Muscles Acting on the Scapula

The scapula is loosely attached to the thoracic cage and is capable of considerable movement—rotation (as in raising and lowering the apex of the shoulder), elevation and depression (as in shrugging and lowering the shoulders), and protraction and retraction (pulling the shoulders forward or back) (fig. 10.21). The clavicle braces the shoulder and moderates these movements.

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Chapter 10 The Muscular System 353

Table 10.7 Muscles of the Back (see figs. 10.17 and 10.18)

O = origin, I = insertion, N = innervation (n. = nerve, nn. = nerves)

Superficial Group—The Erector Spinae (ee-RECK-tur SPY-nee)

Iliocostalis Cervicis (ILL-ee-oh-coss-TAH-liss SIR-vih-sis), Iliocostalis Thoracis (tho-RA-sis), and Iliocostalis Lumborum (lum-BORE-um)

Extend and laterally flex vertebral column; thoracis and lumborum rotate ribs during forceful inspiration O: angles of ribs, sacrum, iliac crest I: cervicis—vertebrae C4-C6; thoracis—vvertebra C7, angles of ribs 1-6; lumborum—angles of ribs 7-12

N: dorsal rami of spinal nn.

Longissimus (lawn-JISS-ih-muss) Cervicis and Longissimus Thoracis

Extend and laterally flex vertebral column

O: cervicis—vertebrae T1 to T4 or T5; I: cervicis—vertebrae C2-C6; thoracis— vertebrae T1-T12, thoracis—sacrum, iliac crest, vertebrae T1-L5 ribs 3 or 4 to 12

N: dorsal rami of spinal nn.

Spinalis (spy-NAY-liss) Cervicis and Spinalis Thoracis

Extend vertebral column

O: cervicis—nuchal ligament, spinous process of I: cerncis—spinous process of axis; thoracis—spinous vertebra C7; thoracis—spinous processes processes of upper thoracic vertebrae of T11-L2

N: dorsal rami of spinal nn.

Superficial Group—Serratus Posterior Muscles

Serratus Posterior Superior (seh-RAY-tus)

Elevates ribs 2-5 during inspiration

O: spines of vertebrae C7-T3 I: ribs 2-5

N: intercostal nn. 2-5

Serratus Posterior Inferior

Depresses ribs 9-12 during inspiration

O: spines of vertebrae T10-L2 I: ribs 9-12

N: ventral rami of T9-T12

Deep Group

Semispinalis Cervicis (SEM-ee-spy-NAY-liss SUR-vih-sis) and Semispinalis Thoracis (tho-RA-sis)

Extend neck; extend and rotate vertebral column

O: transverse processes of vertebrae T1-T10 I: spinous processes of vertebrae C2-T5

N: dorsal rami of spinal nn.

Quadratus Lumborum (quad-RAY-tus lum-BORE-um)

Laterally flexes vertebral column, depresses rib 12

O: iliac crest, lower lumbar vertebrae, I: upper lumbar vertebrae, rib 12 thoracolumbar fascia

N: ventral rami of L1-L3

Multifidus (mul-TIFF-ih-dus)

Extends and rotates vertebral column

O: sacrum, iliac crest, vertebrae C4-L5 I: laminae and spinous processes of vertebrae above origins

N: dorsal rami of spinal nn.

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354 Part Two Support and Movement

Table 10.8 Muscles of the Pelvic Floor (see fig. 10.20)

O = origin, I = insertion, N = innervation (n. = nerve)

Superficial Muscles of the Perineum

Ischiocavernosus (ISS-kee-oh-CAV-er-NO-sus)

Aids in erection of penis and clitoris

O: ischial and pubic rami, ischial tuberosity I: penis, clitoris

N: pudendal n.

Bulbospongiosus (BUL-bo-SPUN-jee-OH-sus)

Male: compresses urethra to expel semen or urine. Female: constricts vaginal orifice.

O: central tendon of perineum, bulb of penis I: fasciae of perineum, penis or clitoris

N: pudendal n.

Superficial Transverse Perineus (PERR-ih-NEE-us)

Fixes central tendon of perineum, supports pelvic floor

O: ischial ramus I: central tendon of perineum

N: pudendal n.

