60
The anterior triangle of the neck
inferior root of the ansa cervicalis arises from the ventral rami of the second and third cervical nerves behind the internal jugular vein. It curves for- wards, usually on the lateral surface of the vein, and passes down on the common carotid artery. Here it joins the superior root from the hypoglossal nerve.
The loop lies at the level of the lower part of the lar- ynx.
The ansa cervicalis consists of nerve fibres from the cervical ventral rami of C. 1, C. 2, and C. 3. It supplies the infrahyoid muscles—sternohyoid, sternothyroid, and omohyoid. The thyrohyoid and geniohyoid are supplied by C. 1 fibres which run along with the hypoglossal nerve [see Fig. 16.3]. (In this way, the hypoglossal nerve forms a secondary plexus with the first three cervical ventral rami.) Together they supply a ventral strip of muscle from the tongue to the sternum.
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Subdivisions of the anterior triangle
attachment of the sternohyoid. It ascends to the oblique line on the lateral surface of the thyroid cartilage [see Fig. 20.2], superficial to the attach- ment of the pre-tracheal fascia and the cricothy- roid muscle. It is anterior to the large vessels of the thorax at the root of the neck, and to the thyroid gland.
The thyrohyoid seems like the upward con- tinuation of the sternothyroid. It extends from the oblique line on the thyroid cartilage to the lower border of the greater horn of the hyoid bone [Fig.
6.5]. It is deep to the omohyoid and sternohyoid, and covers the entry of the internal laryngeal nerve to the larynx.
Nerve supply: the ventral rami of the first three cervical nerves supply the omohyoid, sternohyoid, and sternothyroid muscles through the ansa cervi- calis. The thyrohyoid nerve, C. 1 fibres which run with the hypoglossal, supplies the thyrohyoid. Ac- tions: the infrahyoid muscles move the larynx and hyoid bone in speech and swallowing. They can:
(1) depress the hyoid bone or, when acting with the suprahyoid muscles, fix the hyoid to form a stable base for the tongue; (2) draw the larynx to- wards the hyoid (thyrohyoid), as in the early phase of swallowing; and (3) depress the larynx, leaving the hyoid in position (sternothyroid), as in the last phase of swallowing.
See Clinical Application 6.1 for the practical im- plications of the anatomy discussed in this chapter.
Infrahyoid muscles
The infrahyoid muscles—sternohyoid, sternothy- roid, thyrohyoid, and omohyoid—form two layers of ribbon-like muscles extending from the sternum to the hyoid bone [Fig. 6.1]. The deeper muscle layer (sternothyroid and thyrohyoid) is interrupt- ed at the thyroid cartilage. The muscles lie on the trachea, thyroid gland, larynx, and thyrohyoid membrane. The thin fascia which encloses them is thickened around the intermediate tendon of the omohyoid and holds it down to the sternum and clavicle.
The sternohyoid lies superficial to the sterno- thyroid and thyrohyoid. It extends from the pos- terior surface of the manubrium and the medial end of the clavicle to the lower border of the hyoid bone adjacent to the midline [Fig. 6.1].
The superior belly of the omohyoid is attached to the inferior surface of the body and the greater horn of the hyoid, immediately lateral to the sterno- hyoid [Fig. 6.1]. Inferiorly, it continues with the inferior belly of the omohyoid at the intermediate tendon. The intermediate tendon lies on the inter- nal jugular vein under the sternocleidomastoid, at the level of the cricoid cartilage. The inferior belly of the omohyoid extends from the intermediate tendon to the scapular notch [see Fig. 4.2, Vol. 1].
The sternothyroid is shorter and wider, and is situated deep to the sternohyoid. It arises from the sternum and first costal cartilage below the
CLINICAL APPLICATION 6.1 Carotid artery stenosis A 75-year-old woman was brought to the hospital by rela-
tives, with a history of disorientation and memory loss. On examination, the woman was well oriented in time and space, and could recall events that occurred in the distant past, as well as events that occurred earlier that day. She, however, recounted that she had no clear recollection of events that happened 2 days ago—the day the relatives re- ported disorientation. The physician made a tentative di- agnosis of transient ischaemic attack and ordered a colour Doppler study.
Study question 1: what is a transient ischaemic attack?
(Answer: a sudden, focal loss of neurological function which disappears within 24 hours is a transient ischaemic attack.)
Study question 2: from your knowledge of anatomy, in- volvement of which vessel could have caused this ischaemia?
(Answer: internal carotid artery or vertebral artery. However, disorientation and memory loss is more likely to occur with involvement of the internal carotid artery.)
Colour Doppler of the neck vessels showed stenosis (nar- rowing) of the internal carotid artery in the neck by an athero- sclerotic plaque. A carotid endarterectomy was performed to remove the plaque, and the patient had an uneventful recovery.
Study question 3: what important structures lie close to the internal carotid artery in the neck? (Answer: cranial nerves IX, X, XI, and XII, and the laryngeal and pharyngeal branches of X and XI lie close to the internal carotid artery in the neck.
They are bound to the artery by the carotid sheath.)