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Types of Synovial Joints There are seven types of synovial joints in the body.

Each of the different synovial joints has the basic

structural features common to all synovial joints but is

further classifi ed based on the shape of and motion that occurs at the articular surfaces of the joint. The different types of

synovial joint are depicted below and on the opposite page. Note the shapes of the reciprocal surfaces as you study these

photos.

Hinge joint example

Humero-ulnar joint of elbow

Plane joint examples

Intertarsal joints Pivot joint examples

Proximal radio-ulnar joint of elbow

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Bicondylar joint example

Knee joint

Condylar joint example

Wrist joint

Saddle joint example

Metacarpal-carpal joint of thumb

Ball and socket joint example

Shoulder joint

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Temporomandibular Joint The complex temporomandibular joint differs from

other synovial joints by having an articular disc

that usually separates the joint into two separate

synovial capsules, one above and one below the disc. The articular surfaces have a covering of dense fi brocartilage rather

than the typical hyaline cartilage of most synovial joints. With its associated ligaments this joint structure accounts for the

complex series of movements that are essential during the activities of eating and speech. Each temporomandibular joint is

a condylar joint and both joints together form a bicondylar joint. The fi brous membrane of the articular capsule spans from

temporal bone to mandible only on the lateral side. Anteriorly, medially, and posteriorly the fi bers attach from mandible and

temporal bone to the articular disc. Extrinsic ligaments that help stabilize the joint are the lateral temporomandibular ligament, sphenomandibular ligament, and stylomandibular ligament.

 1 Mandibular condyle

 2 Mandibular ramus

 3 Articular tubercle of temporal bone

 4 Mastoid process of temporal bone

 5 Mastoid air cells

 6 Superior compartment of articular cavity

 7 Inferior compartment of articular cavity

 8 Articular disc

 9 Joint (articular) capsule

10 Masseter muscle

 11 Parotid gland

12 Brain

13 External acoustic meatus

14 Sigmoid venous sinus

Section of right temporomandibular joint

Lateral view of sagittal section

Bones of temporomandibular joint

Lateral view

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Glenohumeral Joint The glenohumeral or shoulder joint is a ball and socket joint and is

the most mobile joint in the body. The tremendous range of motion

at this joint is the result of few external ligaments that present little

limitation to movement, and shallow, ovoid articular surfaces that make movements in all planes of space possible. In fact,

surrounding muscles and tendons play a more signifi cant role in joint support than do the joint structures. The capsular ligament is extremely lax, providing limited support to the joint. Blending with the capsule are the tendons of four muscles.

Together the capsule and tendons form the rotator cuff, which is the major support structure of the joint.

 1 Articular cartilage

 2 Synovial membrane

 3 Fibrous membrane

 4 Glenoid labrum

 5 Acromioclavicular ligament

 6 Clavicle

 7 Humerus

 8 Glenoid of scapula

 9 Acromion of scapula

10 Supraspinatus muscle

 11 Subscapularis muscle

12 Deltoid muscle

13 Tendon of long head of biceps brachii

14 Skin

15 Subcutaneous layer

Section of left glenohumeral joint

Anterior view of frontal section

Bones of glenohumeral joint

Anterior view

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Elbow Joint The elbow joint is a complex joint comprised of multiple articular surfaces within one articular

capsule. The elbow joint can be subdivided into three distinct articular interfaces —

the humero-ulnar joint (hinge), the humeroradial joint (combined hinge and pivot), and

 1 Articular cartilage

 2 Joint (articular) capsule

 3 Articular (synovial) cavity

 4 Capitulum of humerus

 5 Olecranon of ulna

 6 Head of radius

 7 Anular ligament

 8 Biceps brachii muscle

 9 Brachialis muscle

10 Triceps brachii muscle

 11 Brachioradialis muscle

Section of pronated left elbow joint

Medial view of sagittal section

Bones of elbow joint

Anterior view

the proximal radioulnar joint (pivot). Two distinct pairs of movements occur as a result of the articulations within the elbow

joint — the hinged movements of fl exion and extension, and the rotational movements of pronation and supination. Unlike

the shoulder joint, the joints fo the elbow have strong extrinsic ligaments that limit movemnts and stabilize the articulating

bones. The fi brous capsule is thin anteriorly and posteriorly, allowing for free range of motion during fl exion and extension.

On either side the capsule is reinforced by strong extrinsic ligaments, the ulnar collateral and radial collateral ligaments.

