Part 1: Neck

If needed, reflect/remove any remaining skin from both sides of the ventral neck region and at least one side of the trunk.

• Dissection Note: In the neck, be sure you have reflected the skin from the ventral neck all the way up to the base of the head. For the trunk, be sure the skin has been reflected all the way to the dorsal midline on at least one side and that the lumbar region is exposed.

  Related Learning Objective

  • D3.1

   

Hypaxial muscles generally lie ventral to the transverse processes of the vertebrae; in this chapter, we will dissect some hypaxial muscles associated with the neck, thorax, and abdomen. Epaxial muscles generally lie dorsal to the transverse processes of the vertebrae and are associated with the vertebral column and ribs. Later in this chapter, we will dissect some of these muscles that are divided into three parallel muscle systems running longitudinally: iliocostalis system, longissimus system, and transversospinalis system. (See DG Fig. 2-81)

Begin in the ventral neck region and identify the sternocephalicus mm. extending from the sternum toward the head on both left and right sides. (See DG Fig. 2-12) As you begin to dissect the sternocephalicus mm., identify the external jugular veins on both left and right sides of the animal. The external jugular vein crosses the sternocephalicus m. superficially so it will be slightly obstructive as you dissect the muscle. (See DG Figs. 3-2 and 3-3)

• Comment: Note that sternocephalicus has two parts, mastoid and occipital, but you do not need to differentiate these parts for our dissection purposes. These two parts will be dissected on the demonstration animals for you to observe.

• Using the side of the animal that has the dissected forelimb, transect the sternocephalicus m. ~2cm cranial to its origin from the sternum.

  • • Optional: If you feel the external jugular vein is too obstructive, you may transect it at its approximate middle (if it is not already cut).

Identify the sternohyoideus and sternothyroideus mm. deep to the sternocephalicus m. These muscles extend from the sternum toward the head, running along the ventral midline of the neck. (Since these are midline structures, identify these muscles on both left and right sides.) (See DG Fig. 2-12)

• Dissection Note: The sternohyoideus mm. will be the more superficial muscles, running along the midline and coursing cranially into the head. The sternothyroideus m. will be seen coursing slightly laterally toward the thyroid cartilage of the larynx.

  Related Learning Objectives

  • D3.2

   

Spread the (left and right) sternohyoideus muscles apart (along the ventral midline) to identify the trachea, which is made up of C-shaped cartilaginous rings. (See DG Fig. 2-12) Note that this ventral midline approach to the trachea can be used clinically to perform a tracheotomy.

• Comment: You may see the left & right recurrent laryngeal nn. coursing cranially along the ventrolateral aspect of the trachea (on left and right sides, respectively). These nerves will be discussed again in Chapter 4, but they will be dissected out for you to observe on demonstration specimens.

• After exposing the trachea, move cranially and palpate the beginning of the trachea to identify the larynx; this is the ‘enlarged’ region near the base of the head. The larynx is made up of several cartilages which will be identified in a later lab.

Lateral, and slightly caudal, to the larynx, identify the thyroid gland. The thyroid gland consists of two separate lobes, a left and a right (in some they may be connected by a small isthmus ventrally). These lobes of the thyroid gland are found along the first five rings of the trachea. (The lobes of the thyroid are somewhat small, elongated ovals and are usually dark in color.) (See DG Fig. 3-15, A & B)

• Dissection Note: There are two parathyroid glands associated with each thyroid lobe, the external parathyroid and the internal parathyroid. These parathyroid glands will appear as lighter colored, small, spherical bodies. Sometimes the external parathyroid glands can be seen grossly in the fascia at the cranial pole of each lobe of the thyroid gland.

  • • Attempt to identify at least one external parathyroid gland. (You need not look for the internal parathyroid glands.) If you cannot find an external parathyroid gland in your animal, be sure to look at the demonstration animals.

Lateral to the trachea, on both left and right sides of the neck, identify the carotid sheath; this is the deep cervical fascia surrounding the common carotid artery, vagosympathetic nerve trunk, internal jugular vein, and tracheal lymphatic duct.

• Using blunt dissection, separate and identify the common carotid a. and vagosympathetic nerve trunk (running alongside the common carotid a.) within the carotid sheath on the left and right sides of the animal. (See DG Fig. 3-1)

• Dissection Note: In addition to the common carotid a. and vagosympathetic nerve trunk, you may also see an internal jugular vein and tracheal lymphatic duct within the carotid sheath.

On the left or right side, identify the medial retropharyngeal lymph node which lies at the level of the larynx, ventrolateral to the carotid sheath, and deep to the sternocephalicus m. (See DG Figs. 2-12 and 5-44)

• Dissection Note: This may be easiest to do on the side which has the transected sternocephalicus m.

Running caudally, along the left side of the trachea, identify the esophagus. The esophagus is the simple tube that carries food from the oral cavity/pharynx to the stomach.

Along the lateral and ventral surfaces of the cervical vertebrae (neck region), identify the longus mm. (See DG Figs. 2-82, A, and 3-20)

• Comment: There are two muscles included in this group, the longus capitis m. and the longus colli m., but you need not differentiate between the two; they will be dissected on the demonstration animals for you to observe. Note that the longus capitis is lateral to the longus colli, with longus colli being more ventrally located.

• Dissection Note: Reflect/push the trachea and esophagus to one side as needed to facilitate viewing of the longus mm.

If possible, note the median raphe of the neck which is a fibrous septum between the left and right sides of epaxial muscles in the neck (along the dorsal midline).

Re-identify the superficial cervical lymph nodes (aka prescapular lymph nodes) deep to the omotransversarius m., just cranial to the scapula/shoulder region. (See DG Fig. 3-26) These were dissected in Chapter 1 and should be visible on one side of your specimen.

• Dissection Note: If you would like to locate the superficial cervical lnn. on the other side of your specimen, you may need to cut through the brachiocephalicus m. on that side.

Between the cranial edge of the trapezius m. and the cleidocephalicus mm. (previously dissected in Chapter 1), identify the accessory (eleventh) cranial nerve. This is a large nerve that runs caudally and is found deep to the occipital part of the sternocephalicus and the cervical part of the cleidocephalicus mm. It often runs along the dorsal border of the omotransversarius m. on its way to the trapezius m. (See DG Figs. 3-1, 3-2, 3-3 and 5-44)