) as you open and close your mouth. The muscle you feel contracting is the temporalis. The masseter elevates and protracts (pulls anteriorly) the mandible. It is the most powerful and important of the masticatory muscles. This short, thick muscle is superficial to the temporalis. You can feel the contraction of the masseter by palpating near the angle of the mandible as you open and close your mouth. The lateral and medial pterygoid muscles arise from the lateral pterygoid plates of the sphenoid bone and insert on the mandible. Both pterygoids protract the mandible and move it from side to side during chewing. These movements maximize the efficiency of the teeth while chewing or grinding foods of various consistencies. The medial pterygoid also elevates the mandible. Table 11.3 summarizes the characteristics of the muscles of mastication. 11.1d Muscles That Move the Tongue The tongue is an agile, highly mobile organ. It consists of intrinsic muscles that curl, squeeze, and fold the tongue during chewing and speaking. Thus, the tongue itself acts like a big muscle. The extrinsic muscles of the tongue have their origin on other head and neck structures and insert on the tongue. The 2/14/11 3:06 PM Chapter Eleven 333 Axial Muscles Temporalis (cut) Temporalis Lateral pterygoid Medial pterygoid Masseter Buccinator Buccinator Orbicularis oris Orbicularis oris (a) Superficial lateral view (b) Deep lateral view Figure 11.5 Muscles of Mastication. (a) Superficial and (b) deep lateral views of the muscles of mastication (shown in bold), which move the mandible. extrinsic muscles end in the suffix -glossus, meaning “tongue” (figure 11.6). These extrinsic tongue muscles are used in various combinations to accomplish the precise, complex, and delicate tongue movements required for proper speech. Additionally, they manipulate food within the mouth in preparation for swallowing. Most of these muscles are innervated by CN XII, the hypoglossal nerve. The left and right genioglossus muscles have their origin on the mandible and protract the tongue. You use these muscles when you stick out your tongue. The left and right styloglossus muscles originate on the styloid processes of the temporal bone. These muscles elevate and retract the tongue (pull the tongue posteriorly, back into the mouth). The left and right hyoglossus muscles originate at the hyoid bone and insert on the sides of the tongue. These muscles depress and retract the tongue. The left and right palatoglossus muscles originate on the soft palate and elevate the posterior portion of the tongue. Table 11.3 Muscles of Mastication Muscle Action Origin/Insertion Innervation Temporalis (tem-pō-rā ́lis) tempora = pertaining to temporal bone Elevates and retracts mandible O: Superior and inferior temporal lines I: Coronoid process of mandible CN V3 (trigeminal nerve, mandibular division) Masseter (ma ś e-ter) maseter = chewer Elevates and protracts mandible; prime mover of jaw closure O: Zygomatic arch I: Coronoid process, lateral surface and angle of mandible CN V3 (trigeminal nerve, mandibular division) Medial pterygoid (mē ́dē-ăl ter í goyd) Elevates and protracts mandible; produces side-to-side movement of mandible O: Maxilla, palatine, and medial surface of lateral pterygoid plate I: Medial surface of mandibular ramus CN V3 (trigeminal nerve, mandibular division) Lateral pterygoid (lat ́er-ăl ter ́igoyd) pterygoid = winglike Protracts mandible; produces side-toside movement of mandible O: Greater wing of sphenoid and lateral surface of lateral pterygoid plate I: Condylar process of mandible CN V3 (trigeminal nerve, mandibular division) mck78097_ch11_322-353.indd 333 2/14/11 3:06 PM 334 Chapter Eleven Axial Muscles Figure 11.6 Muscles That Move the Tongue. Extrinsic tongue muscles (shown in bold) originate on structures other than the tongue and insert onto it to allow gross tongue movement. Styloid process Tongue Palatoglossus Styloglossus Genioglossus Stylohyoid Mandible (cut) Hyoglossus Geniohyoid Hyoid bone Thyroid cartilage Right lateral view Table 11.4 summarizes the characteristics of the muscles that move the tongue. 11.1e Muscles of the Pharynx The pharynx, commonly called the “throat,” is a funnelshaped tube that lies posterior to and extends inferiorly from both the oral and nasal cavities. Several muscles help form this muscular tube or attach to it and aid in swallowing (figure 11.7). Most pharyngeal muscles are innervated by CN X (vagus nerve). The primary pharyngeal muscles are the pharyngeal constrictors (superior, middle, and inferior). When a bolus of food enters the pharynx, these muscles contract sequentially to initiate swallowing and force the bolus inferiorly into the esophagus. Other pharyngeal muscles help elevate or tense the palate when swallowing. These muscles are summarized in table 11.5. Table 11.4 Muscles That Move the Tongue Muscle Action Origin/Insertion Innervation Genioglossus (jē ́nı̄-ō-glos ́ŭs) geni = chin glossus = tongue Protracts tongue O: Mental spines of mandible I: Inferior region of tongue; hyoid bone CN XII (hypoglossal nerve) Styloglossus (stı̄ ́lō-glos ́ŭs) stylo = pertaining to styloid process of temporal bone Elevates and retracts tongue O: Styloid process of temporal bone I: Sides and inferior aspect of tongue CN XII (hypoglossal nerve) Hyoglossus (hı̄ ́ō-glos ́ŭs) hyo = pertaining to hyoid bone Depresses and retracts tongue O: Hyoid bone I: Inferolateral side of tongue CN XII (hypoglossal nerve) Palatoglossus (pal-ā-tō-glos ́ŭs) palato = palate Elevates posterior part of tongue O: Anterior surface of soft palate I: Side and posterior aspect of tongue CN X (vagus nerve) via pharyngeal plexus of nerves mck78097_ch11_322-353.indd 334 2/14/11 3:06 PM Chapter Eleven Axial Muscles 335 Figure 11.7 Pharyngeal Constrictors, Palate Muscles, and Laryngeal Elevators. A right lateral view reveals some of the muscles that constrict the pharynx when swallowing, move the palate, and elevate the larynx (palatopharyngeus and salpingopharyngeus not shown). Tensor veli palatini Levator veli palatini Superior constrictor 11.1f Muscles of the Anterior Neck Stylopharyngeus The muscles of the anterior neck are divided into the suprahyoid muscles, which are superior to the hyoid bone, and the infrahyoid muscles, which are inferior to the hyoid bone (figure 11.8). The suprahyoid muscles are associated with the floor of the mouth. In general, these muscles act as a group to elevate the hyoid bone during swallowing or speaking. Some of these muscles perform additional functions: The digastric has two bellies, anterior and posterior. One belly extends from the mental protuberance to the hyoid, and the other continues from the hyoid to the mastoid portion of the temporal bone. The two bellies are united by an intermediate tendon that is held in position by a fascia sling (fibrous loop). In addition to elevating the hyoid bone, this muscle can also Middle constrictor Inferior constrictor Esophagus Right lateral view Table 11.5 Muscles of the Pharynx1 Region/Muscle Action Origin/Insertion Innervation Levator veli palatini (lē-vā ́tor, le-vā ́ter vel ı̄́ pal ́ă-tē ́nı̄) levator = elevates velum = veil Elevates soft palate when swallowing O: Petrous part of temporal bone I: Soft palate CN X (vagus nerve) Tensor veli palatini (ten ́sōr vel ı̄́ pal ́ă-tē ́nı̄) tensus = to stretch Tenses soft palate and opens auditory tube when swallowing or yawning O: Sphenoid bone; region around auditory tube I: Soft palate CN V3 (trigeminal nerve, mandibular division) Superior constrictor (kon-strik ́ter, -tor) constringo = to draw together Constricts pharynx in sequence to force bolus into esophagus; superior is innermost O: Pterygoid process of sphenoid bone; medial surface of mandible I: Posterior median raphe (muscle fiber union from both sides) CN