The discussion assignment provides a forum for discussing the relevant topics for this week based on the course competencies covered. You will participate in a weekly discussion with your classmates t

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The discussion assignment provides a forum for discussing the relevant topics for this week based on the course competencies covered. You will participate in a weekly discussion with your classmates that will help with your overall understanding of weekly topics of study.

For this assignment, make sure you post your initial response to the Discussion Area by the due date assigned.

Start reviewing and responding to the postings of your classmates as early in the week as possible. Respond to at least two of your classmates. Participate in the discussions (which are your peer responses) by providing a statement of clarification, providing a point of view with a rationale, challenging an aspect of the discussion, or by indicating a relationship between two or more lines of reasoning in the discussion. Your peer responses should be about 150 words each, follow APA format, and include academic citations. Complete your participation for this assignment by the end of the week.

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To successfully complete this assignment, first read the following exercise from the Laboratory Manual: Exercise 13: Gross Anatomy of the Muscular System.

Student Discussion Assignment

  1. Briefly identify, list, and describe the action of at least two of the following muscle types:

    • superficial (i.e., two anterior and two posterior) muscles
    • head and neck
    • anterolateral neck and throat
    • thorax and shoulder
    • abdominal wall
  2. Briefly identify and list antagonists of the major prime movers associated with each muscle described above.

Write your responses in a minimum of 250 words in APA format.

As in all assignments, cite your sources in your work and provide references for the citations in APA format. Support your work, using your course lectures and textbook readings. Helpful APA guides and resources are available in the South University Online Library. Below are guides that are located in the library and can be accessed and downloaded via the South University Online Citation Resources: APA Style page. The American Psychological Association website also provides detailed guidance on formatting, citations, and references at APA Style.

The discussion assignment provides a forum for discussing the relevant topics for this week based on the course competencies covered. You will participate in a weekly discussion with your classmates t
Classification of Skeletal Muscles Types of Muscles Muscles that are most responsible for producing a particular movement are called prime movers, or agonists. Muscles that oppose or reverse a movement are called antagonists. When a prime mover is active, the fibers of the antagonist are stretched and in the relaxed state. Antagonists can be prime movers in their own right. For example, the biceps muscle of the arm (a prime mover of flexion at the elbow) is antagonized by the triceps (a prime mover of extension at the elbow). Synergists aid the action of agonists either by assisting with the same movement or by reducing undesirable or unnecessary movement. Contraction of a muscle crossing two or more joints would cause movement at all joints spanned if the synergists were not there to stabilize them. For example, the muscles that flex the fingers cross both the wrist and finger joints, but you can make a fist without bending at the wrist because synergist muscles stabilize the wrist joint. Fixators, or fixation muscles, are specialized synergists. They immobilize the origin of a prime mover so that all the tension is exerted at the insertion. Muscles that help maintain posture are fixators; so too are muscles of the back that stabilize or “fix” the scapula during arm movements. Naming Skeletal Muscles Remembering the names of the skeletal muscles is a monumental task, but certain clues help. Muscles are named on the basis of the following criteria: Figure 13.1 Patterns of fascicle arrangement in muscles. Direction of muscle fibers: Some muscles are named in reference to some imaginary line, usually the midline of the body. A muscle with fibers running parallel to that imaginary line will have the term rectus (straight) in its name. For example, the rectus abdominis is the straight muscle of the abdomen. Likewise, the terms transverse and oblique indicate that the muscle fibers run at right angles and obliquely (respectively) to the imaginary line. Muscle structure is determined by fascicle arrangement (Figure 13.1). Relative size of the muscle: Terms such as maximus (largest), minimus (smallest), longus (long), and brevis (short) are often used in naming muscles—as in gluteus maximus and gluteus minimus. Location of the muscle: Some muscles are named for the bone with which they are associated. For example, the temporalis muscle overlies the temporal bone. Number of origins: When the term biceps, triceps, or quadriceps forms part of a muscle name, you can generally assume that the muscle has two, three, or four origins (respectively). For example, the biceps brachii has two origins. Location of the muscle’s origin and insertion: For example, the sternocleidomastoid muscle has its origin on the sternum (sterno) and clavicle (cleido), and inserts on the mastoid process of the temporal bone. Shape of the muscle: For example, the deltoid muscle is roughly triangular (deltoid = triangle), and the trapezius muscle resembles a trapezoid. Action of the muscle: For example, all the adductor muscles of the anterior thigh bring about its adduction, and all the extensor muscles of the wrist extend the hand. Identification of Human Muscles While reading the tables and identifying the various human muscles in the figures, try to visualize what happens when the muscle contracts. Since muscles often have many actions, we have indicated