Wrapping from the back of the ulna at the base of the olecranon to the front of the ulna at the lateral surface of the coronoid

process is the semicircular anular ligament. With the radial notch of the ulna this ligament forms a fi bro-osseous ring for the

pivoting action of the radial head.

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Hip Joint Like the shoulder joint the hip joint, also a ball and socket joint, allows for great freedom of motion, although the range of motion is not quite as great as that of the shoulder. This comparative

decrease in mobility results from the deep hip socket with its extended labrum, which almost

 1 Ligament of head of femur

 2 Joint (articular) capsule

 3 Articular cartilage of acetabulum

 4 Articular cartilage of femur

 5 Articular (synovial) cavity

 6 Acetabular labrum

 7 Fovea capitis of femur

 8 Head of femur

 9 Greater trochanter of femur

10 Os coxae

 11 Psoas major muscle

12 Iliacus muscle

13 Adductor muscles

14 Vastus lateralis muscle

15 Gluteus medius muscle

16 Gluteus minimis muscle

17 Obturator internus muscle

18 Obturator externus muscle

19 Skin

20 Subcutaneous layer

21 External iliac artery

22 Intestine

Bones of hip joint

Anterior view

Section of right hip joint

Anterior view of frontal section

completely engulfs the head of the femur. In addition, thick extrinsic ligaments tightly surround the joint to form a strong,

reinforced capsule. The three major ligaments of the hip joint, the iliofemoral, pubofemoral, and ischiofemoral, form a sheath

around the fi brous capsule. The iliofemoral ligament is argued to be the strongest ligament in the human body. Often called

the Y-shaped ligament it passes superior and anterior to the joint, running from the anterior inferior iliac spine to the intertrochanteric line. With the thinner pubofemoral and ischiofemoral ligaments it spirals around the joint to stabilize this powerful

joint. In additon to these large ligaments, a triangular fl at band, the ligament of the head of the femur, extends from the fovea

of the femoral head to the margins of the acetabular fossa. This ligament is also important because it functions as a pathway

for blood vessels that supply the bone tissue in the head of the femur.

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Knee Joint The knee joint is a combined bicondylar and saddle joint. The relationships between the

femur and the tibia provide no interlocking joint mechanisms or stability between the neighboring bones, and from this perspective the knee joint is completely unstable. The strength

of the knee joint is dependent on strong ligaments and surrounding muscles. Although its primary motions are of a hinge

nature, it is a complex joint with subtle rotational and sliding movements also. The major stabilizers of the joint are four

strong ligaments. Two collateral ligaments support the joint on either side, while two cruciate ligaments criss-cross through

the middle of the joint. The tibial or medial collateral ligament is a strong, fl at band that stretches from the femoral epicondyle

to the tibial condyle. Posteriorly it fi rmly attaches to the joint capsule and the medial meniscus, while anteriorly bursae

separate it from these structures. The fi bular or lateral collateral ligament is a strong cord that runs from the lateral femoral

 1 Articular (synovial) cavity

 2 Articular cartilage

 3 Medial meniscus

 4 Suprapatellar bursa

 5 Prepatellar bursa

 6 Infrapatellar bursa

 7 Infrapatellar fat pad

 8 Fibrous membrane of joint capsule

 9 Synovial membrane of joint capsule

10 Lateral meniscus

 11 Fibular collateral ligament

12 Tibial collateral ligament

13 Anterior cruciate ligament

14 Posterior cruciate ligament

15 Oblique popliteal ligament

16 Patellar ligament

17 Quadriceps tendon

18 Femur

19 Tibia

20 Fibula

21 Patella

22 Periosteum

23 Semimembranosus muscle

24 Gastrocnemius muscle

25 Soleus muscle

26 Popliteal fat

Bones of knee joint

Anterior view

Section of right knee joint

Lateral view of sagittal section

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epicondyle to the head of the fi bula. Unlike the tibial collateral ligament it does not attach to the lateral meniscus or joint

capsule. The cruciate ligaments stabilize the knee from excessive anterior-posterior and rotational movements. The anterior cruciate ligament ascends posterolaterally from the medial aspect of the intercondylar area to the medial aspect of the

lateral condyle of the femur. The shorter posterior cruciate ligament ascends from the posterior intercondylar area to the

medial femoral condyle. Both cruciates have fi bers that blend with the lateral meniscus. In additon to these ligamentous

structures, two semilunar menisci project into the capsule between the femoral condyles and the articular plateaus of the

tibia. The large, extensive articular capsule connects the femur, patella, and tibia.