X (vagus nerve) via branches of pharyngeal plexus Middle constrictor Constricts pharynx in sequence O: Hyoid bone I: Posterior median raphe CN X (vagus nerve) via branches of pharyngeal plexus Inferior constrictor Constricts pharynx in sequence; inferior is outermost O: Thyroid and cricoid cartilage I: Posterior median raphe CN X (vagus nerve) via branches of pharyngeal plexus PALATE MUSCLES PHARYNGEAL CONSTRICTORS LARYNGEAL (VOICE BOX) ELEVATORS 1 Palatopharyngeus (păl ắ -tō-far-in -́ jē-ŭs) pharynx = pharynx Elevates pharynx and larynx O: Soft palate I: Side of pharynx and thyroid cartilage of larynx CN X (vagus nerve) via branches of pharyngeal plexus Salpingopharyngeus (sal-ping ́gō-făr-in ́jē-ŭs) salpinx = trumpet Elevates pharynx and larynx O: Auditory tube I: Blends with palatopharyngeus on lateral wall of pharynx CN X (vagus nerve) via branches of pharyngeal plexus Stylopharyngeus (stı̄ ́lō-far-in j́ ē-ŭs) stylo = styloid process Elevates pharynx and larynx O: Styloid process of temporal bone I: Side of pharynx and thyroid cartilage of larynx CN IX (glossopharyngeal nerve) via branches of pharyngeal plexus Only the pharyngeal constrictors are discussed in the text. mck78097_ch11_322-353.indd 335 2/14/11 3:06 PM 336 Chapter Eleven Axial Muscles Deep Superficial Mylohyoid Suprahyoid muscles Genioglossus Stylohyoid Digastric: Anterior belly Posterior belly Geniohyoid Hyoglossus Suprahyoid muscles Hyoid bone Omohyoid Infrahyoid muscles Omohyoid Thyrohyoid Sternohyoid Sternothyroid Infrahyoid muscles Sternocleidomastoid Scalene muscles Trapezius Clavicle Mylohyoid Stylohyoid Suprahyoid muscles Digastric: Anterior belly Posterior belly Hyoid bone Infrahyoid muscles Omohyoid Thyrohyoid Omohyoid Sternohyoid Sternocleidomastoid Sternothyroid Infrahyoid muscles Scalene muscles Trapezius Clavicle Anterior view Figure 11.8 Muscles of the Anterior Neck. An illustration and a cadaver photo show the anterior neck muscles, which move the hyoid bone and the thyroid cartilage. Superficial muscles are shown on the right side, while deeper muscles are shown on the left side. mck78097_ch11_322-353.indd 336 2/14/11 3:06 PM Chapter Eleven depress the mandible. The geniohyoid originates from the mental spines of the mandible and inserts on the hyoid bone. This muscle elevates the hyoid bone. The broad, flat mylohyoid provides a muscular floor to the mouth. When this muscle contracts, it both elevates the hyoid bone and raises the floor of the mouth. The muscle fibers of the left and right mylohyoid are aligned in a V shape. The stylohyoid originates from the styloid process of the skull and inserts on the hyoid. Upon contraction, it elevates the hyoid bone, causing the floor of the oral cavity to elongate during swallowing. As swallowing ends, the infrahyoid muscles contract to influence the position of the hyoid bone and the larynx. In general, these muscles either depress the hyoid bone or depress the thyroid cartilage of the larynx. The omohyoid contains two thin muscle bellies anchored in place by a connective tissue “sling.” This muscle is lateral to the sternohyoid and extends from the superior border of the scapula and inserts on the hyoid, where it depresses the hyoid bone. The sternohyoid extends from the sternum to the hyoid, where it depresses the hyoid bone. The sternothyroid is deep to the sternohyoid. It extends from the sternum to the thyroid cartilage of the larynx. It depresses the thyroid cartilage to return it to its original position after swallowing. The thyrohyoid extends from the thyroid cartilage of the larynx to the hyoid. It depresses the hyoid bone and elevates the thyroid cartilage to close off the larynx during swallowing. In addition, the omohyoid, sternohyoid, and thyrohyoid help anchor the hyoid so the digastric can depress the mandible. Table 11.6 summarizes the characteristics of the muscles of the anterior neck. W H AT D O Y O U T H I N K ? 2 ● Since muscles frequently are named for their attachment sites, what do you think the prefix -omo in “omohyoid” means? Axial Muscles 337 11.1g Muscles That Move the Head and Neck Muscles that move the head and neck originate on the vertebral column, the thoracic cage, and the pectoral girdle, and insert on bones of the cranium (figure 11.9; see figure 11.8). Anterolateral Neck Muscles The anterolateral neck muscles flex the head and/or neck. The main muscles in this group are the sternocleidomastoid and the three scalenes. The sternocleidomastoid is a thick, cordlike muscle that extends from the sternum and clavicle to the mastoid process posterior to the ear. Contraction of both sternocleidomastoid muscles (called bilateral contraction) flexes the neck. Contraction of just one sternocleidomastoid muscle (called unilateral contraction) results in lateral flexion of the neck and rotation of the head to the opposite side. Thus, if the left sternocleidomastoid muscle contracts, it rotates the head to the right side of the body. The three scalene muscles (anterior, middle, and posterior) work with the sternocleidomastoid to flex the neck. In addition, the scalene muscles elevate the first and second ribs during forced inhalation. Posterior Neck Muscles Several muscles work together to extend the head and/or neck (figure 11.10). The trapezius attaches to the skull and helps extend the head and/or neck. The primary function of the trapezius is to help move the pectoral girdle, so it is discussed in greater detail in chapter 12. When the left and right splenius capitis, splenius cervicis, semispinalis capitis, and longissimus capitis muscles bilaterally contract, they extend the neck. Unilateral contraction turns the head and neck to the same side. A group of muscles called the suboccipital muscles includes the Table 11.6 Muscles of the Anterior Neck Region/Muscle Action Origin/Insertion Innervation Digastric (dı̄-gas t́ rik) di = two gaster = belly Depresses mandible; elevates hyoid bone O: Anterior belly, mandible near mental protuberance; posterior belly, mastoid process I: Hyoid bone via fascia sling Anterior belly: CN V3 (trigeminal nerve, mandibular division) Posterior belly: CN VII (facial nerve) Geniohyoid (jĕ ́nı̄-ō-hı̄ ́-oyd) hyoid = hyoid bone Elevates hyoid bone O: Mental spines of mandible I: Hyoid bone First cervical spinal nerve (C1) via CN XII (hypoglossal nerve) Mylohyoid (mı̄ ́lō-hı̄ ́oyd) myle = molar Elevates hyoid bone; elevates floor of mouth O: Mylohyoid line of mandible I: Hyoid bone CN V3 (trigeminal nerve, mandibular division) Stylohyoid (stı̄ ́lō-hı̄ ́oyd) Elevates hyoid bone O: Styloid process of temporal bone I: Hyoid bone CN VII (facial nerve) Omohyoid (ō ́mō-hı̄ ́oyd) omo = shoulder Depresses hyoid bone; fixes hyoid during opening of mouth O: Superior border of scapula I: Hyoid bone Cervical spinal nerves C1–C3 through ansa cervicalis (from cervical plexus) Sternohyoid (ster n ́ ō-hı̄ ́oyd) sterno = sternum Depresses hyoid bone O: Manubrium of sternum and medial end of clavicle I: Hyoid bone Cervical spinal nerves C1–C3 through ansa cervicalis (from cervical plexus) Sternothyroid (ster n ́ ō-thı̄ ́royd) thyro = thyroid cartilage Depresses thyroid cartilage of larynx O: Posterior surface of manubrium of sternum I: Thyroid cartilage of larynx Cervical spinal nerves C1–C3 through ansa cervicalis (from cervical plexus) Thyrohyoid (thı̄ ́rō-hı̄ ́oyd) Depresses hyoid bone and elevates thyroid cartilage of larynx O: Thyroid cartilage of larynx I: Hyoid bone First cervical spinal nerve C1 via CN XII (hypoglossal nerve) SUPRAHYOID MUSCLES INFRAHYOID MUSCLES mck78097_ch11_322-353.indd 337 2/14/11 3:06 PM 338 Chapter Eleven Axial Muscles Stylohyoid Posterior belly of digastric Semispinalis capitis Mylohyoid Splenius capitis Levator scapulae Anterior belly of digastric Thyrohyoid Inferior constrictor Sternocleidomastoid