the primary action of each muscle in blue type in the tables. Then, use a torso model or an anatomical chart to identify as many muscles as possible. If a human cadaver is available, your instructor will provide specific instructions. Then, carry out the instructions for demonstrating and palpating muscles. Figures 13.2 and 13.3 are summary figures illustrating the superficial musculature of the body. Muscles of the Head and Neck The muscles of the head serve many specific functions. For instance, the muscles of facial expression differ from most skeletal muscles because they insert into the skin or other muscles rather than into bone. As a result, they move the facial skin, allowing a wide range of emotions to be expressed. Other muscles of the head are the muscles of mastication, which move the mandible during chewing, and the six extrinsic eye muscles located within the orbit, which aim the eye. (Orbital muscles are studied in Exercise 23.) Neck muscles provide for the movement of the head and shoulder girdle. Activity 1 Identifying Head and Neck Muscles Read the descriptions of specific head and neck muscles and identify the various muscles in the figures (Tables  13.1 and 13.2, pp. 204–208 and Figures 13.4 and 13.5), trying to visualize their action when they contract. Then identify them on a torso model or anatomical chart. Demonstrating Operations of Head Muscles Raise your eyebrow to wrinkle your forehead. You are using the frontal belly of the epicranius muscle. Blink your eyes; wink. You are contracting orbicularis oculi. Close your lips and pucker up. This requires contraction of orbicularis oris. Smile. You are using zygomaticus. To demonstrate the temporalis, place your hands on your temples and clench your teeth. The masseter can also be palpated now at the angle of the jaw. Muscles of the Trunk The trunk musculature includes muscles that move the vertebral column; anterior thorax muscles that act to move ribs, head, and arms; and muscles of the abdominal wall that play a role in the movement of the vertebral column but more importantly form the “natural girdle,” or the major portion of the abdominal body wall. Activity 2 Identifying Muscles of the Trunk Read the descriptions of specific trunk muscles and identify them in the figures (Tables 13.3, pp. 209–210 and 13.4, pp. 212–214 and Figures 13.6– 13.9, pp. 209–215), visualizing their action when they contract. Then identify them on a torso model or anatomical chart. Demonstrating Operations of Trunk Muscles Now, work with a partner to demonstrate the operation of the following muscles. One of you can demonstrate the movement; the following steps are addressed to this partner. The other can supply resistance and palpate the muscle being tested. Fully abduct the arm and extend at the elbow. Now adduct the arm against resistance. You are using the latissimus dorsi. To observe the action of the deltoid, try to abduct your arm against resistance. Now attempt to elevate your shoulder against resistance; you are contracting the upper portion of the trapezius. The pectoralis major is used when you press your hands together at chest level with your elbows widely abducted. Figure 13.2 Anterior view of superficial muscles of the body. The abdominal surface has been partially dissected on the left side of the body to show somewhat deeper muscles. Figure 13.3 Posterior view of superficial muscles of the body. Table 13.1 Major Muscles of the Human Head (Figure 13.4) Muscle Comments Origin Insertion Action Facial Expression (Figure 13.4a) Epicranius—frontal and occipital bellies Bipartite muscle consisting of frontal and occipital bellies, which covers dome of skull Frontal belly—epicranial aponeurosis; occipital belly—occipital and temporal bones Frontal belly—skin of eyebrows and root of nose; occipital belly—epicranial aponeurosis With aponeurosis fixed, frontal belly raises eyebrows; occipital belly fixes aponeurosis and pulls scalp posteriorly Orbicularis oculi Tripartite sphincter muscle of eyelids Frontal and maxillary bones and ligaments around orbit Tissue of eyelid Closes eye, produces blinking, squinting, and draws eyebrows inferiorly Corrugator supercilii Small muscle; acts with orbicularis oculi Arch of frontal bone above nasal bone Skin of eyebrow Draws eyebrows medially and inferiorly; wrinkles skin of forehead vertically Levator labii superioris Thin muscle between orbicularis oris and inferior eye margin Zygomatic bone and infraorbital margin of maxilla Skin and muscle of upper lip Raises and furrows upper lip; opens lips Zygomaticus—major and minor Extends diagonally from corner of mouth to cheekbone Zygomatic bone Skin and muscle at corner of mouth Raises lateral corners of mouth upward (smiling muscle) Risorius Slender muscle; runs inferior and lateral to zygomaticus Fascia of masseter muscle Skin at angle of mouth Draws corner of lip laterally; tenses lip; zygomaticus synergist Depressor labii inferioris Small muscle running from lower lip to mandible Body of mandible lateral to its midline Skin and muscle of lower lip Draws lower lip inferiorly Depressor anguli oris Small muscle lateral to depressor labii inferioris Body of mandible below incisors Skin and muscle at angle of mouth below insertion of zygomaticus Zygomaticus antagonist; draws corners of mouth downward and laterally Orbicularis oris Multilayered muscle of lips with fibers that run in many different directions; most run circularly Arises indirectly from maxilla and mandible; fibers blended with fibers of other muscles associated with lips Encircles mouth; inserts into muscle and skin at angles of mouth Closes lips; purses and protrudes lips (kissing and whistling muscle) Mentalis One of muscle pair forming V-shaped muscle mass on chin Mandible below incisors Skin of chin Protrudes lower lip; wrinkles chin Buccinator Principal muscle of cheek; runs horizontally, deep to the masseter Molar region of maxilla and mandible Orbicularis oris Draws corner of mouth laterally; compresses cheek (as in whistling); holds food between teeth during chewing Mastication (Figure 13.4c, d) Masseter Covers lateral aspect of mandibular ramus; can be palpated on forcible closure of jaws Zygomatic arch and maxilla Angle and ramus of mandible Prime mover of jaw closure; elevates mandible Temporalis Fan-shaped muscle lying over parts of frontal, parietal, and temporal bones Temporal fossa Coronoid process of mandible Closes jaw; elevates and retracts mandible Buccinator (See muscles of facial expression.)       