Dissection of left knee joint

Anterior view

Dissection of left knee joint

Anterior view

Dissection of left knee joint

Posterior view

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Synovial Bursae and Sheaths A synovial bursa is a small sac-like structure interposed between structures that

generate signifi cant amounts of friction.

Bursae have a similar design to the articular capsule of a synovial joint. These small bags have an outer fi brous membrane

of dense irregular collagenous connective tissue and an inner lining of synovial membrane. The synovial membrane produces

a small amount of synovia as a lubricant inside the sac. The fi brous membrane binds to surrounding tissues, allowing the

juxtaposed walls of synovial membrane to rub together in a frictionless manner. Many bursae arise as outgrowths of synovial joint cavities. In some cases these pinch off from the joint forming sacs that are independent from the joint, while other

bursal sacs retain their connections with the joint cavity. A synovial sheath is a modifi ed bursa that wraps around a tendon

to protect it from friction on all sides. In the tight confi nes of the wrist, ankle, and digits, tendons often pass beneath fi brous

bands called retinacula. The retinaculum is a connective tissue band that crosses over the tendons and keeps them from

being displaced upward when the muscle shortens and bends the joints. Because the retinaculum and bone create a fi broosseous tunnel around the tendon, considerable friction can occur on all surfaces of the tendon at these locations. As the

tendon moves through the tunnel, the juxtaposed synovial membranes smoothly glide over each other with minimal friction.

 1 Suprapatellar bursa

 2 Prepatellar bursa

 3 Infrapatellar bursa

 4 Synovial (tendon) sheath

 5 Retinaculum

 6 Flexor digitorum superficialis tendon

 7 Flexor digitorum profundus tendon

 8 Lumbrical muscles

 9 Flexor digiti minimi brevis muscle

10 Abductor digiti minimi muscle

Synovial bursae around the knee joint

Medial view of sagittal section

Tendon sheath of fi ngers

Anterior view, pin inserted into tendon sheath

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Bodies are designed to move! We move when we walk, jog,

or run, activities that transport our bodies from one location to another. In addition to moving

from location to location we also move in other ways. For example, think about grasping something with your hands and placing it in your mouth, or protecting yourself by kicking at

something with your lower limb. How about throwing something? All of

these activities are forms of movement that occur without moving

from one location to another, yet they are movements nonetheless. Like moving about, these other types of movements are

not only essential for survival, but defi ne the broad spectrum for the majority of human movement. Refl ect for

a moment on the wide variety of movements that you

make without moving from place to place. For example, think about the variety of intricate movements

required to eat a meal, movements such as grasping,

manipulating, cutting, chewing, and swallowing.

Another example is getting dressed for the day. From

the simple movements of pulling on clothing to the

intricate movements of buttoning shirts and tying

shoelaces, getting dressed involves a wide variety of

movements. And here is something else to ponder —

how about all the movements involved in communication? Th ink of the wide array of movements

that you produce as you communicate with

others — whether the communication involves

writing a note on a piece of paper, typing a letter

on the keyboard of a computer, signaling pleasure

and happiness with a smile, or using your voice to

talk to a friend on the telephone.

We could go on and on discussing the wide

 variety of movement and its importance, but the bottom

line is all movement results from the combined activity

of individual muscles. Th e most detailed movements you

make can be broken down into the simple actions of individual muscles moving the

bones of the skeleton at the joints. Th is chapter introduces the muscular system. On the

pages that follow you will see the structural design of a typical muscle and whole body views of

the muscles of the body. Our approach to the skeletal muscles of the body is based on their embryonic origins. Th e four chapters that follow this chapter cover each of the developmental

groups of muscles — muscles of the head, muscles of the trunk, muscles of the upper limb, and

muscles of the lower limb. Th e logic of this approach will be further discussed as we introduce

each chapter.