Sternothyroid Superior belly of omohyoid Scalene muscles Sternohyoid Trapezius Inferior belly of omohyoid Platysma (cut) Anterolateral view Figure 11.9 Muscles That Move the Head and Neck. and/or neck. Anterolateral muscles collectively flex the neck, while posterior neck muscles extend the head CLINICAL VIEW Congenital Muscular Torticollis Congenital muscular torticollis (CMT), often known as wryneck, is a condition where a newborn presents with a shortened and tightened sternocleidomastoid muscle. CMT is relatively common with estimates between 0.4% and 2% of births. It is thought to be a result of trauma resulting from either a difficult birth or prenatal position of the fetus. The trauma causes a hematoma and fibrosing of the muscle tissue. Pediatricians also have seen an increase in acquired muscular torticollis among newborns who are kept in their infant seats for extended periods of time outside of the car. Infants and children with CMT often tilt their heads to the affected side and their chins to the unaffected side. Since the infant favors a particular head position, plagiocephaly (flattening of the head) often accompanies CMT. CMT treatment typically involves repeated daily stretching of the affected muscle, changing sleeping positions, and making the child use the affected side while playing and interacting with others. More severe cases may require surgery. A newer approach to treatment of CMT is the use of botulinum toxin (Botox) combined with stretching. Botox is injected into the muscle to impair contraction in response to nerve stimulation, and thus the muscle responds better to stretching. mck78097_ch11_322-353.indd 338 Photo of a 7-year-old boy with CMT. Notice the prominence of the sternocleidomastoid on the left (affected) side. 2/14/11 3:06 PM Chapter Eleven Deep Axial Muscles 339 Deeper Rectus capitis posterior minor Rectus capitis posterior major Semispinalis capitis Obliquus capitis superior Sternocleidomastoid Obliquus capitis inferior Longissimus capitis Splenius capitis Semispinalis capitis (cut) Levator scapulae Splenius capitis (cut) Splenius cervicis Scalene (posterior) Rectus capitis posterior minor Rectus capitis posterior major Semispinalis capitis Obliquus capitis superior Sternocleidomastoid Obliquus capitis inferior Longissimus capitis Splenius capitis (cut) Splenius capitis Semispinalis capitis (cut) Splenius cervicis (cut) Levator scapulae Splenius cervicis Posterior view Figure 11.10 Posterior Neck Muscles. mck78097_ch11_322-353.indd 339 An illustration and a cadaver photo show the deep and deeper muscles that extend and rotate the head and neck. 2/14/11 3:06 PM 340 Chapter Eleven Axial Muscles Table 11.7 Muscles That Move the Head and Neck Muscle Action Origin/Insertion Innervation Sternocleidomastoid (ster ́nō-klı̄ ́dōmăs ́toyd) sterno = sternum cleido = clavicle masto = mastoid process Unilateral action : Lateral flexion, rotation of head to opposite side Bilateral action2: Flexes neck O: Manubrium and sternal end of clavicle I: Mastoid process CN XI (accessory nerve) Scalene muscles (anterior, middle, posterior) (see also table 11.9) (skā ́lēnz) scalene = uneven Flex neck (when 1st rib is fixed); elevate 1st and 2nd ribs during forced inhalation when neck is fixed O: Transverse processes of cervical vertebrae I: Superior surface of 1st and 2nd ribs Cervical spinal nerves Splenius capitis and cervicis (splē ́nē-ŭs ka p ́ ı̆-tis) (ser v ́ i-sis) splenion = bandage Unilateral action: Turns head to same side Bilateral action: Extends head/neck O: Ligamentum nuchae I: Occipital bone and mastoid process of temporal bone Cervical spinal nerves Longissimus capitis (lon-jis ́i-mŭs ka p ́ ı̆-tis) longissimus = longest caput = head Unilateral action: Turns (rotates) head toward same side Bilateral action: Extends head/neck O: Transverse process of T1–T4 and articular processes of C4–C7 vertebrae I: Mastoid process Cervical and thoracic spinal nerves Obliquus capitis superior (ob-lı̄ ́kŭs ka p ́ ı̆-tis soo-pēr ́ē-ōr) Turns head to same side O: Transverse process of atlas I: Inferior nuchal line Suboccipital nerve (posterior ramus of 1st cervical spinal nerve) Obliquus capitis inferior (ob-lı̄ ́kŭs ka p ́ ı̆-tis in-fē ́rē-ōr) obliquus = slanting Turns head to same side O: Spinous process of axis I: Transverse process of atlas Suboccipital nerve (posterior ramus of 1st cervical spinal nerve) Rectus capitis posterior major (rek ́tŭs ka ́pı̆-tis pos-tēr ḗ -ŏr) Extends head/neck O: Spinous process of axis I: Inferior nuchal line of occipital bone Suboccipital nerve (posterior ramus of 1st cervical spinal nerve) Rectus capitis posterior minor Extends head/neck O: Posterior tubercle of atlas I: Inferior nuchal line of occipital bone Suboccipital nerve (posterior ramus of 1st cervical spinal nerve) 1 1 Unilateral action means only one muscle (either the left or right muscle) is contracting. 2 Bilateral action means both the left and right muscles are contracting together. obliquus capitus superior, obliquus capitus inferior, rectus capitis posterior major, and rectus capitis posterior minor. The obliquus muscles turn the head to the same side, while the rectus muscles extend the head and neck. Table 11.7 summarizes the characteristics of the muscles of the head and neck. W H AT D I D Y O U L E A R N? 4 ● 5 ● 6 ● 7 ● What movements do the medial and lateral pterygoids perform? Which muscle protracts the tongue? List the suprahyoid muscles. What is their common function? The unilateral contraction of which muscle causes lateral flexion of the neck and rotation of the head to the opposite side? 11.2 Muscles of the Vertebral Column Learning Objective: 1. Detail the muscles involved in the movements of the vertebral column. The muscles of the vertebral column are very complex; they have multiple origins and insertions, and they exhibit extensive overlap (figure 11.11). All of these muscles are covered by the mck78097_ch11_322-353.indd 340 most superficial back muscles, which actually move the upper limb, including the trapezius and the latissimus dorsi. Note that the “neck” is actually the cervical portion of the vertebral column. Thus, the muscles discussed previously in connection with neck extension (splenius cervicis, splenius capitis, longissimus capitis, semispinalis capitis) extend the cervical portion of the vertebral column. The erector spinae function to maintain posture and to help an individual stand erect. When the left and right erector spinae muscles contract together, they extend the vertebral column. If the erector spinae muscles on only one side contract, the vertebral column flexes laterally toward that same side. The erector spinae muscles are organized into three groups; a series of multipart, overlapping muscles compose each of these groups. These muscles share a common tendinous insertion from the posterior part of the iliac crest, posterior sacrum, and spinous processes of the lumbar vertebrae. The muscles are named based on the body region with which they are associated. ■ ■ The iliocostalis group is the most laterally placed of the three erector spinae components. It is composed of three parts: cervical, thoracic, and lumbar. The longissimus group is medial to the iliocostalis group. The longissimus muscle group inserts on the transverse processes of the vertebrae. The longissimus group is composed of three parts: capitis, cervical, and thoracic. 