Medial pterygoid Runs along internal (medial) surface of mandible (thus largely concealed by that bone) Sphenoid, palatine, and maxillary bones Medial surface of mandible, near its angle Synergist of temporalis and masseter; elevates mandible; in conjunction with lateral pterygoid, aids in grinding movements of teeth Lateral pterygoid Superior to medial pterygoid Greater wing of sphenoid bone Condylar process of mandible Protracts jaw (moves it anteriorly); in conjunction with medial pterygoid, aids in grinding movements of teeth Figure 13.4 Muscles of the head (left lateral view). (a) Superficial muscles. (b) Photo of superficial structures of head and neck. Figure 13.4 Muscles of the head mastication. (c) Lateral view of the temporalis, masseter, and buccinator muscles. (d) Lateral view of the deep chewing muscles, the medial and lateral pterygoid muscles. Table 13.2 Anterolateral Muscles of the Human Neck (Figure 13.5) Muscle Comments Origin Insertion Action Superficial Platysma (Figure 13.4a) Unpaired, thin, sheetlike superficial neck muscle, plays role in facial expression Fascia of chest (over pectoral muscles) and deltoid Lower margin of mandible, skin, and muscle at corner of mouth Tenses skin of neck; depresses mandible; pulls lower lip back and down Sternocleidomastoid Two-headed muscle located deep to platysma on anterolateral surface of neck; indicate limits of anterior and posterior triangles of neck Manubrium of sternum and medial portion of clavicle Mastoid process of temporal bone and superior nuchal line of occipital bone Simultaneous contraction of both muscles of pair causes flexion of neck, acting independently, rotate head toward shoulder on opposite side Scalenes—anterior, middle, and posterior (Figure 13.5c) Located more on lateral than anterior neck; deep to platysma and sternocleidomastoid Transverse processes of cervical vertebrae Anterolaterally on ribs 1–2 Flex and slightly rotate neck; elevate ribs 1–2 (aid in inspiration) Deep (Figure 13.5a, b) Digastric Consists of two bellies united by an intermediate tendon; forms a V-shape under chin Lower margin of mandible (anterior belly) and mastoid process (posterior belly) By a connective tissue loop to hyoid bone Acting together, elevate hyoid bone; open mouth and depress mandible Stylohyoid Slender muscle parallels posterior border of digastric; below angle of jaw Styloid process of temporal bone Hyoid bone Elevates and retracts hyoid bone Mylohyoid Just deep to digastric; forms floor of mouth Medial surface of mandible Hyoid bone and median raphe Elevates hyoid bone and base of tongue during swallowing Sternohyoid Runs most medially along neck; straplike Manubrium and medial end of clavicle Lower margin of hyoid bone Acting with sternothyroid and omohyoid, depresses larynx and hyoid bone if mandible is fixed; may also flex skull Sternothyroid Lateral and deep to sternohyoid Posterior surface of manubrium Thyroid cartilage of larynx (See Sternohyoid above) Omohyoid Straplike with two bellies; lateral to sternohyoid Superior surface of scapula Hyoid bone; inferior border (See Sternohyoid above) Thyrohyoid Appears as a superior continuation of sternothyroid muscle Thyroid cartilage Hyoid bone Depresses hyoid bone; elevates larynx if hyoid is fixed Figure 13.5 Muscles of the anterolateral neck and throat. (a) Photo of the anterior and lateral regions of the neck. Muscles of the anterolateral neck and throat. (b) Anterior view of deep neck muscles (suprahyoid and infrahyoid). (c) Sternocleidomastoid and scalenes shown in an isolated view. Table 13.3 Anterior Muscles of the Human Thorax, Shoulder, and Abdominal Wall (Figures 13.6, 13.7, and 13.8) Muscle Comments Origin Insertion Action * The linea alba (white line) is a narrow, tendinous sheath that runs along the middle of the abdomen from the sternum to the pubic symphysis. It is formed by the fusion of the aponeurosis of the external oblique and transversus muscles. Thorax and Shoulder, Superficial (Figure 13.6) Pectoralis major Large fan-shaped muscle covering upper portion of chest Clavicle, sternum, cartilage of ribs 1–6 (or 7), and aponeurosis of external oblique muscle Fibers converge to insert by short tendon into intertubercular sulcus of humerus Prime mover of arm flexion; adducts, medially rotates arm; with arm fixed, pulls chest upward (thus also acts in forced inspiration) Serratus anterior Fan-shaped muscle deep to scapula; deep and inferior to pectoral muscles on lateral rib cage Lateral aspect of ribs 1–8 (or 9) Vertebral border of anterior surface of scapula Prime mover to protract and hold scapula against chest wall; rotates scapula, causing inferior angle to move laterally and upward; essential to raising arm; fixes scapula for arm abduction Deltoid (see also Figure 13.9) Fleshy triangular muscle forming shoulder muscle mass; intramuscular injection site Lateral 1/3 of clavicle; acromion and spine of scapula Deltoid tuberosity of humerus Acting as a whole, prime mover of arm abduction; when only specific fibers are active, can act as a synergist in flexion, extension, and rotation of arm Thorax and Shoulder, Superficial (continued) Pectoralis minor Flat, thin muscle deep to pectoralis major Anterior surface of ribs 3–5, near their costal cartilages Coracoid process of scapula With ribs fixed, draws scapula forward and inferiorly; with scapula fixed, draws rib cage superiorly Thorax, Deep: Muscles of Respiration (Figure 13.7) External intercostals 11 pairs lie between ribs; fibers run obliquely downward and forward toward sternum Inferior