Find more information

about the muscular system in

REAL ANATOMY

8 Muscular System

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 1 Muscle belly or body

 2 Tendon of origin

 3 Tendon of insertion

 4 Collagen fiber

 5 Muscle cell or fiber

 6 Nucleus

Dissection of brachium highlighting biceps brachii

as example of muscle anatomy

Anterior view

Dense regular connective tissue of tendon

200x

While there is a wide variety to the shape, size, and architecture of the skeletal muscles of the body, most muscles share

a common basic design — a tendon of origin, a muscle body

Anatomy of a Muscle

 7 Biceps brachii muscle

 8 Brachialis muscle

 9 Triceps brachii muscle

10 Epimysium

 11 Perimysium

12 Endomysium

or belly, and a tendon of insertion. The tendons, projecting from the muscle belly, are a continuation of the connective tissue

surrounding the muscle cells within the belly of the muscle. As the connective tissue projects beyond the muscle cells, it

condenses to become the tendons, which merge and blend with the periosteum to attach the muscle to bone.

13 Blood vessels in perimysium

14 Nerve in perimysium

15 Fascia

16 Sucutaneous layer

17 Skin

18 Periosteum

Skeletal muscle tissue of muscle belly

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Photomicrograph of muscle fasciculus

Transverse section, 100x

Transverse section of left brachium at level of dashed line

Inferior (distal) view, anterior at top

Dashed line shows level of transverse section

Anterior view

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Skeletal muscles of the body

Anterior view

Skeletal muscles of the body

Lateral view

Skeletal muscles of the body

Posterior view

In the dissections below, the integument and fascia were removed to reveal

the superfi cial skeletal muscles. Some of the larger muscles are identifi ed

here. More detailed muscle labeling will occur in the next four chapters.

Skeletal Muscles

 1 Platysma

 2 Pectoralis major

 3 Deltoid

 4 Rectus abdominis

 5 External oblique

 6 Biceps brachii

 7 Triceps brachii

 8 Trapezius

 9 Brachioradialis

10 Latissimus dorsi

 11 Gluteus maximus

12 Biceps femoris

13 Sartorius

14 Vastus medialis

15 Rectus femoris

16 Adductor magnus

17 Tibialis anterior

18 Gastrocnemius

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Head muscles, like the platysma and risorius seen in

the photo on this page, arise from two sources during embryonic development. One source

is the pharyngeal arches, which give rise to the majority of the head muscles. Muscles of

the pharyngeal

arches include

the muscles

of mastication,

muscles of the

middle ear, muscles of facial expression, muscles

of the palate, muscles of the pharynx,

muscles of the larynx, and the sternocleidomastoid and

trapezius. Th e second

category of head muscles

includes those muscles that

arise from the pre-otic and occipital

 somites. Th e pre-otic somites give rise to the extraocular muscles, and the occipital somites give rise

to the tongue muscles. Grouping muscles by their developmental origin is an eff ective way to understand the

muscles because muscles that share a developmental origin share a common nerve supply. For example, during

development all the muscles of the fi rst pharyngeal arch

are innervated by the mandibular branch of the trigeminal nerve; therefore the mandibular nerve and its

branches innervate all eight muscles that arise from the

fi rst pharyngeal arch. Th e same is true for each of the

other arches, as well as the head somites. Th is chapter

will showcase the muscles of the head and emphasize

their developmental origin and neuromuscular pairing. With a few exceptions, all of the head muscles are

depicted in the photos throughout this chapter. Th e

following page outlines the developmental groups of

head musculature and their nerve associations.

Find more information

about the muscles of the

head in

REAL ANATOMY

9 Head Muscles

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of these muscles arise from the paraxial mesoderm of the embryonic head. Unlike many anatomy sources that mix these

muscles into multiple groups, with no logic to their innervation, we choose to present them based on their embryonic origins.

Taking this approach makes it very easy to learn the innervation patterns of the head muscles because each developmental

group is associated with a distinct cranial nerve or set of cranial nerves (see groups below). Accompanying each labeled

dissection photograph on the pages that follow are small reference photos that clearly depict each of the developmental

muscle groups of the head. Since some of the head muscles migrate into the neck, we also depict the somitic muscles of

the neck in the reference photos, to help distinguish them from the true head muscles. The somitic muscles of the neck will

be the subject of the next chapter. For example, the fi rst photo (see opposite page) labels numerous head muscles. The

reference photos clearly reveal that the labeled muscles are primarily from two sources — the fi rst pharyngeal arch and the

second pharyngeal arch (accounting for the majority of the muscles). The third reference photo shows that some muscles

are from neck somites.