2/14/11 3:06 PM Chapter Eleven 341 Axial Muscles Deeper Deep Semispinalis capitis Longissimus capitis Semispinalis cervicis Splenius capitis Serratus posterior superior External intercostals Splenius cervicis Transversospinalis Iliocostalis group Erector spinae Semispinalis thoracis Longissimus group Spinalis group Serratus posterior inferior Internal oblique (cut) Multifidus Quadratus lumborum External oblique (cut) Semispinalis cervicis Splenius capitis Serratus posterior superior Splenius cervicis Iliocostalis group Erector spinae Semispinalis thoracis Longissimus group Transversospinalis Spinalis group Multifidus Serratus posterior inferior Internal oblique Quadratus lumborum External oblique (cut) Figure 11.11 Deep Muscles of the Vertebral Column. the vertebral column, neck, and ribs. mck78097_ch11_322-353.indd 341 Posterior view An illustration and a cadaver photo show the muscles that affect, modify, and stabilize the positions of 2/14/11 3:06 PM 342 ■ Chapter Eleven Axial Muscles The spinalis group is the most medially placed of the erector spinae muscles. The spinalis muscle fibers insert on the spinous processes of the vertebrae (hence, the name of this muscle group). The spinalis group is composed of cervical and thoracic parts. The cervical part originates from the C7 spinous process. Deep to the erector spinae, a group of muscles collectively called the transversospinalis muscles connect and stabilize the vertebrae (figure 11.12). There are several specific muscles in this group (table 11.8). In addition, minor deep back muscles called interspinales and intertransversarii assist the transversospinalis muscles with moving the vertebral column. A final pair of muscles helps move the vertebral column. The quadratus lumborum muscles are located primarily in the lumbar region (see figure 11.11). When the left and right quadratus lumborum muscles bilaterally contract, they extend the vertebral column. When either the left or right quadratus lumborum muscle unilaterally contracts, it laterally flexes the vertebral column. Table 11.8 summarizes the characteristics of the muscles of the vertebral column. Intertransversarius Rotatores thoracis Transverse process of vertebra Interspinales Spinous process of vertebra W H AT D I D Y O U L E A R N? 8 ● Figure 11.12 Describe the erector spinae, and briefly discuss their function. Transversospinalis Muscles and Minor Deep Back Muscles. These muscles affect and modify the positions of the vertebral column. Table 11.8 Muscles of the Vertebral Column Group/Muscle Action Origin/Insertion Innervation Iliocostalis group (il-ē-ō-kos-tā ́lis) ilio = ilium cost = rib Extends neck and vertebral column; maintains posture O: Tendon from posterior part of iliac crest, posterior sacrum, and lumbar spinous processes I: Angles of ribs; transverse processes of cervical vertebrae Cervical, thoracic, and lumbar spinal nerves Longissimus group (lon-jis í -mŭs) longissimus = longest Extends neck and vertebral column and rotates head; maintains posture O: Tendon from posterior part of iliac crest, posterior sacrum, and lumbar spinous processes I: Mastoid process of temporal bone and transverse processes of cervical and thoracic vertebrae Cervical and thoracic spinal nerves Spinalis group (spı̄-nā ́lis) spin = spine Extends neck and vertebral column; maintains posture O: Lumbar spinous processes (thoracic part) and C7 spinous process (cervical part) I: Spinous process of axis and thoracic vertebrae Cervical and thoracic spinal nerves Multifidus (mul-tif í -dŭs) multus = much findo = to cleave Extends vertebral column; rotates vertebral column toward opposite side O: Sacrum and transverse processes of each vertebra I: Spinous processes of vertebrae located 2–4 segments superior to origin Cervical, thoracic, and lumbar spinal nerves Rotatores (rō-tā ́tōrz) rotatus = to revolve Extends vertebral column; rotates vertebral column toward opposite side O: Transverse processes of each vertebra I: Spinous process of immediately superior vertebra Cervical, thoracic, and lumbar spinal nerves Semispinalis group (sem ́ē-spı̄nā ́lis) Bilateral action: Extends vertebral column/neck Unilateral action: Laterally flexes vertebral column/neck O: Transverse processes of C4–T12 vertebrae I: Occipital bone and spinous processes of cervical and thoracic vertebrae Cervical and thoracic spinal nerves ERECTOR SPINAE TRANSVERSOSPINALIS GROUP mck78097_ch11_322-353.indd 342 2/14/11 3:06 PM Chapter Eleven Axial Muscles 343 Table 11.8 Muscles of the Vertebral Column (continued) Group/Muscle Action Origin/Insertion Innervation Interspinales (in-ter-spı̄-nā ́lēz) inter = between Extends vertebral column; rotates vertebral column to opposite side O: Spinous processes of each vertebra I: Spinous processes of more superior vertebrae Cervical, thoracic, and lumbar spinal nerves Intertransversarii (in-ter-trans v ́ ersăr ́ē-ı̄) Lateral flexion of vertebral column O: Transverse processes of each vertebra I: Transverse process of more superior vertebrae Cervical, thoracic, and lumbar spinal nerves O: Iliac crest and iliolumbar ligament I: Last rib; transverse processes of lumbar vertebrae Thoracic and lumbar spinal nerves MINOR DEEP BACK MUSCLES SPINAL EXTENSORS AND LATERAL FLEXORS Quadratus lumborum (kwah-drā ́tūs lŭm-bōr ́ŭm) quad = four-sided lumb = lumbar region Bilateral action: Extends vertebral column Unilateral action: Laterally flexes vertebral column 11.3 Muscles of Respiration Learning Objective: 1. Name the muscles of respiration, and explain their functions. The process of respiration involves inhalation and exhalation. When an individual inhales, several muscles contract to increase the dimensions of the thoracic cavity to allow the lungs to fill with air. When an individual exhales, some respiratory muscles contract and others relax, collectively decreasing the dimensions of the thoracic cavity and forcing air out of the lungs. The muscles of respiration are on the anterior and posterior surfaces of the thorax. These muscles are covered by more superficial muscles (such as the pectoral muscles, trapezius, and latissimus dorsi) that move the upper limb. Two posterior thorax muscles assist with respiration. These muscles are located deep to the trapezius and latissimus dorsi, but superficial to the erector spinae muscles. The serratus posterior superior attaches to ribs 2–5 (see figure 11.11) and elevates these ribs during inspiration, thereby increasing the lateral dimensions of the thoracic cavity. The serratus posterior inferior attaches to ribs 8–12 and depresses those ribs during expiration. Several groups of anterior thorax muscles change the dimensions of the thorax during respiration (figure 11.13). The scalene muscles (discussed previously with other neck muscles) help elevate the first and second ribs during forced inspiration, thereby increasing the dimensions of the thoracic cavity. The external intercostals extend inferomedially from the superior rib to the adjacent inferior rib. The external intercostals assist in expanding the thoracic cavity by elevating the ribs during inhalation. This movement is like lifting a bucket handle—that is, as CLINICAL VIEW Paralysis of the Diaphragm Injury to critical parts of the brain, spinal cord, or phrenic nerves can result in the loss of diaphragmatic innervation and cause paralysis. When the diaphragm becomes paralyzed, it cannot contract, and thus no air is exchanged in the lungs. The patient cannot breathe, and death is inevitable unless artificial breathing measures are implemented. The most common cause of diaphragmatic paralysis today is spinal cord injury at or superior to the fourth cervical vertebra, where the motor neurons that innervate the diaphragm are located. In years past, infection with the poliovirus was a common cause of diaphragmatic paralysis, as the viral infection destroyed brain and spinal cord motor neurons. Prior to the development of the modern ventilator (a device to help the patient breathe) and other respiratory assistance techniques, the polio patient with diaphragmatic paralysis was placed in a device known as an iron lung. This was a chamber in which the air pressure surrounding the patient was cyclically decreased to facilitate inhalation and then increased to facilitate exhalation. Lying prone, mck78097_ch11_322-353.indd 343 with only the head extending from this large, tubular apparatus, a patient spent weeks or even months while the iron lung supplied the needed respiratory assistance. A patient in an iron lung due to diaphragm paralysis. 