border of rib above (not shown in figure) Superior border of rib below Pull ribs toward one another to elevate rib cage; aid in inspiration Internal intercostals 11 pairs lie between ribs; fibers run deep and at right angles to those of external intercostals Superior border of rib below Inferior border of rib above (not shown in figure) Draw ribs together to depress rib cage; aid in forced expiration; antagonistic to external intercostals Diaphragm Broad muscle; forms floor of thoracic cavity; dome-shaped in relaxed state; fibers converge from margins of thoracic cage toward a central tendon Inferior border of rib and sternum, costal cartilages of last six ribs and lumbar vertebrae Central tendon Prime mover of inspiration flattens on contraction, increasing vertical dimensions of thorax; increases intra-abdominal pressure Abdominal Wall (Figure 13.8a and b) Rectus abdominis Medial superficial muscle, extends from pubis to rib cage; ensheathed by aponeuroses of oblique muscles; segmented Pubic crest and symphysis Xiphoid process and costal cartilages of ribs 5–7 Flexes and rotates vertebral column; increases abdominal pressure; fixes and depresses ribs; stabilizes pelvis during walking; used in sit-ups and curls External oblique Most superficial lateral muscle; fibers run downward and medially; ensheathed by an aponeurosis Anterior surface of lower eight ribs Linea alba,* pubic crest and tubercles, and iliac crest See rectus abdominis, above; compresses abdominal wall; also aids muscles of back in trunk rotation and lateral flexion; used in oblique curls Internal oblique Most fibers run at right angles to those of external oblique, which it underlies Lumbar fascia, iliac crest, and inguinal ligament Linea alba, pubic crest, and costal cartilages of last three ribs As for external oblique Transversus abdominis Deepest muscle of abdominal wall; fibers run horizontally Inguinal ligament, iliac crest, cartilages of last five or six ribs, and lumbar fascia Linea alba and pubic crest Compresses abdominal contents Figure 13.6 Muscles of the thorax and shoulder acting on the scapula and arm (anterior view). The superficial muscles, which effect arm movements, are shown on the left side of the figure. These muscles have been removed on the right side of the figure to show the muscles that stabilize or move the pectoral girdle. Figure 13.7 Deep muscles of the thorax: muscles of respiration. (a) The external intercostals (inspiratory muscles) are shown on the left and the internal intercostals (expiratory muscles) are shown on the right. These two muscle layers run obliquely and at right angles to each other. (b) Inferior view of the diaphragm, the prime mover of inspiration. Notice that its muscle fibers converge toward a central tendon, an arrangement that causes the diaphragm to flatten and move inferiorly as it contracts. The diaphragm and its tendon are pierced by the great vessels (aorta and inferior vena cava) and the esophagus. Figure 13.8 Anterior view of the muscles forming the anterolateral abdominal wall. (a) The superficial muscles have been partially cut away on the left side of the diagram to reveal the deeper internal oblique and transversus abdominis muscles. Table 13.4 Posterior Muscles of the Human Trunk (Figure 13.9) Muscle Comments Origin Insertion Action * The quadriceps form the flesh of the anterior thigh and have a common insertion in the tibial tuberosity via the patellar ligament. They are powerful leg extensors, enabling humans to kick a football, for example. Muscles of the Neck, Shoulder, and Thorax (Figure 13.9a) Trapezius Most superficial muscle of posterior thorax; very broad origin and insertion Occipital bone; ligamentum nuchae; spines of C7 and all thoracic vertebrae Acromion and spinous process of scapula; lateral third of clavicle Extends head; raises, rotates, and retracts (adducts) scapula and stabilizes it; superior fibers elevate scapula (as in shrugging the shoulders); inferior fibers depress scapula Latissimus dorsi Broad flat muscle of lower back (lumbar region); extensive superficial origins Indirect attachment to spinous processes of lower six thoracic vertebrae, lumbar vertebrae, last three to four ribs, and iliac crest Floor of intertubercular sulcus of humerus Prime mover of arm extension; adducts and medially rotates arm; brings arm down in power stroke, as in striking a blow Infraspinatus A rotator cuff muscle; partially covered by deltoid and trapezius Infraspinous fossa of scapula Greater tubercle of humerus Lateral rotation of arm; helps hold head of humerus in glenoid cavity; stabilizes shoulder Teres minor A rotator cuff muscle; small muscle inferior to infraspinatus Lateral margin of posterior scapula Greater tubercle of humerus Same as for infraspinatus Teres major Located inferiorly to teres minor Posterior surface at inferior angle of scapula Intertubercular sulcus of humerus Extends, medially rotates, and adducts arm; synergist of latissimus dorsi Supraspinatus A rotator cuff muscle; obscured by trapezius Supraspinous fossa of scapula Greater tubercle of humerus Initiates abduction of arm; stabilizes shoulder joint Levator scapulae Located at back and side of neck, deep to trapezius Transverse processes of C1–C4 Medial border of scapula superior to spine Elevates and adducts scapula; with fixed scapula, laterally flexes neck to the same side Rhomboids—major and minor Beneath trapezius and inferior to levator scapulae; rhomboid minor is the more superior muscle Spinous processes of C7 and T1–T5 Medial border of scapula Pull scapulae medially (retraction); stabilize scapulae; rotate glenoid cavity downward Muscles Associated with the Vertebral Column (Figure 13.9b) Semispinalis Deep composite muscle of the back—thoracis, cervicis, and capitis portions Transverse processes of C7–T12 Occipital bone and spinous processes of cervical vertebrae and T1–T4 Acting together, extend head and vertebral column; acting independently (right vs. left), causes rotation toward the opposite side Muscles Associated with the