This chapter presents numerous dissections of the head and neck that depict the

muscles of the head. We defi ne the head muscles as all muscles that arise from

the pharyngeal (branchial) arches or the head somites (pre-otic and occipital). All

Head Muscles

Muscles of the First Pharyngeal Arch

(Nerve supply - mandibular branch of the trigeminal nerve CN V)

 Temporalis

 Masseter

 Medial pterygoid

 Lateral pterygoid

 Anterior digastricus

 Mylohyoid

 *Tensor tympani

 Tensor veli palatini

Muscles of the Second Pharyngeal Arch

(Nerve supply - facial nerve CN VII)

 Occipitofrontalis

 Temporoparietalis

 Transversus nuchae

 Procerus

 Nasalis

 *Depressor septi nasi

 Orbicularis oculi

 Corrugator supercilii

 Depressor supercilii

 Auricularis anterior

 Auricularis superior

 Auriculalris posterior

 Intrinsic auricular muscles

 Helicis major muscle

 Helicis minor muscle

 Tragicus muscle

 *Pyramidal muscle of auricle

 Antitragicus muscle

 *Transverse muscle of auricle

 *Oblique muscle of auricle

 Orbicularis oris

 Depressor anguli oris

 Transversus menti

 Risorius

 Zygomaticus major

 Zygomaticus minor

 Levator labii superioris

 Levator labii superioris alaeque nasi

 Depressor labii inferioris

 Levator anguli oris

 Buccinator

 Mentalis

 *Stapedius

 Stylohyoid

 Posterior digastricus

 Platysma

Muscle of the Third Pharyngeal Arch

(Nerve supply - glossopharyngeal nerve CN IX)

 Stylopharyngeus

Muscles of the Fourth Pharyngeal Arch

(Nerve supply - vagus nerve CN X)

 Levator veli palatini

 Palatoglossus

 Palatopharyngeus

 Musculus uvulae

 Superior pharyngeal constrictor

 Middle pharyngeal constrictor

 Inferior pharyngeal constrictor

 Cricothyroid

 Salpingopharyngeus

Muscles of the Sixth Pharyngeal Arch

(Nerve supply - vagus nerve CN X)

 Posterior crico-arytenoid

 Lateral crico-arytenoid

 Vocalis

 Thyro-arytenoid

 Oblique arytenoid

 Transverse arytenoid

Muscles of the Posterior Pharyngeal Arch

(Nerve supply - accessory nerve CN XI)

 Sternocleidomastoid

 Trapezius

Muscles of the Pre-otic Somites

(Nerve supply - oculomotor CN III, trochlear CN IV, and abducens CVI)

 Superior rectus

 Inferior rectus

 *Medial rectus

 Lateral rectus

 Superior oblique

 Inferior oblique

 Levator palpebrae superioris

Muscles of the Occipital Somites

(Nerve supply - hypoglossal nerve CN XII)

 Genioglossus

 Hyoglossus

 Styloglossus

 Superior longitudinal muscle

 Inferior longitudinal muscle

 Transverse muscle

 Vertical muscle

All the muscles listed above are depicted in photos in this chapter except those marked with an asterisk.

145

First arch muscles

Somitic muscles

Superfi cial head muscles of neck

Anterolateral view

 1 Masseter

 2 Anterior belly of digastricus (cut)

 3 Mylohyoid

 4 Frontal belly of occipitofrontalis

 5 Temporoparietalis

 6 Procerus

 7 Nasalis

 8 Orbicularis oculi

 9 Corrugator supercilii

10 Depressor supercilii

 11 Auricularis anterior

12 Auricularis superior

13 Orbicularis oris

14 Depressor anguli oris

15 Transversus menti

16 Zygomaticus major

17 Zygomaticus minor

18 Levator labii superioris

19 Levator labii superioris alaeque nasi

20 Depressor labii inferioris

21 Levator anguli oris

22 Buccinator

23 Mentalis

24 Posterior digastricus

25 Epicranial aponeurosis

26 Temporal fascia

27 Parotid gland (cut)

Second arch muscles

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The dissections depicted on this page and the facing page represent two stages Head Muscles in a dissection of the head. Below is a superfi cial dissection with the integument

 1 Temporalis

 2 Masseter

 3 Mylohyoid

 4 Anterior belly of digastricus

 5 Frontal belly of occipitofrontalis

 6 Temporoparietalis

 7 Orbicularis oculi

 8 Procerus

 9 Levator labii superioris alaeque nasi

10 Nasalis

 11 Levator labii superioris

12 Zygomaticus major

13 Levator anguli oris

14 Orbicularis oris

15 Buccinator

16 Depressor anguli oris

17 Depressor labii inferioris

18 Mentalis

First arch muscles

Second arch muscles

Somitic muscles

of neck

Fourth arch muscles

Posterior arch muscles

Head muscles, superfi cial dissection

Lateral view

and some fascia removed. On the opposing page some superfi cial muscles were removed. Most of the

head muscle groups are represented. Note also the somitic muscles of the neck that are visible.