2/14/11 3:06 PM 344 Chapter Eleven Axial Muscles Scalene muscles Scalene muscles Sternum External intercostals Serratus anterior Internal intercostals Internal intercostals Transversus thoracis External intercostals Ribs (cut) Diaphragm Lung Diaphragm (a) Anterior view (b) Anterolateral view Sternum Xiphoid process Costal cartilage Sternum Caval opening Central tendon of diaphragm Sternum Esophageal opening External intercostals Internal intercostals (c) Lateral view Aortic opening (hiatus) 12th rib Right crus L2 Left crus L3 L4 Quadratus lumborum muscle (cut) Psoas major muscle (cut) (d) Diaphragm, inferior view Figure 11.13 Muscles of Respiration. These skeletal muscles contract rhythmically to alter the size of the thoracic cavity and facilitate respiration. (a) Anterior view. (b) A cadaver photo provides an anterolateral view, with the inferior ribs cut to expose the thoracic cavity and the superior surface of the diaphragm. (c) Lateral views demonstrate fiber directions of the external and internal intercostals. (d) Inferior view of the diaphragm. mck78097_ch11_322-353.indd 344 2/14/11 3:06 PM Chapter Eleven Axial Muscles Table 11.9 Muscles of Respiration Muscle Action Description Origin/Insertion Innervation Serratus posterior superior (sĕr-ā ́tŭs pos-tēr-ē-ōr soo-pēr ́ē-ōr) serratus = a saw Elevates ribs during inhalation Thin, superior intermediate back muscle; splits into four separate muscle segments O: Spinous processes of C7–T3 vertebrae I: Lateral borders of ribs 2–5 Thoracic spinal nerves Serratus posterior inferior (sĕr-ā ́tŭs pos-tēr ḗ -ōr infē ́rē-ōr) Depresses ribs during exhalation Thin, inferior intermediate back muscle; splits into four separate muscle segments O: Spinous processes of T11–L2 vertebrae I: Inferior borders of ribs 8–12 Thoracic spinal nerves External intercostals (eks-ter n ́ ăl in t́ er-kos t́ alz) inter = between cost = rib Elevates ribs during inhalation 11 pairs of oblique fibers between ribs; project anteroinferiorly O: Inferior border of superior rib I: Superior border of inferior rib Thoracic spinal nerves Internal intercostals (in-ter ́nal in ́ter-kos t́ alz) Depresses ribs during forced exhalation; antagonistic to external intercostals 11 pairs of oblique fibers between ribs; project posteroinferiorly O: Superior border of inferior rib I: Inferior border of superior rib Thoracic spinal nerves Transversus thoracis (trans-ver ś ŭs thō-ra ́sis) Depresses ribs during exhalation Assist in decreasing diameter of thoracic cavity O: Posterior surface of xiphoid process and inferior region of sternum I: Costal cartilages 2–6 Thoracic spinal nerves Diaphragm (dı̄ ́ă-fram) dia = across phragm = partition Contraction causes flattening of diaphragm (moves inferiorly), and thus expansion of thoracic cavity; increases pressure in abdominopelvic cavity Dome-shaped, broad muscle; separates thoracic and abdominopelvic cavities O: Inferior internal surface of ribs 7–12; xiphoid process of sternum and costal cartilages of inferior 6 ribs; lumbar vertebrae I: Central tendon Phrenic nerves (C3–C5) 345 Scalene muscles (anterior, middle, posterior) (see table 11.7 for description) the bucket handle (rib) is elevated, its distance from the center of the bucket (thorax) increases. Thus, contraction of the external intercostals increases the transverse dimensions of the thoracic cavity. The internal intercostals lie deep to the external intercostals, and their muscle fibers are at right angles to the external intercostals. The internal intercostals depress the ribs, but only during forced exhalation; normal exhalation takes no active muscular effort. A small transversus thoracis extends across the inner surface of the thoracic cage and inserts on ribs 2–6. It helps depress the ribs. Finally, the diaphragm is an internally placed, domeshaped muscle that forms a partition between the thoracic and abdominopelvic cavities. It is the most important muscle associated with breathing. The muscle fibers of the diaphragm converge from its margins toward a fibrous central tendon, a strong aponeurosis that is the insertion tendon for all peripheral muscle fibers of the diaphragm. When the diaphragm contracts, the central tendon is pulled inferiorly toward the abdominopelvic cavity, thereby increasing the vertical dimensions of the thoracic cavity. As it compresses the abdominopelvic cavity, it also increases intra-abdominal pressure, an event that is necessary for urination, defecation, and childbirth. Beyond respiration, diaphragm movements are also important in helping return venous blood to the heart from the inferior half of the body. Table 11.9 summarizes the characteristics of the muscles of respiration. mck78097_ch11_322-353.indd 345 W H AT D O Y O U T H I N K ? 3 ● After you’ve eaten a very large meal, it is sometimes difficult to take a big, deep breath. Why is it more difficult to breathe deeply with a full GI tract? W H AT D I D Y O U L E A R N? 9 ● Compare the functions of the external intercostals and the internal intercostals. 10 ● Identify the muscle of respiration that partitions the thoracic and abdominopelvic cavities. What is the name of the structure to which all fibers of this muscle converge? 11.4 Muscles of the Abdominal Wall Learning Objective: 1. Illustrate the organization and function of the muscles of the abdominal wall. The anterolateral wall of the abdomen is reinforced by four pairs of muscles that collectively compress and hold the abdominal organs in place: the external oblique, internal oblique, transversus abdominis, and rectus abdominis (figure 11.14). These muscles also work together to flex and stabilize the vertebral column. 2/14/11 3:06 PM 346 Chapter Eleven Axial Muscles Superficial Deep Pectoralis minor Pectoralis major Serratus anterior External oblique Tendinous intersections Rectus sheath Rectus abdominis Umbilicus Transversus abdominis Internal oblique (cut) Linea alba External oblique (cut) Aponeurosis of external oblique Inguinal ligament Internal oblique and rectus abdominis (a) Anterior view External intercostal Tendinous intersections Internal intercostal Inguinal ligament Rectus abdominis Rectus sheath Transversus abdominis Umbilicus (c) Linea alba Transversus abdominis Internal oblique (cut) Aponeurosis of external oblique External oblique (cut) Inguinal ligament (b) Anterolateral view mck78097_ch11_322-353.indd 346 Figure 11.14 Muscles of the Abdominal Wall. The abdominal muscles compress abdominal contents and flex the vertebral column. (a) An illustration provides an anterior view of some superficial and deep muscles. (b) A cadaver photo provides an anterolateral view of the muscles of the abdominal wall. (c) Diagrams show some individual abdominal muscles, ranging from superficial to deep. 2/14/11 3:06 PM Chapter Eleven Table 11.10 Muscles of the Abdominal Wall Muscle Action 1 347 Axial Muscles Description Origin/Insertion Innervation External oblique (eks-ter n ́ ăl ob-lēk ́) Unilateral action : Lateral flexion of vertebral column; rotation of vertebral column to opposite side Bilateral action2: Flexes vertebral column and compresses abdominal wall Large superficial muscle sheet; fibers project inferomedially; forms inguinal ligament O: External and inferior borders of the inferior 8 ribs I: Linea alba by a broad aponeurosis; some to iliac crest Spinal nerves T8–T12, L1 Internal oblique (in-ter n ́ al ob-lēk ́) Unilateral action: Lateral flexion of vertebral column; rotation of vertebral column to opposite side Bilateral