Vertebral Column (continued) Erector spinae A long tripartite muscle composed of iliocostalis (lateral), longissimus, and spinalis (medial) muscle columns; superficial to semispinalis muscles; extends from pelvis to head Iliac crest, transverse processes of lumbar, thoracic, and cervical vertebrae, and/or ribs 3–6 depending on specific part Ribs and transverse processes of vertebrae about six segments above origin; longissimus also inserts into mastoid process Extend and bend the vertebral column laterally; fibers of the longissimus also extend and rotate head Splenius (Figure 13.9c) Superficial muscle (capitis and cervicis parts) extending from upper thoracic region to skull Ligamentum nuchae and spinous processes of C7–T6 Mastoid process, occipital bone, and transverse processes of C2–C4 As a group, extend or hyperextend head; when only one side is active, head is rotated and bent toward the same side Quadratus lumborum Forms greater portion of posterior abdominal wall Iliac crest and lumbar fascia Inferior border of rib 12; transverse processes of lumbar vertebrae Each flexes vertebral column laterally; together extend the lumbar spine and fix rib 12; maintains upright posture Figure 13.8 Anterior view of the muscles forming the anterolateral abdominal wall. (b) Photo of the anterolateral abdominal wall. Figure 13.9 Muscles of the neck, shoulder, and thorax (posterior view). (a) The superficial muscles of the back are shown for the left side of the body, with a corresponding photograph. The superficial muscles are removed on the right side of the illustration to reveal the deeper muscles acting on the scapula and the rotator cuff muscles that help to stabilize the shoulder joint. Muscles of the neck, shoulder, and thorax (posterior view). (b) The erector spinae and semispinalis muscles, which respectively form the intermediate and deep muscle layers of the back associated with the vertebral column. Figure 13.9  Muscles of the neck, shoulder, and thorax (posterior view). (c) Deep (splenius) muscles of the posterior neck. Superficial muscles have been removed. Muscles of the Upper Limb The muscles that act on the upper limb fall into three groups: those that move the arm, those causing movement of the forearm, and those moving the hand and fingers. The muscles that cross the shoulder joint to insert on the humerus and move the arm (subscapularis, supraspinatus and infraspinatus, deltoid, and so on) are primarily trunk muscles that originate on the axial skeleton or shoulder girdle. These muscles are included with the trunk muscles. The second group of muscles, which cross the elbow joint and move the forearm, consists of muscles forming the musculature of the humerus. These muscles arise mainly from the humerus and insert in forearm bones. They are responsible for flexion, extension, pronation, and supination. The third group forms the musculature of the forearm. For the most part, these muscles cross the wrist to insert on the digits and produce movements of the hand and fingers. Activity 3 Identifying Muscles of the Upper Limb Study the origins, insertions, and actions of muscles that move the forearm, and identify them (Table 13.5, p. 216 and Figure 10.10, p. 216). Do the same for muscles acting on the wrist and hand (Table 13.6, pp. 217–218 and Figure 10.11, pp. 217–219). They are more easily identified if you locate their insertion tendons first. Then see if you can identify the upper limb muscles on a torso model, anatomical chart, or cadaver. Complete this portion of the exercise with palpation demonstrations as outlined next. Demonstrating Operations of Upper Limb Muscles To observe the biceps brachii, attempt to flex your forearm (hand supinated) against resistance. The insertion tendon of this biceps muscle can also be felt in the lateral aspect of the cubital fossa (where it runs toward the radius to attach). If you acutely flex at your elbow and then try to extend the forearm against resistance, you can demonstrate the action of your triceps brachii. Strongly flex your hand, and make a fist. Palpate your contracting wrist flexor muscles’ origins at the medial epicondyle of the humerus and their insertion tendons, at the anterior aspect of the wrist. Flare your fingers to identify the tendons of the extensor digitorum muscle on the dorsum of your hand. Table 13.5 Muscles of the Human Humerus That Act on the Forearm (Figure 13.10) Muscle Comments Origin Insertion Action Triceps brachii Large fleshy muscle of posterior humerus; three-headed origin Long head—inferior margin of glenoid cavity; lateral head—posterior humerus; medial head—distal radial groove on posterior humerus Olecranon of ulna Powerful forearm extensor; antagonist of forearm flexors (brachialis and biceps brachii) Anconeus Short triangular muscle blended with triceps Lateral epicondyle of humerus Lateral aspect of olecranon of ulna Abducts ulna during forearm pronation; synergist of triceps brachii in forearm extension Biceps brachii Most familiar muscle of anterior humerus because this two-headed muscle bulges when forearm is flexed Short head: coracoid process; long head: supraglenoid tubercle and lip of glenoid cavity; tendon of long head runs in intertubercular sulcus and within capsule of shoulder joint Radial tuberosity Flexion (powerful) and supination of forearm; “it turns the corkscrew and pulls the cork”; weak arm flexor Brachioradialis Superficial muscle of lateral forearm; forms lateral boundary of cubital fossa Lateral ridge at distal end of humerus Base of radial styloid process Synergist in forearm flexion Brachialis Immediately deep to biceps brachii Distal portion of anterior humerus Coronoid process of ulna Flexor of forearm Figure 13.10 Muscles acting on the arm and forearm. (a) Superficial muscles of the anterior thorax, shoulder, and arm, anterior view. (b) Posterior aspect of the arm showing the lateral and long heads of the triceps brachii muscle. Table 13.6 Muscles of the Human Forearm That Act on the Hand and Fingers (Figure 13.11) Muscle Comments