Muscles of the Urogenital Diaphragm

Deep Transverse Perineus

Fixes central tendon of perineum; supports pelvic floor; expels last drops of urine in both sexes and semen in male O: ischial ramus I: central tendon of perineum

N: pudendal n.

External Urethral Sphincter

Compresses urethra to voluntarily inhibit urination

O: ischial and pubic rami I: medial raphe of male, vaginal wall of female

N: pudendal n.

Muscle of the Anal Triangle

External Anal Sphincter

Compresses anal canal to voluntarily inhibit defecation

O: anococcygeal raphe I: central tendon of perineum

N: pudendal n., S4

Muscles of the Pelvic Diaphragm

Levator Ani (leh-VAY-tur AY-nye)

Supports viscera; resists pressure surges in abdominal cavity; elevates anus during defecation; forms vaginal and anorectal sphincters

O: os coxae from pubis to ischial spine I: coccyx, anal canal, anococcygeal raphe N: pudendal n., S3-S4

Coccygeus (coc-SIDJ-ee-us)

Draws coccyx anteriorly after defecation or childbirth; supports and elevates pelvic floor; resists abdominal pressure surges O: ischial spine I: lower sacrum to upper coccyx

N: S3 or S4

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Chapter 10 The Muscular System 355

Male

Female Bulbospongiosus Muscle

Ischiocavernosus Raphe

Bulbospongiosus

Superficial transverse perineus

Levator ani

-Gluteus maximus -

Female

-Gluteus maximus -

Perineus Transversus

Urogenital triangle

Urethra Vagina

Anus

Anal triangle

- Pubic symphysis -

- Pubic ramus-

- External urethral sphincter- Deep transverse-

perineus Central tendon

- External anal sphincter-

Urogenital Diaphragm

Urethra

-'Vagina

-Anus

Urogenital diaphragm .

Pelvic diaphragm Levator ani -

Coccygeus Piriformis

Urogenital Triangle Muscles

Urethra Vagina

Anus Coccyx

Pelvic Scapular Girdle

Figure 10.20 Muscles of the Pelvic Floor. (a, b) The superficial perineal space, inferior view. Triangles of the perineum are marked in b. (c, d) The urogenital diaphragm, inferior view; this is the next deeper layer after the muscles in a and b. (e) The pelvic diaphragm, the deepest layer, superior view (seen from within the pelvic cavity).

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

356 Part Two Support and Movement

Lateral rotation

Trapezius (superior part) Serratus anterior

Elevation

Levator scapulae Trapezius (superior part) Rhomboideus major Rhomboideus minor

Lateral rotation

Trapezius (superior part) Serratus anterior

Medial rotation

Levator scapulae Rhomboideus major Rhomboideus minor

Medial rotation

Levator scapulae Rhomboideus major Rhomboideus minor

Depression

Trapezius (inferior part) Serratus anterior

Retraction Protraction

Rhomboideus major Pectoralis minor Rhomboideus minor Serratus anterior Trapezius

Retraction Protraction

Rhomboideus major Pectoralis minor Rhomboideus minor Serratus anterior Trapezius

Figure 1 0.21 Actions of Some Thoracic Muscles on the Scapula. Note that an individual muscle can contribute to multiple actions, depending on which fibers contract and what synergists act with it. In the two upper figures, mark the insertion of each of the named muscles.

The muscles that act on the pectoral girdle originate on the axial skeleton and insert on the clavicle and scapula. They are divided into anterior and posterior groups (table 10.9). The important muscles of the anterior group are the pectoralis minor and serratus anterior (see fig. 10.15b). In the posterior group, we have the large, superficial trapezius, already studied, and three deep muscles, the levator scapulae, rhomboideus major, and rhomboideus minor. The action of the trapezius depends on whether its superior, middle, or inferior fibers contract and whether it acts alone or with other muscles. The lev-

ator scapulae and superior fibers of the trapezius rotate the scapula in opposite directions if either of them acts alone. If both act together, their opposite rotational effects balance each other and they elevate the scapula and shoulder, as when you carry a heavy weight on your shoulder. Depression of the scapula occurs mainly by gravitational pull, but the trapezius and serratus anterior can cause faster, more forcible depression, as in swimming, hammering, and rowing.

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