action: Flexes vertebral column and compresses abdominal wall Middle muscle sheet, deep to external oblique and superficial to transversus abdominis; fibers primarily project superomedially O: Lumbar fascia, inguinal ligament, and iliac crest I: Linea alba, pubic crest, inferior rib surfaces (last 4 ribs); costal cartilages of ribs 8–10 Spinal nerves T8–T12, L1 Transversus abdominis (trans-ver ́sŭs ab-dom í -nis) Unilateral action: Lateral flexion of vertebral column Bilateral action: Flexes vertebral column; compresses abdominal wall Deepest of the three muscle sheets; fibers project horizontally O: Iliac crest, cartilages of inferior 6 ribs; lumbar fascia; inguinal ligament I: Linea alba and pubic crest Spinal nerves T8–T12, L1 Rectus abdominis (rek t́ ŭs ab-dom í -nis) Flexes vertebral column; compresses abdominal wall Paired anterior and medial surface muscles; extend from thoracic cage to pubis; wrapped by aponeuroses of lateral muscles; segmented by three perpendicular tendinous intersections O: Superior surface of pubis near symphysis I: Xiphoid process of sternum; inferior surfaces of ribs 5–7 Spinal nerves T7–T12 1 Unilateral action means only one muscle (either the left or right muscle) is contracting. 2 Bilateral action means both the left and right muscles are contracting together. When these muscles unilaterally contract, they laterally flex the vertebral column. The muscle fibers of the superficial external oblique are directed inferomedially. The external oblique is muscular along the lateral abdominal wall and forms an aponeurosis as it projects anteriorly. Inferiorly, the aponeurosis of the external oblique forms a strong, cordlike inguinal ligament that extends from the anterior superior iliac spine to the pubic tubercle. Immediately deep to the external oblique is the internal oblique. Its muscle fibers project superomedially, which is at right angles to the external oblique. Like the external oblique, this muscle forms an aponeurosis as it projects anteriorly. Unilaterally, the external and internal oblique also rotate the vertebral column to the opposite side of the contracting muscle. The deepest muscle is the transversus abdominis, whose fibers project transversely across the abdomen. The rectus abdominis is a long, straplike muscle that extends vertically the entire length of the anteromedial abdominal wall between the sternum and the pubic symphysis. It is partitioned into four segments by three fibrous tendinous intersections, which form the traditional “six-pack” of a muscular, toned abdominal wall. The rectus abdominis is enclosed within a fibrous sleeve called the rectus sheath, which is formed from the aponeuroses of the external oblique, internal oblique, and transversus abdominis muscles. The left and right rectus sheaths are connected by a vertical fibrous strip termed the linea alba. Table 11.10 summarizes the characteristics of the muscles of the abdominal wall. mck78097_ch11_322-353.indd 347 Study Tip! Knowing the direction of the oblique and intercostal muscle fibers can help you identify these specific muscles on models and cadavers: ■ The fibers of the external intercostals and external oblique muscles run in the same direction—inferomedially. This is the same direction that you put your hands in your pockets. ■ The fibers of the internal intercostals and internal oblique muscles run perpendicular (in the opposite direction) to the external muscles—superomedially. We have seen that multiple muscles may work together to perform a common function. For example, several neck muscles and back muscles work together to extend the vertebral column. Learning muscles in groups according to common function helps most students assimilate the anatomy information. Table 11.11 summarizes the actions of various axial muscles and groups them according to common function. Note that a muscle that has multiple functions is listed in more than one group. W H AT D I D Y O U L E A R N? 11 ● Identify the muscles of the abdominal wall. 2/14/11 3:06 PM 348 Chapter Eleven Axial Muscles Table 11.11 Muscle Actions on the Axial Skeleton Extend the Head, Neck, and/or Vertebral Column Flex the Head, Neck, and/or Vertebral Column Laterally Flex the Vertebral Column Rotate the Head and/or Neck to One Side Elevate the Ribs Depress the Ribs Splenius muscles2 Sternocleidomastoid2 Quadratus lumborum1 Sternocleidomastoid1 Serratus posterior superior Serratus posterior inferior Erector spinae2 (iliocostalis, longissimus, spinalis) Scalenes2 External oblique1 Splenius muscles1 External intercostals Internal intercostals Quadratus lumborum2 External oblique2 Internal oblique1 Longissimus capitis1 Scalene muscles (ribs 1 and 2 only) Transversus thoracis Transversospinalis group2 Internal oblique2 Transversus abdominis1 Obliquus capitis inferior1 Minor deep back muscles2 Transversus abdominis2 Rectus capitis posterior major and minor2 Rectus abdominis2 1 Unilateral action of muscles 2 Bilateral action Obliquus capitis superior1 11.5 Muscles of the Pelvic Floor Learning Objective: 1. Identify the muscles that form the pelvic floor and perineum. The floor of the pelvic cavity is formed by three layers of muscles and associated fasciae, collectively known as the pelvic diaphragm. (The term diaphragm refers to a muscle or group CLINICAL VIEW: In Depth Hernias The condition in which a portion of the viscera protrudes through a weakened point of the muscular wall of the abdominopelvic cavity is called a hernia (her ń ē -ă; rupture). A significant medical problem may develop if the herniated portion of the intestine swells, becoming trapped. Blood flow to the trapped segment may diminish, causing that portion of the intestine to die. This condition, called a strangulated intestinal hernia, is very painful and can be life-threatening if not treated promptly. Two common types of hernias are inguinal hernias and femoral hernias. An inguinal hernia is the most common type of hernia to require treatment. The inguinal region is one of the weakest areas of the abdominal wall. Within this region is a canal (inguinal canal) that allows the passage of the spermatic cord in males, and a smaller structure in females called the round ligament of the uterus. The inguinal canal, or the superficial inguinal ring associated with it, is often the site of a rupture or separation of the abdominal wall. Males are more likely to develop inguinal hernias than females, because their inguinal canals and superficial inguinal rings are larger to allow room for the spermatic cord. Rising pressure in the abdominopelvic cavity, as might develop while straining to lift a heavy object, provides the mck78097_ch11_322-353.indd 348 of muscles that covers or partitions an opening.) The pelvic diaphragm extends from the ischium and pubis of the ossa coxae across the pelvic outlet to the sacrum and coccyx. These muscles collectively form the pelvic floor and support the pelvic viscera (figure 11.15). The most superior(deepest) layer of the pelvic diaphram consists of the coccygeus and the levator ani. The coccygeus pulls the coccyx anteriorly after its posterior deflection during defecation or force to push a segment of the small intestine into the canal. There are two types of inguinal hernia: ■ ■ In a direct inguinal hernia, the loop of small intestine protrudes directly through the superficial inguinal ring, but not through the entire length of the inguinal canal, and creates a bulge in the lower anterior abdominal wall. This type of hernia is typically seen in middle-aged males with poorly developed abdominal muscles and protruding abdomens. In an indirect inguinal hernia, the herniation travels through the entire inguinal canal and may even extend all the way into the scrotum, since the path of the herniation follows the path of the spermatic cord. This type of hernia tends to occur in younger males or male children who have a congenital anomaly called patent process vaginalis, in which the embryonic path taken by the testis into the scrotum fails to regress. A femoral hernia occurs in the upper thigh, just inferior to the inguinal ligament, originating in a region called the femoral triangle. The medial part of the femoral triangle is relatively weak and prone to stress injury, thus allowing a loop of small intestine to protrude. Women more commonly develop femoral hernias because of the greater width of their femoral triangle, which equates to the wider hip span of the female anatomy. 