Origin Insertion Action Anterior Compartment, Superficial (Figure 13.11a, b, c) Pronator teres Seen in a superficial view between proximal margins of brachioradialis and flexor carpi radialis Medial epicondyle of humerus and coronoid process of ulna Midshaft of radius Acts synergistically with pronator quadratus to pronate forearm; weak forearm flexor Flexor carpi radialis Superficial; runs diagonally across forearm Medial epicondyle of humerus Base of metacarpals II and III Powerful flexor and abductor of the hand Palmaris longus Small fleshy muscle with a long tendon; medial to flexor carpi radialis Medial epicondyle of humerus Palmar aponeurosis; skin and fascia of palm Flexes hand (weak); tenses skin and fascia of palm Anterior Compartment, Superficial (continued) Flexor carpi ulnaris Superficial; medial to palmaris longus Medial epicondyle of humerus and olecranon and posterior surface of ulna Base of metacarpal V; pisiform and hamate bones Flexes and adducts hand Flexor digitorum superficialis Deeper muscle (deep to muscles named above); visible at distal end of forearm Medial epicondyle of humerus, coronoid process of ulna, and shaft of radius Middle phalanges of fingers 2–5 Flexes hand and middle phalanges of fingers 2–5 Anterior Compartment, Deep (Figure 13.11a, b, c) Flexor pollicis longus Deep muscle of anterior forearm; distal to and paralleling lower margin of flexor digitorum superficialis Anterior surface of radius, and interosseous membrane Distal phalanx of thumb Flexes thumb (pollex is Latin for “thumb”) Flexor digitorum profundus Deep muscle; overlain entirely by flexor digitorum superficialis Anteromedial surface of ulna, interosseous membrane, and coronoid process Distal phalanges of fingers 2–5 Sole muscle that flexes distal phalanges; assists in hand flexion Pronator quadratus Deepest muscle of distal forearm Distal portion of anterior ulnar surface Anterior surface of radius, distal end Pronates forearm Posterior Compartment, Superficial (Figure 13.11d, e, f) Extensor carpi radialis longus Superficial; parallels brachioradialis on lateral forearm Lateral supracondylar ridge of humerus Base of metacarpal II Extends and abducts hand Extensor carpi radialis brevis Deep to extensor carpi radialis longus Lateral epicondyle of humerus Base of metacarpal III Extends and abducts hand; steadies wrist during finger flexion Extensor digitorum Superficial; medial to extensor carpi radialis brevis Lateral epicondyle of humerus By four tendons into distal phalanges of fingers 2–5 Prime mover of finger extension; extends hand; can flare (abduct) fingers Extensor carpi ulnaris Superficial; medial posterior forearm Lateral epicondyle of humerus; posterior border of ulna Base of metacarpal V Extends and adducts hand Posterior Compartment, Deep (Figure 13.11d, e, f) Extensor pollicis longus and brevis Muscle pair with a common origin and action; deep to extensor carpi ulnaris Dorsal shaft of ulna and radius, interosseous membrane Base of distal phalanx of thumb (longus) and proximal phalanx of thumb (brevis) Extend thumb Abductor pollicis longus Deep muscle; lateral and parallel to extensor pollicis longus Posterior surface of radius and ulna; interosseous membrane Metacarpal I and trapezium Abducts and extends thumb Supinator Deep muscle at posterior aspect of elbow Lateral epicondyle of humerus; proximal ulna Proximal end of radius Synergist of biceps brachii to supinate forearm; antagonist of pronator muscles Figure 13.11 Muscles of the forearm and wrist. (a) Superficial anterior view of right forearm and hand. (b) The brachioradialis, flexors carpi radialis and ulnaris, and palmaris longus muscles have been removed to reveal the position of the somewhat deeper flexor digitorum superficialis. (c) Deep muscles of the anterior compartment. Superficial muscles have been removed. (Note: The thenar muscles of the thumb and the lumbricals that help move the fingers are illustrated here but are not described in   Table 13.6.) Muscles of the forearm and wrist. (d) Superficial muscles, posterior view. (e) Deep posterior muscles; superficial muscles have been removed. The interossei, the deepest layer of intrinsic hand muscles, are also illustrated. (f) Photo of posterior muscles of the right forearm. Muscles of the Lower Limb Table 13.7 Muscles Acting on the Human Thigh and Leg, Anterior and Medial Aspects (Figure 13.12) Muscle Comments Origin Insertion Action * The iliotibial tract, a thickened lateral portion of the fascia lata, ensheathes all the muscles of the thigh. It extends as a tendinous band from the iliac crest to the knee. †The hamstrings are the fleshy muscles of the posterior thigh. As a group, they are strong extensors of the thigh; they counteract the powerful quadriceps by stabilizing the knee joint when standing. Origin on the Pelvis Iliopsoas—iliacus and psoas major Two closely related muscles; fibers pass under inguinal ligament to insert into femur via a common tendon; iliacus is more lateral Iliacus—iliac fossa and crest, lateral sacrum; psoas major—transverse processes, bodies, and discs of T12 and lumbar vertebrae On and just below lesser trochanter of femur Flex trunk at hip joint, flex thigh; lateral flexion of vertebral column (psoas) Sartorius Straplike superficial muscle running obliquely across anterior surface of thigh to knee Anterior superior iliac spine By an aponeurosis into medial aspect of proximal tibia Flexes, abducts, and laterally rotates thigh; flexes leg; known as “tailor’s muscle” because it helps effect cross-legged position in which tailors are often depicted Medial Compartment Adductors—magnus, longus, and brevis Large muscle mass forming medial aspect of thigh; arise from front of pelvis and insert at various levels on femur Magnus—ischial and pubic rami and ischial tuberosity; longus—pubis near pubic symphysis; brevis—body and inferior pubic ramus Magnus—linea aspera and adductor tubercle of femur; longus and brevis—linea aspera Adduct and medially rotate and flex thigh; posterior part of magnus is also