Lab Manual - Anterior Triangle of the Neck

Learning Objectives:

Upon completion of this session, the student will be able to:

  1. Identify and list the attachments, innervation and action of the sternocleidomastoid, digastric and infrahyoid (strap) muscles. (explanation)
  2. Identify the boundaries of the anterior and posterior cervical triangles and their subdivisions. (explanation)
  3. Describe the cutaneous branches of the cervical plexus and identify their areas of distribution. (explanation)
  4. Identify the deep cervical fascia, its various component layers and the resulting compartmentalization of the neck. (explanation)
  5. Locate and describe the specific features of the thyroid gland. (explanation)
  6. Give the position of the parathyroid glands and consider the thyroid/parathyroid gland relationship in terms of vascular supply and surgical intervention. (explanation)
  7. Discuss the general features of endocrine system and specifically those of the thyroid and parathyroid glands. (explanation)
  8. Recognize and describe the contents of the carotid sheath and their relationships with surrounding structures (below the level of the hyoid bone). (explanation)
  9. Locate the vagus nerve and give its relationships to the fascia, vessels and viscera of the region. (explanation)

Readings and Modules:

Procedure:

1. Review the bony landmarks. (Play movie; View images: N 1, 2, 4, 8, 13, 15, 28, 29, 77A, 77D, 185, 419, TG 4-4, 7-01, 7-03, 7-04A, 7-05A, 7-06, 7-09, 7-12, 7-13, 7-25, 7-26)

On the cadaver, yourself, or partner, palpate the following: superior border of the clavicle; inferior border and angle of mandible, mental protuberance; jugular notch and mastoid process. Also, locate the hyoid bone, laryngeal prominence, thyroid cartilage, cricoid cartilage and trachea.

Skull structures

2. Remove any remaining skin in the neck region and expose the platysma. (Play movie; View images: N 26, TG 7-11, 7-16)

Using a sharp scalpel, remove the remaining skin of the neck below the mandible, being careful to preserve the delicate underlying muscle layer.

3. Reflect the platysma superiorly to expose the superficial veins and cutaneous nerves of the neck. (Play movie; View images: N 28, 31, 32A, 32B, 33, 35, 36, 74, 129, 135, 185, 256, TG 7-10, 7-11, 7-16A, 7-16B, 7-31, 7-87)

Clean and expose the platysma muscle. The platysma is a tissue-paper-thin subcutaneous muscle. Carefully reflect the muscle from the clavicle upward toward the mandible. The cervical branch of the facial nerve (CN VII) enters the muscle on its deep surface just posterior to the angle of the mandible; it will be seen better in a later dissection. Locate and trace the distribution of the great auricular, transverse cervical, lesser occipital (may not be able to see yet) and supraclavicular nerves (cutaneous branches of the cervical plexus) which emerge along the posterior border of the sternocleidomastoid muscle. Removal of the platysma exposes superficial veins of the neck in the subcutaneous tissue; however, these veins may enter and lie in the deep or investing fascia. Identify and trace the following veins: external jugular, anterior jugular, jugular venous arch and communicating veins. Are they bilaterally symmetrical in arrangement or size? Are they all present?

Read about the deep cervical fascia, its various component layers and the resulting compartmentalization of the neck, specifically: the superficial (investing) layer of deep cervical fascia, infrahyoid (muscular) fascia, visceral fascia (composed of pretracheal fascia and buccopharyngeal fascia), the carotid sheath and prevertebral fascia.

4. Identify the triangles of the neck and clean the strap muscles and reflect them to expose the ansa cervicalis and the thyroid gland. (Play movie; View images: N 28, 29, 31, 32, 127, 128, 129, TG 7-02A, 7-02B, 7-12, 7-13, 7-17)

Define the two major triangles of the neck (and their subdivisions): (1) the posterior triangle (omoclavicular or subclavian triangle and the occipital triangle); and (2) the anterior triangle (muscular, carotid, submandibular and submental triangles). If you have not already done so, on one side open the superficial layer of deep cervical fascia investing the sternocleidomastoid muscle. Expose the muscle throughout its length but do not destroy the superficial veins or nerves. Reflect it upward from its origins to the level of the hyoid bone.