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19 Auricularis anterior

20 Auricularis superior

21 Auricularis posterior

22 Occipital belly of occipitofrontalis

23 Transversus nuchae

24 Epicranial aponeurosis

25 Helicis major

26 Helicis minor

27 Tragicus

28 Antitragicus

29 Posterior belly of digastricus

30 Stylohyoid

31 Middle pharyngeal constrictor

32 Inferior pharyngeal constrictor

33 Sternocleidomastoid

34 Trapezius

35 Styloglossus

36 Temporal fascia

37 Parotid gland

38 Submandibular gland

39 Thyroid cartilage

40 Sternohyoid

41 Omohyoid

42 Thyrohyoid

43 Longus colli

44 Middle scalene

45 Posterior scalene

46 Levator scapulae

47 Splenius capitis

48 Deltoid

First arch muscles

Second arch muscles

Somitic muscles

of head and neck

Fourth arch muscles

Posterior arch muscles

Head muscles, masticatory muscles exposed

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148

The lateral head dissections below and opposite are deeper

dissections that expose the deep masticatory muscles (below) and the extraocular muscles (opposite).

Head Muscles

Head muscles, deep masticatory muscles exposed

Lateral view, portion of mandible removed

First arch muscles

Second arch muscles

Somitic muscles

of head and neck

Fourth arch muscles

Posterior arch muscles

 1 Temporalis

 2 Masseter

 3 Medial pterygoid

 4 Lateral pterygoid

 5 Anterior belly of digastricus

 6 Mylohyoid

 7 Frontal belly of occipitofrontalis

 8 Occipital belly of occipitofrontalis

 9 Transversus nuchae

10 Procerus

 11 Nasalis

12 Orbicularis oculi

13 Auricularis anterior (cut)

14 Auricularis superior (cut)

15 Auricularis posterior

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Head muscles, extraocular muscels exposed

Lateral view, lateral wall of orbit removed

First arch muscles

Second arch muscles

Posterior arch muscles

Somitic muscles

of head and neck

16 Helicis major

17 Helicis minor

18 Tragicus

19 Antitragicus

20 Orbicularis oris

21 Depressor anguli oris

22 Transversus menti

23 Zygomaticus major

24 Zygomaticus minor

25 Levator labii superioris

26 Levator labii superioris alaeque nasi

27 Depressor labii inferioris

28 Levator anguli oris

29 Buccinator

30 Mentalis

31 Stylohyoid

32 Posterior belly of digastricus

33 Middle pharyngeal constrictor

34 Inferior pharyngeal constrictor

35 Sternocleidomastoid

36 Trapezius

37 Superior rectus

38 Inferior rectus

39 Lateral rectus

40 Supra-orbital nerve

41 Inferior oblique

42 Levator palpebrae superioris

43 Styloglossus

44 Hyoglossus

45 Sternohyoid

46 Omohyoid

47 Thyrohyoid

48 Longus colli

49 Splenius capitis

50 Levator scapulae

51 Posterior scalene

52 Middle scalene

53 Epicranial aponeurosis

54 Temporal fascia (cut)

55 Parotid gland

56 Submandibular gland

57 Skin

58 Subcutaneous layer

59 Thyroid cartilage

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150

The dissections on this and the opposing page are

deep dissections of the head and neck that expose

many of the muscles of the palate, pharynx, and

Head Muscles

tongue. The palatal and pharyngeal muscles, along with the muscles of the larynx, are the deepest of the head muscles. These groups arise from the third, fourth, and sixth arches and form the

muscular walls to the upper regions of the embryonic gut tube. All of the “true” tongue muscles

(the palatoglossus is included by many with the tongue muscles, but it is a muscle of the palate

from fourth arch origin) arise from the occipital somites and are innervated by the cranial nerve

XII, the hypoglossal nerve. The hypoglossal nerve is the lowest of the ventral motor nerves arising from the brainstem and is developmentally paired with the occipital somites.

First arch muscles

Second arch muscles

Somitic muscles

of head

Third arch muscles

Fourth arch muscles

Head muscles, palatal and pharyngeal muscles exposed

Lateral view, mandibular ramus removed

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