a synergist in thigh extension Pectineus Overlies adductor brevis on proximal thigh Pectineal line of pubis (and superior pubic ramus) Inferior from lesser trochanter to linea aspera of femur Adducts, flexes, and medially rotates thigh Gracilis Straplike superficial muscle of medial thigh Inferior ramus and body of pubis Medial surface of tibia just inferior to medial condyle Adducts thigh; flexes and medially rotates leg, especially during walking Anterior Compartment Quadriceps femoris*  Rectus femoris Superficial muscle of thigh; runs straight down thigh; only muscle of group to cross hip joint Anterior inferior iliac spine and superior margin of acetabulum Tibial tuberosity and patella Extends leg and flexes thigh  Vastus lateralis Forms lateral aspect of thigh; intramuscular injection site Greater trochanter, intertrochanteric line, and linea aspera Tibial tuberosity and patella Extends leg and stabilizes knee  Vastus medialis Forms inferomedial aspect of thigh Linea aspera and intertrochanteric line Tibial tuberosity and patella Extends leg; stabilizes patella  Vastus intermedius Obscured by rectus femoris; lies between vastus lateralis and vastus medialis on anterior thigh Anterior and lateral surface of femur Tibial tuberosity and patella Extends leg Tensor fasciae latae Enclosed between fascia layers of thigh Anterior aspect of iliac crest and anterior superior iliac spine Iliotibial tract (lateral portion of fascia lata) Flexes, abducts, and medially rotates thigh; steadies trunk Muscles that act on the lower limb cause movement at the hip, knee, and ankle joints. Since the human pelvic girdle is composed of heavy, fused bones that allow very little movement, no special group of muscles is necessary to stabilize it. This is unlike the shoulder girdle, where several muscles (mainly trunk muscles) are needed to stabilize the scapulae. Muscles acting on the thigh (femur) cause various movements at the multiaxial hip joint. These include the iliopsoas, the adductor group, and others. Muscles acting on the leg form the major musculature of the thigh. (Anatomically, the term leg refers only to that portion between the knee and the ankle.) The thigh muscles cross the knee to allow flexion and extension of the leg. They include the hamstrings and the quadriceps. The muscles originating on the leg cross the ankle joint and act on the foot and toes. Activity 4 Identifying Muscles of the Lower Limb Read the descriptions of specific muscles acting on the thigh and leg and identify them (Tables 13.7, pp. 220–222 and 13.8, pp. 222–223 and Figures 10.12 and 10.13, p. 223), trying to visualize their action when they contract. Since some of the muscles acting on the leg also have attachments on the pelvic girdle, they can cause movement at the hip joint. Do the same for muscles acting on the foot and toes (Table 13.9 and Figures 10.14 and 10.15). Then identify all the muscles on a model or anatomical chart. Demonstrating Operations of Lower Limb Muscles Go into a deep knee bend and palpate your own gluteus maximus muscle as you extend your thigh to resume the upright posture. Demonstrate the contraction of the anterior quadriceps femoris by trying to extend your leg against resistance. Do this while seated and note how the quadriceps tendon reacts. The biceps femoris of the posterior thigh comes into play when you flex your leg against resistance. Now stand on your toes. Have your partner palpate the lateral and medial heads of the gastrocnemius and follow it to its insertion in the calcaneal tendon. Dorsiflex and invert your foot while palpating your tibialis anterior muscle (which parallels the sharp anterior crest of the tibia laterally). Figure 13.12 Anterior and medial muscles promoting movements of the thigh and leg. (a) Anterior view of the deep muscles of the pelvis and superficial muscles of the right thigh. (b) Adductor muscles of the medial compartment of the thigh. (c)   The vastus muscles (isolated) of the quadriceps group. Table 13.8 Muscles Acting on the Human Thigh and Leg, Posterior Aspect (Figure 13.13) Muscle Comments Origin Insertion Action Origin on the Pelvis Gluteus maximus Largest and most superficial of gluteal muscles (which form buttock mass); intramuscular injection site Dorsal ilium, sacrum, and coccyx Gluteal tuberosity of femur and iliotibial tract* Complex, powerful thigh extensor (most effective when thigh is flexed, as in climbing stairs—but not as in walking); antagonist of iliopsoas; laterally rotates and abducts thigh Gluteus medius Partially covered by gluteus maximus; intramuscular injection site Upper lateral surface of ilium Greater trochanter of femur Abducts and medially rotates thigh; steadies pelvis during walking Gluteus minimus (not shown in figure) Smallest and deepest gluteal muscle External inferior surface of ilium Greater trochanter of femur Abducts and medially rotates thigh; steadies pelvis Posterior Compartment Hamstring†  Biceps femoris Most lateral muscle of group; arises from two heads Ischial tuberosity (long head); linea aspera and distal femur (short head) Tendon passes laterally to insert into head of fibula and lateral condyle of tibia Extends thigh; laterally rotates leg; flexes leg  Semitendinosus Medial to biceps femoris Ischial tuberosity Medial aspect of upper tibial shaft Extends thigh; flexes leg; medially rotates leg  Semimembranosus Deep to semitendinosus Ischial tuberosity Medial condyle of tibia; lateral condyle of femur Extends thigh; flexes leg; medially rotates leg Figure 13.13 Muscles of the posterior aspect of the right hip and thigh. (a) Superficial view showing the gluteus muscles of the buttock and hamstring muscles of the thigh. (b) Photo of muscles of the posterior thigh. Group Challenge Name That Muscle Table 13.9 Muscles Acting on the Human Foot and Ankle (Figures 13.14 and 13.15) Muscle Comments Origin Insertion Action Lateral Compartment (Figure 13.14a, b and Figure 13.15b) Fibularis (peroneus) longus Superficial lateral muscle; overlies fibula Head and upper portion of