2/14/11 3:06 PM Chapter Eleven Axial Muscles 349 Figure 11.15 Muscles of the Pelvic Floor. The pelvic cavity floor is composed of muscle layers that form the urogenital and anal triangles, extend across the pelvic outlet, and support the organs in the pelvic cavity (puborectalis not shown). (a) Superior view of the female pelvic cavity. (b, c) Inferior views show male and female perineal regions, respectively. Sacrum Sacroiliac articulation Ilium Coccyx Piriformis Coccygeus Ischial spine Obturator internus Anal canal Iliococcygeus Levator Pubococcygeus ani Obturator canal Vagina Urethra Urogenital diaphragm Pubic symphysis (a) Female, superior view Pubic symphysis Pubic ramus External urethral sphincter Urethra Deep transverse perineal muscle Raphe Bulbospongiosus Urogenital triangle Ischiocavernosus Superficial transverse perineal muscle Perineal body Anus External anal sphincter Levator ani Anal triangle Gluteus maximus Superficial Deep (b) Male, inferior view Pubic symphysis Pubic ramus External urethral sphincter Urethra Vagina Deep transverse perineal muscle Perineal body Anus External anal sphincter Urethra Vagina Urogenital triangle Bulbospongiosus Ischiocavernosus Superficial transverse perineal muscle Levator ani Anal triangle Gluteus maximus Superficial Deep (c) Female, inferior view mck78097_ch11_322-353.indd 349 2/14/11 3:06 PM 350 Chapter Eleven Axial Muscles childbirth. The external anal sphincter (located within a region called the anal triangle) assists in defecation. The largest and most important collection of muscles in the pelvic floor is the levator ani. It supports the pelvic viscera and functions as a sphincter at the anorectal junction, urethra, and vagina. The levator ani is formed by the iliococcygeus, the pubococcygeus, and the puborectalis (not shown in figures). The puborectalis muscle forms a circular “sling” around the anorectal junction. When this muscle is contracted, it increases the bend or angle of the anorectal junction. When an individual wishes to defecate, the puborectalis muscle must be consciously relaxed to decrease the bend of the anorectal junction and allow for the easy passage of feces through the rectum. The diamond-shaped region between the lower appendages is called the perineum. The perineum has four significant bony landmarks: the pubic symphysis anteriorly, the coccyx posteriorly, and both ischial tuberosities laterally. A transverse line drawn between the ischial tuberosities partitions the perineum into an anterior urogenital triangle that contains the external genitalia and urethra, and a posterior anal triangle that contains the anus (figure 11.15b, c). The urogenital triangle is subdivided into a superficial layer and a deep layer. The superficial layer of the urogenital triangle is composed of three muscles: bulbospongiosus, ischiocavernosus, and superficial transverse perineal (table 11.12). The ischiocavernosus inserts on the pubic symphysis, while the bulbospongiosus and superficial transverse perineal muscles insert on a tendinous central structure called the perineal body (or central tendon of the perineum). The deep layer of the urogenital triangle is composed of two muscles: the deep transverse perineal and the external CLINICAL VIEW Episiotomy An episiotomy (e-piz-ē-ot ṓ -mē; epicios = vulvar, otomy = to cut) is a surgical incision made in the perineal skin and soft tissues between the vagina and the anus during childbirth to prevent tearing of the mother’s tissues and to minimize fetal injury. It was long believed that a clean surgical incision heals more rapidly and effectively than a laceration or tear, but lately its routine use has come into question. Large studies have indicated that serious perineal lacerations may be more common in women who have had an episiotomy. Furthermore, in some European countries where the procedure is rarely done, no significant increase in serious maternal or fetal complications has been seen. Medical studies now indicate that, at most, 25–30% of women would benefit from the procedure, and that the selective use of episiotomy may be a better health-care option. Women approaching childbirth are advised to talk in advance with their health-care provider about the necessity of having an episiotomy. urethral sphincter. These muscles are collectively referred to as the urogenital diaphragm, because they serve as a partition for the urogenital portion of the pelvic floor. Table 11.12 summarizes the characteristics of the muscles of the pelvic floor. Table 11.12 Muscles of the Pelvic Floor Group/Muscle Action Origin/Insertion Innervation Coccygeus (kok-si ́jē-ŭs) coccy = coccyx Forms pelvic floor and supports pelvic viscera O: Ischial spine I: Lateral and inferior borders of sacrum Spinal nerves (S4–S5) External anal sphincter (eks-ter ́năl ā ́năl sfingk ́ter) anal = referring to anus sphin = squeeze Closes anal opening; must relax to defecate O: Perineal body I: Encircles anal opening Pudendal nerve (S2–S4) ANAL TRIANGLE Levator ani (lē-vā ́tor, le-vā ́-ter ā ́nı̄) levator = raises ani = anus Iliococcygeus (il ́ē-ō-kok-si j́ ē-ŭs) ilio = ilium Pubococcygeus (pū ́bō-kok-si ́ jē-ŭs) pubes = pubis Puborectalis (pū ́-bō-rek t́ ăl-is) rectal = rectum Group of muscles that form the anterior and lateral parts of the pelvic diaphragm Forms pelvic floor and supports pelvic viscera Pubis and ischial spine Coccyx and median raphe Pubis and ischial spine Coccyx and median raphe Pudendal nerve (S2–S4) Forms pelvic floor and supports pelvic viscera O: I: O: I: Supports anorectal junction; must relax to defecate O: Pubis and ischial spine I: Coccyx and median raphe Pudendal nerve (S2–S4) Bulbospongiosus (female) (bul ́bō-spŭn j́ ē-ō ́sŭs) bulbon = bulb spongio = sponge Narrows vaginal opening; compresses and stiffens clitoris O: Sheath of collagen fibers at base of clitoris I: Perineal body Pudendal nerve (S2–S4) Bulbospongiosus (male) Ejects urine or semen; compresses base of penis; stiffens penis O: Sheath of collagen fibers at base of penis I: Median raphe and perineal body Pudendal nerve (S2–S4) Pudendal nerve (S2–S4) UROGENITAL TRIANGLE SUPERFICIAL LAYER mck78097_ch11_322-353.indd 350 2/14/11 3:06 PM Chapter Eleven Table 11.12 Muscles of the Pelvic Floor (continued) Group/Muscle Action Axial Muscles Origin/Insertion Innervation 351 UROGENITAL TRIANGLE (continued) SUPERFICIAL LAYER Ischiocavernosus (ish ́ē-ō-kav ́ernō ́sŭs) ischi = hip caverna = hollow chamber Assists erection of penis or clitoris O: Ischial tuberosities and ischial ramus I: Pubic symphysis Pudendal nerve (S2–S4) Superficial transverse perineal muscle (soo-per-fish ́ăl trans-vers ́ per-i-nē ́-ăl) Supports pelvic organs O: Ramus of ischium I: Perineal body Pudendal nerve (S2–S4) DEEP LAYER (UROGENITAL DIAPHRAGM) Deep transverse perineal muscle (dēp trans-vers ́ per-i-nē ́ăl) Supports pelvic organs O: Ischial ramus I: Median raphe of urogenital diaphragm Pudendal nerve (S2–S4) External urethral sphincter (eks-ter n ́ ăl ū-rē ́thrăl sfingk t́ er) sphin = squeeze Constricts urethra to voluntarily inhibit urination O: Rami of ischium and pubis I: Median raphe of urogenital diaphragm Pudendal nerve (S2–S4) Clinical Terms linea nigra Condition sometimes seen in pregnant females. The linea alba of the rectus sheath darkens (hence the term “nigra”), forming a line that extends along the midline of the abdomen. rectus sheath separation Separation of the left and right rectus muscles/sheaths due to great expansion of the abdomen (as occurs in some pregnancies). If severe, the separation may have to be repaired surgically. Chapter Summary 11.1 Muscles of the Head and Neck 323 ■ The axial muscles attach to components of the axial skeleton, whereas the appendicular muscles stabilize or move components of the appendicular skeleton. ■ Axial musculature functions include supporting and positioning the head, vertebral column, and thoracic cage; controlling movements associated with respiration; and forming part of the floor of the pelvic cavity. ■ Muscles of the head and neck are separated into groups based on their specific activities. 