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

Learning Objectives and Explanations:

1. Identify and list the attachments, innervation and action of the sternocleidomastoid muscle and infrahyoid (strap) muscles. (N 27,28,29,31, TG 7-12, 7-13, 7-16)

The names of these muscles are helpful in identifying their locations. For example: "omo" means shoulder, and omohyoid goes from the shoulder to the hyoid bone.

Muscles	Origin	Insertion	Innervation	Action
Omohyoid	Inferior belly: upper border of scapula medial to scapular notch Superior belly: intermediate tendon	Superior belly: lower border of hyoid lateral to sternohyoid insertion Inferior belly: intermediate tendon	Ansa cervicalis	Depresses/stabilizes hyoid bone
Sternohyoid	Posterior surfaces of manubrium and sternal end of clavicle	Lower border of hyoid bone, medial to omohyoid insertion	Ansa cervicalis	Depresses/stabilizes hyoid bone
Sternothyroid	Posterior surface of manubrium below sternohyoid origin	Oblique line of thyroid cartilage	Ansa cervicalis	Depresses/stabilizes thyroid cartilage
Thyrohyoid	Oblique line of thyroid cartilage	Lower border of hyoid bone	Ansa cervicalis (via fibers running with hypoglossal nerve that leave XII distal to the superior limb of ansa)	Elevates larynx; Depresses/stabilizes hyoid bone
Sternocleidomastoid	Sternal head: anterior surface of manubrium; Clavicular head: medial one-third of clavicle	Mastoid process and lateral aspect of superior nuchal line	Spinal accessory nerve (XI), with sensory supply from C2 & C3 (for proprioception)	Draws mastoid process down to same side; turns chin up toward opposite side

2. Identify the boundaries of the anterior and posterior cervical triangles and their subdivisions. (N 28, TG 7-02A, 7-02B)

• Anterior cervical triangle
  • boundaries:
    • medial: midline
    • lateral: anterior border of the sternocleidomastoid muscle
    • superior: lower border of the mandible
  • subdivisions:
    • muscular triangle
      • medial: midline
      • superolateral: superior belly of the omohyoid muscle
      • inferolateral: sternocleidomastoid muscle
    • submandibular triangle
      
      • anterior: anterior belly of the digastric muscle (this muscle is not in this lab, so for now it is sufficient to know its location as a reference for triangle borders).
      • posterior: posterior belly of the digastric muscle
      • superior: lower border of the mandible
    • submental triangle
      
      • lateral: anterior belly of digastric muscle
      • medial: midline
      • inferior: hyoid bone
    • carotid triangle
      
      • lateral: sternocleidomastoid muscle
      • superior: posterior belly of the digastric muscle
      • anterior: superior belly of the omohyoid muscle
• Posterior cervical triangle
  
  • boundaries:
    • anterior: sternocleidomastoid muscle
    • posterior: trapezius muscle
    • inferior: clavicle
  • subdivisions:
    • subclavian/omoclavicular triangle
      • superior: inferior belly of omohyoid muscle
      • anterior: sternocleidomastoid muscle
      • inferior: clavicle
    • occipital triangle
      
      • anterior: sternocleidomastoid muscle
      • posterior: trapezius muscle
      • inferior: omohyoid muscle

3. Describe the cutaneous branches of the cervical plexus and identify their areas of distribution. (W & B 191 and Fig 190; N 24,31,32,129,178, TG 7-11, 7-16A,7-16B, 7-13)

The cutaneous branches of the cervical plexus include the lesser occipital, the great auricular, the transverse cervical, and the supraclavicular nerves. They emerge along the lateral border of the sternocleidomastoid muscle in the order of lesser occipital, great auricular, transverse cervical, and supraclavicular (superior to inferior). (Note: Netter 31 incorrectly indicates that the great auricular is superior to the lesser occipital. The lesser occipital comes from C2 and the great auricular comes from branches of C2 and C3.)

Nerve	Source	Location	Area of distribution
Lesser occipital	C2	Ascends in the neck along the posterior border of the sternocleidomastoid muscle; pierces the cervical fascia near the muscle and divides into branches	Skin and subcutaneous tissue behind the ear
Great auricular	C2, C3	Appears at the lateral border of the sternocleidomastoid muscle just below the lesser occipital nerve and goes toward the auricle and the angle of the mandible	Skin of the ear and below the ear
Transverse cervical	C2, C3	Appears below the great auricular nerve at the lateral border of the sternocleidomastoid muscle; crosses the muscle horizontally to reach the anterior triangle deep to the platysma muscle and the external jugular vein	Skin of the neck anteriorly
Supraclavicular	C3, C4	Emerges below the transverse cervical nerve at the lateral edge of the sternocleidomastoid muscle; descends through the inferior part of the posterior triangle and divides into three branches that pierce the platysma near the clavicle	Skin of the root of the neck; upper chest and upper shoulder anteriorly

4. Identify the deep cervical fascia, its various component layers and the resulting compartmentalization of the neck. (W & B 191-195 and Fig 3-6, N 35, TG 7-10,7-11)

Deep cervical fascia components:

• superficial (investing) layer: (Note: this is the superficial layer of the deep fascia, which is different from superficial fascia) extends between the trapezius and the sternocleidomastoid muscles in the posterior triangle and between the paired sternocleidomastoid muscles in the anterior triangle. It surrounds all the deeper parts of the neck and splits to enclose the trapezius and sternocleidomastoid muscles. It also splits above the manubrium to create the suprasternal space (the jugular venous arch connecting the anterior jugular veins goes through this space).
• infrahyoid (muscular) fascia: has a superficial layer which encloses the sternohyoid and omohyoid muscles and a deeper lamina which encloses the sternothyroid and thyrohyoid muscles. Both layers create a semi-circle on the anterior side of the neck and end superiorly at the hyoid bone. Inferiorly they pass behind the sternum onto the left brachiocephalic vein and the pericardium.
• visceral fascia: encloses the pharynx, esophagus, larynx, trachea, thyroid, and parathyroid glands. It has two components: pretracheal fascia anteriorly and the buccopharyngeal fascia posteriorly.
• carotid sheath: encloses the internal and common carotid arteries, internal jugular vein, and vagus nerve.
• prevertebral fascia: forms a complete enclosure of the cervical vertebrae and their associated longitudinal musculature, vessels, and nerves. There are also some interfascial spaces. The most important is the retropharyngeal space. This space is between the buccopharyngeal and prevertebral fascias and accommodates the movements of the pharynx and associated parts during swallowing.

5. Locate and describe the specific features of the thyroid gland. (W & B 197-8, N 74, TG 7-13, 7-14)

The thyroid gland is H-shaped with lateral lobes making up the vertical lines and the isthmus making up the middle bar. There also sometimes is a pyramidal lobe which extends upward from the isthmus or from the junction of the isthmus and one of the lateral lobes. The thyroid gland arches over the trachea and is bound posterolaterally by the carotid sheath contents and anterolaterally by the sternothyroid muscles. The upper parts of the lateral lobes are molded against the cricoid and thyroid cartilages.

6. Give the position of the parathyroid glands and consider the thyroid/parathyroid gland relationship in terms of vascular supply and surgical intervention. (W & B 201; N 74, 75, TG 7-14)

The parathyroid glands are usually four (but may be two to six) small glands lying posterior (superior parathyroids) or inferior (inferior parathyroids) to the thyroid gland. Blood supply comes from branches of the inferior or superior thyroid arteries, or from the longitudinal anastomosis between these vessels. Venous drainage flows into the thyroid plexus of veins. Inadvertent removal or damage of the parathyroid glands can occur in surgery on the thyroid gland because of variable positions of the parathyroid glands. If the parathyroid glands atrophy or are all removed during surgery, the patient suffers from tetany, severe convulsive muscle spasms resulting from a fall in serum calcium levels.

7. Discuss the general features of endocrine system and specifically those of the thyroid and parathyroid glands.

You will get much more on this in Physiology and Histology, but for now this summary is sufficient.

Thyroid gland

• composed of two populations of cells: follicles and the surrounding parafollicular cells
• produces two hormones
  • calcitonin: lowers calcium and phosphate levels in the blood and is regulated by serum calcium levels
  • thyroxine: increases basal metabolic rate. Also feeds back to decrease TSH and TRH synthesis and release from the pituitary and hypothalamus.

Parathyroid gland

• produces parathyroid hormone - increases blood calcium levels (opposite of calcitonin) and lowers phosphate levels (same as calcitonin). It is regulated by serum calcium levels

8. Recognize and describe the contents of the carotid sheath and their relationships with surrounding structures (below the level of the hyoid bone). (N 32, 33, TG 7-17, 7-18, 7-13)

The carotid sheath contains the internal and common carotid arteries, the internal jugular vein, and the vagus nerve. Through the middle neck levels, the superior root of the ansa cervicalis lies in the sheath anteriorly. The carotid sheath is posterolateral to the thyroid gland and anterior to the prevertebral fascia. The sympathetic trunk lies behind the medial portion of the sheath. (N 75, TG 7-14,7-15)

9. Locate the vagus nerve and give its relationships to the fascia, vessels and viscera of the region. (W & B 201-3; N 32, 33, TG 7-13, 7-14)

The vagus nerve runs between and posterior to the common carotid artery and the internal jugular vein in the carotid sheath (the artery is medial, the vein lateral). The rest of the question is answered above.

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Questions and Answers:

1. Are the external jugular, anterior jugular, jugular venous arch, and communicating veins bilaterally symmetrical in arrangement or size? Are they all present?

These veins are quite variable and are often asymmetrical. It will be unlikely that you will find all of them in one specimen. (N 31, 256, TG 7-11)

2. How does the innervation to the thyrohyoid muscle differ from the other strap muscles?

Thyrohyoid innervation comes from C1 & C2 via the hypoglossal nerve, arising after the superior root of the ansa cervicalis leaves the hypoglossal nerve. (N 32, 135, TG 7-13, 7-18)

3. Trauma to the external branch of the superior laryngeal nerve to the cricothyroid muscle during thyroid surgery may result in changes in voice quality. Why?

Because cricothyroid muscle tenses the vocal cords by pulling the thyroid and cricoid cartilages closer together anteriorly. (N 74,78, 126, TG 7-26, 7-18)

4. Is there a thyroidea ima artery present?

Only if your cadaver is in the lucky 10% containing this artery.

5. To what vessels do the middle and inferior thyroid veins drain?

Middle thyroid: internal jugular vein
Inferior thyroid: left and right brachiocephalic veins (N 74, TG 7-13)