fibula By long tendon under foot to metatarsal I and medial cuneiform Plantar flexes and everts foot Fibularis (peroneus) brevis Smaller muscle; deep to fibularis longus Distal portion of fibula shaft By tendon running behind lateral malleolus to insert on proximal end of metatarsal V Plantar flexes and everts foot Anterior Compartment (Figure 13.14a, b) Tibialis anterior Superficial muscle of anterior leg; parallels sharp anterior margin of tibia Lateral condyle and upper 2/3 of tibia; interosseous membrane By tendon into inferior surface of first cuneiform and metatarsal I Prime mover of dorsiflexion; inverts foot; supports longitudinal arch of foot Extensor digitorum longus Anterolateral surface of leg; lateral to tibialis anterior Lateral condyle of tibia; proximal ¾ of fibula; interosseous membrane Tendon divides into four parts; inserts into middle and distal phalanges of toes 2–5 Prime mover of toe extension; dorsiflexes foot Fibularis (peroneus) tertius Small muscle; often fused to distal part of extensor digitorum longus Distal anterior surface of fibula and interosseous membrane Tendon inserts on dorsum of metatarsal V Dorsiflexes and everts foot Extensor hallucis longus Deep to extensor digitorum longus and tibialis anterior Anteromedial shaft of fibula and interosseous membrane Tendon inserts on distal phalanx of great toe Extends great toe; dorsiflexes foot Posterior Compartment, Superficial (Figure 13.15a; also Figure 13.14) Triceps surae Refers to muscle pair below that shapes posterior calf   Via common tendon (calcaneal) into calcaneus of the heel Plantar flex foot Gastrocnemius Superficial muscle of pair; two prominent bellies By two heads from medial and lateral condyles of femur Calcaneus via calcaneal tendon Plantar flexes foot when leg is extended; crosses knee joint; thus can flex leg (when foot is dorsiflexed) Soleus Deep to gastrocnemius Proximal portion of tiba and fibula; interosseous membrane Calcaneus via calcaneal tendon Plantar flexion; an important muscle for locomotion Posterior Compartment, Deep (Figure 13.15b– e) Popliteus Thin muscle at posterior aspect of knee Lateral condyle of femur and lateral meniscus Proximal tibia Flexes and rotates leg medially to “unlock” knee when leg flexion begins Tibialis posterior Thick muscle deep to soleus Superior portion of tibia and fibula and interosseous membrane Tendon passes obliquely behind medial malleolus and under arch of foot; inserts into several tarsals and metatarsals II–IV Prime mover of foot inversion; plantar flexes foot; stabilizes longitudinal arch of foot Flexor digitorum longus Runs medial to and partially overlies tibialis posterior Posterior surface of tibia Distal phalanges of toes 2–5 Flexes toes; plantar flexes and inverts foot Flexor hallucis longus (see also Figure 13.14a) Lies lateral to inferior aspect of tibialis posterior Middle portion of fibula shaft; interosseous membrane Tendon runs under foot to distal phalanx of great toe Flexes great toe (hallux = great toe); plantar flexes and inverts foot; the “push-off muscle” during walking Work in groups of three or four to fill out the Group Challenge chart for muscle IDs. Refrain from looking back at the tables. Use the “brain power” of your group and the appropriate muscle models. All members of the group discuss where the origins and insertions in the table are located. One student in the group acts as the “muscle model.” The “model” points to the appropriate attachment locations on his or her body and performs the primary action as advised by the group. To assist in this task, recall that when a muscle contracts, the muscle’s insertion moves toward the muscle’s origin. Also, in the muscles of the limbs, the origin typically lies proximal to the insertion. Sometimes the origin and insertion are even part of the muscle’s name! Group Challenge: Muscle IDs Origin Insertion Muscle Primary action Zygomatic arch and maxilla Angle and ramus of the mandible     Anterior surface of ribs 3–5 Coracoid process of the scapula     Inferior border of rib above Superior border of rib below     Distal portion of anterior humerus Coronoid process of the ulna     Anterior inferior iliac spine and superior margin of acetabulum Tibial tuberosity and patella     By two heads from medial and lateral condyles of femur Calcaneus via calcaneal tendon     Figure 13.14 Muscles of the anterolateral aspect of the right leg. (a) Superficial view of lateral aspect of the leg, illustrating the positioning of the lateral compartment muscles (fibularis longus and brevis) relative to anterior and posterior leg muscles. (b) Superficial view of anterior leg muscles. Activity 5 Review of Human Musculature Review the muscles by watching the Human Musculature video. Activity 6 Making a Muscle Painting Choose a male student to be “muscle painted.” Obtain brushes and water-based paints from the supply area while the “volunteer” removes his shirt and rolls up his pant legs (if necessary). Using different colored paints, identify the muscles listed below by painting his skin. If a muscle covers a large body area, you may opt to paint only its borders. biceps brachii deltoid erector spinae pectoralis major rectus femoris tibialis anterior triceps brachii vastus lateralis biceps femoris extensor carpi radialis longus latissimus dorsi rectus abdominis sternocleidomastoid trapezius triceps surae vastus medialis Check your “human painting” with your instructor before cleaning your bench and leaving the laboratory. For instructions on animal dissections, see the dissection exercises (starting on p. 705) in the cat and fetal pig editions of this manual. Figure 13.15 Muscles of the posterior aspect of the right leg. (a) Superficial view of the posterior leg. (b)   The triceps surae has been removed to show the deep muscles of the posterior compartment. Muscles of the posterior aspect of the right leg. (c–e) Individual muscles are shown in isolation so that their origins and insertions may be observed.

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