Muscles Acting on the Head

Muscles that move the head originate on the vertebral column, thoracic cage, and pectoral girdle and insert on the cranial bones (table 10.4). The principal flexors of the neck are the sternocleidomastoid34 and three scalenes on each side (fig. 10.10). The prime mover is the sternocleidomas-toid, a thick cordlike muscle that extends from the sternum and clavicle to the mastoid process behind the ear. It is most easily seen and palpated when the head is turned to one side and slightly extended. As it passes obliquely across the neck, the sternocleidomastoid divides it into anterior and posterior triangles. Other muscles and landmarks subdivide each of these into smaller triangles of surgical importance (fig. 10.11).

When both sternocleidomastoids contract, the neck flexes forward; for example, when you look down at something between your feet. When only the left one contracts, the head tilts down and to the right, and when the right one acts alone, it draws the head down and to the left. To visualize this action, hold the index finger of sterno = sternum + cleido = clavicle + mastoid = mastoid process of skull

Table 10.4 Muscles Acting on the Head (see figs. 10.10 and 10.17)

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

Flexors of the Neck

Sternocleidomastoid (STIR-no-CLY-doe-MASS-toyd)

Contraction of either one draws head down and toward the side opposite the contracting muscle; contraction of both draws head forward and down, as in looking between the feet O: clavicle, manubrium I: mastoid process

Scalenes (SCAY-leens) (three muscles)

Flex neck laterally; elevate ribs 1 and 2 in inspiration O: vertebrae C2-C6 I: ribs 1-2

Extensors of the Neck

Trapezius (tra-PEE-zee-us)

Abducts and extends neck (see other functions in table 10.9) O: external occipital protuberance, nuchal I: clavicle, acromion, scapular spine ligament, spinous processes of vertebrae C7-T12

Splenius Capitis (SPLEE-nee-us CAP-ih-tis) and Splenius Cervicis (SIR-vih-sis)

Rotate head, extend neck

O: capitis—spinous processes of vertebrae I: capitis—mastoid process, superior nuchal

N: dorsal rami of middle and lower cervical nn.

C7-T3 or T4; cervicis—spinous processes of T3-T6

line; cervicis—transverse processes of C1-C2 or C3

Semispinalis (SEM-ee-spy-NAY-liss) Capitis

Rotates and extends head (see other parts of semispinalis in table 10.7) O: transverse processes of vertebrae T1-T6, I: occipital bone articular processes of C4-C7

N: dorsal rami of cervical nn.

your left hand on your left mastoid process and the index finger of your right hand on your sternal notch. Now contract the left sternocleidomastoid in a way that brings the two fingertips as close together as possible. You will note that this action causes you to look downward and to the right.

The extensors are located in the back of the neck. Their actions include extension (holding the head erect), hyperextension (as in looking upward toward the sky), abduction (tilting the head to one side), and rotation (as in looking to the left and right). Extension and hyperextension involve equal action of the right and left members of a pair; the other actions require the muscle on one side to contract more strongly than the opposite muscle. Many head movements result from a combination of these actions—for example, looking up over the shoulder involves a combination of rotation and extension.

We will consider only three primary extensors: the trapezius, splenius capitis, and semispinalis capitis (figs. 10.12 and 10.17). The trapezius is a vast triangular muscle of the upper back and neck; together, the right and left trapezius muscles form a trapezoid. The origin of the trapezius extends from the occipital protuberance of the skull to thoracic vertebra 12. The trapezius con-

Superior nuchal line -

Semispinalis capitis -Sternocleidomastoid

Longissimus capitis Longissimus cervicis

Trapezius

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

344 Part Two Support and Movement

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Anterior triangles

A1. Muscular

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A2. Carotid

A3. Submandibular

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A4. Suprahyoid

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Posterior triangles

P1. Occipital

P2. Omoclavicular

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Sternocleidomastoid

Figure 10.11 Triangles of the Neck. The sternocleidomastoid muscle separates the anterior triangles from the posterior triangles.

Figure 10.11 Triangles of the Neck. The sternocleidomastoid muscle separates the anterior triangles from the posterior triangles.

Images Posterior Triangle Neck

Figure 10.12 Muscles of the Shoulder and Nuchal Regions.

Saladin: Anatomy & I 10. The Muscular System I Text I © The McGraw-Hill

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

Chapter 10 The Muscular System 345

verges to an insertion on the shoulder. The splenius35 capitis, which lies just deep to the trapezius on the neck, has oblique fascicles that diverge from the vertebral column toward the ears. It is nicknamed the "bandage muscle" because of the way it tightly binds deeper neck muscles. The semispinalis capitis is slightly deeper, and its fascicles travel vertically up the back of the neck to insert on the occipital bone. A complex array of smaller, deeper extensors are synergists of these prime movers; they extend the head, rotate it, or both.

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Responses

  • isaias
    What is the function of the sterno Saladin mastoid?
    7 years ago
  • marco
    What musle draws the head down?
    7 years ago

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