11.1a Muscles of Facial Expression ■ 11.1b Extrinsic Eye Muscles ■ 328 The six extrinsic eye muscles attach to the eye and control the movements and position of the eyes. 11.1c Muscles of Mastication ■ 323 The muscles of facial expression arise from the skull and often attach to the skin. 332 The muscles of mastication move the mandible during chewing. 11.1d Muscles That Move the Tongue ■ 11.1e Muscles of the Pharynx ■ 334 Muscles of the pharynx function in swallowing. 11.1f Muscles of the Anterior Neck ■ 332 The muscles of the tongue are divided into intrinsic muscles, which function during chewing and speaking, and extrinsic muscles, which function during food manipulation, swallowing, and some speech-related activities. 335 Anterior neck muscles are the suprahyoid muscles, superior to the hyoid bone, and the infrahyoid muscles, inferior to the hyoid bone. These move the hyoid bone or thyroid cartilage during swallowing or speaking. 11.1g Muscles That Move the Head and Neck 11.2 Muscles of the Vertebral Column 340 337 ■ Muscles that move the head and neck originate on the vertebral column, the thoracic cage, and the pectoral girdle, and insert on bones of the cranium. ■ Deep back muscles extend the vertebral column. (continued on next page) mck78097_ch11_322-353.indd 351 2/14/11 3:06 PM 352 Chapter Eleven Axial Muscles Chapter Summary (continued) 11.3 Muscles of Respiration 343 11.4 Muscles of the Abdominal Wall 345 11.5 Muscles of the Pelvic Floor 348 ■ The muscles of respiration are located on the anterior and posterior surfaces of the thorax and are covered by more superficial muscles. Their contraction either increases or decreases the size of the thoracic cavity. ■ The muscles of the abdominal wall compress the abdomen, help hold the abdominal organs in place, and assist the stabilization and lateral flexion of the vertebral column. ■ The muscles of the pelvic floor extend from the pubis and ischium anteriorly to the sacrum and coccyx posteriorly. They support the pelvic cavity organs and control the evacuation of waste materials from the digestive and urinary systems. Challenge Yourself Matching Match each numbered item with the most closely related lettered item. ______ 1. platysma a. moves eye laterally ______ 2. buccinator b. elevates and retracts tongue ______ 3. lateral rectus c. elevates and retracts mandible ______ 4. temporalis ______ 5. levator ani ______ 6. digastric ______ 7. external intercostal ______ 8. styloglossus ______ 9. zygomaticus major ______ 10. spinalis group d. tenses skin of neck e. extends vertebral column f. elevates angles of mouth g. compresses cheeks h. supports pelvic floor and viscera i. depresses mandible j. elevates ribs Multiple Choice Select the best answer from the four choices provided. ______ 1. The geniohyoid muscle a. depresses the hyoid bone and larynx. b. elevates the floor of the mouth. c. elevates the hyoid bone. d. depresses the larynx. ______ 2. When the left and right ______ contract, they flex the neck. a. sternocleidomastoid b. longissimus group c. splenius d. rectus abdominis ______ 3. When this large muscle contracts, the vertical dimensions of the thoracic cavity increase. a. external intercostal b. internal intercostal c. diaphragm d. transversus thoracis mck78097_ch11_322-353.indd 352 ______ 4. Which of the following is not a muscle within the urogenital triangle? a. bulbospongiosus b. coccygeus c. superficial transverse perineal d. ischiocavernosus ______ 5. The muscle that does not cause some lateral movement in the eye is the a. inferior rectus. b. inferior oblique. c. lateral rectus. d. superior oblique. ______ 6. Which muscle allows you to stick out your tongue? a. palatoglossus b. genioglossus c. lateral pterygoid d. hyoglossus ______ 7. Each of these muscles can laterally flex the vertebral column, except the a. external oblique. b. transversus abdominis. c. spinalis. d. internal oblique. ______ 8. Which muscle is not involved in extending the head or neck? a. rectus capitis posterior major b. longissimus capitis c. sternocleidomastoid d. splenius cervicis ______ 9. One function of the transversus abdominis muscle is to a. elevate the ribs. b. compress the abdominal wall. c. extend the vertebral column. d. increase the dimensions of the thoracic cavity. ______ 10. Which muscle protrudes the lower lip (as when you “pout”)? a. risorius b. levator labii superioris c. mentalis d. zygomaticus major 2/14/11 3:06 PM Chapter Eleven Content Review 1. Describe which muscles of facial expression you use to (a) smile and (b) frown. 2. Compare and contrast the functions of the extrinsic muscles of the tongue. 3. Discuss why the eye moves slightly medially during the contraction of either the superior or inferior rectus muscle. 4. Discuss the effect of contracting the three pharyngeal constrictors during swallowing. 5. Distinguish between suprahyoid and infrahyoid muscles, and describe the functions of each group. 6. Describe the differences in action between bilateral and unilateral contraction of the splenius muscles. 7. Describe the functions of the thoracic diaphragm. 8. What is the effect of contracting the abdominal oblique muscles? 9. What structures form the rectus sheaths, and how do the left and right rectus sheaths relate to the linea alba? Axial Muscles 353 10. What are the general functions of the pelvic diaphragm muscles, and what specific muscles form the pelvic diaphragm? Developing Critical Reasoning 1. Albon is a 45-year-old male who characterizes himself as a “couch potato.” He exercises infrequently and has a rounded abdomen (“beer belly”). While helping a friend move some heavy furniture, he felt a sharp pain deep within his abdominopelvic cavity. An emergency room resident told Albon that he had suffered an inguinal hernia. What is this injury, how did it occur, and how might Albon’s poorly developed abdominal musculature have contributed to it? 2. While training on the balance beam, Pat slipped during her landing from a back flip and fell, straddling the beam. Although only slightly sore from the fall, she became concerned when she suddenly lost the ability to completely control her urination. What might have happened to Pat’s pelvic floor structures during the fall? Answers to “What Do You Think?” 1. The zygomaticus major, zygomaticus minor, levator anguli oris, risorius, and levator labii superioris all contract when you smile. 2. Since the omohyoid attaches to the scapula, the prefix -omo means “shoulder.” 3. When you breathe deeply, the diaphragm contracts and pushes down on the GI tract (abdominal viscera). If these viscera are bulging with food, the diaphragm has difficulty contracting fully, making it hard to take deep breaths. www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more. mck78097_ch11_322-353.indd 353 2/14/11 3:06 PM

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