Surface anatomy of the cervical triangles

The remainder of the dissection is carried out below the level of the hyoid bone. Dissect the infrahyoid (strap) muscles. The nerves and arteries enter the strap muscles on their lateral borders, so care must be taken while cleaning the muscles to preserve their nerves. It is best to clean them in the medial to lateral direction. First clean the superficial layer, composed of the sternohyoid and omohyoid muscles. Consider this layer as a musculofascial sheet, noting its continuity across the mid-line and attachments. Remove the fascia and examine individual muscles. Note the superior and inferior bellies of omohyoid, its intermediate tendon, and the attachments of bellies and tendon. Carefully clean and identify branches of the ansa cervicalis (C1-3) which innervate the muscles along their lateral borders (the origins of the ansa cervicalis will be seen later). Reflect the omohyoid laterally and the sternohyoid superiorly. Now, examine the sternothyroid and thyrohyoid muscles. Find the nerve to the sternothyroid. How does the innervation to the thyrohyoid muscle differ from the other strap muscles?

In order to expose the thyroid gland, reflect the sternocleidomastoid upward from its origins and reflect the strap muscles superiorly.

5. Open the carotid sheath to expose the associated vessels and nerves. (Play movie; View images: N 32, 34, 72, 74, 126, 226, TG 7-13, 7-14, 7-17, 7-74, 7-92)

With the sternocleidomastoid reflected, the carotid sheath becomes evident. Define it, consider its contents. Identify the ansa cervicalis and define its superior and inferior roots. The easiest way to find it is to carefully clean along the lateral margin of any of the strap muscles and identify the nerve entering it. Any nerve to a strap muscle can be followed back to the ansa. If possible trace the superior root of the ansa to the hypoglossal nerve and the inferior root to the cervical plexus (C2-3). Now open the carotid sheath below the level of the hyoid bone and identify the internal jugular vein, the common carotid artery, the beginning of the external carotid artery and the vagus nerve. You may find small cardiac nerves in the connective tissue of the sheath. These are branches of the sympathetic cervical chain or vagus nerve which end in the cardiac plexus.

Angiogram of neck vessels

Look for lymph nodes along the internal jugular vein. These belong to a chain of deep cervical nodes that drain into the venous circulation at the root of the neck. They will be mentioned again in more detail in the next dissection.

Ultrasound of a lymph node

6. Dissect the thyroid gland, its vascular supply, the recurrent laryngeal nerves and the parathyroid glands. (Play movie; View images: N 29, 31, 33, 69, 74A, 74B, 75, 76, 78A, 78B, 79, 232, TG 4-38, 7-11, 7-13, 7-14, 7-15, 7-18, 7-19, 7-21, 7-26B, 7-26C)

Examine the thyroid gland. Note the fascial investment and capsule. Identify lobes, isthmus, and if present, the pyramidal lobe (consider thyroid development). Note specific relations to overlying muscles (sternothyroid), to the trachea, cricoid and thyroid cartilages, and carotid sheath contents (common carotid artery, internal jugular vein, and vagus nerve).

Locate and trace the superior thyroid artery (a branch of the external carotid artery). Trace branches to the gland and larynx. Trace the superior thyroid vein to the internal jugular vein. Find the external branch of the superior laryngeal nerve to the cricothyroid muscle. This nerve may accompany the superior thyroid artery or approach the muscle from behind by lying on the inferior pharyngeal constrictor muscle, which it also innervates. Trauma to this nerve during thyroid surgery may result in changes in voice quality. Why?

Cut through the isthmus of the thyroid and reflect the lobes laterally from the trachea (is there a thyroidea ima artery present?). Note ligaments attaching the gland to the cricoid cartilage and trachea. Locate the middle and inferior thyroid veins draining the middle portion and inferior poles (of lobes). To what do they drain? Locate the inferior thyroid artery, looping superiorly from the thyrocervical trunk and then passing downward to approach the inferior pole of the gland from its posterior lateral surface.

Find the recurrent laryngeal nerve (of the vagus) in the tracheo-esophageal groove. This nerve is in close proximity to the inferior thyroid artery and is an important relationship in thyroid surgery. You may also find some deep cervical lymph nodes in this area.

Parathyroid glands may be noted posterior and inferior to the thyroid gland. They are usually attached to branches of the inferior thyroid artery, with superior parathyroid gland within a centimeter of where it crosses the recurrent laryngeal nerve. Inferior parathyroid is usually below the thyroid gland. They are difficult to distinguish in preserved material. Consider the importance of the thyroid/ parathyroid gland relationship in terms of vascular supply, innervation, and surgical intervention.