Part 1: Head Removal, Skinning, and Superficial Structures

Locate the larynx and thyroid gland (previously identified in Chapter 3) in the ventral neck region of the specimen.

• Just CAUDAL to the larynx and thyroid gland, transect the trachea, and other soft tissues of the neck, to facilitate removal of the head (keep the thyroid gland and larynx with the head).

• Use a hand saw to cut through the neck, caudal to the larynx, and remove the head.

• Discard or store the body of the specimen (as directed by your lab instructors) and keep only the head out for dissection.

Remove the skin on both halves of the specimen head, but leave muscles and cutaneous structures in place wherever possible.

• Leave a rim of skin around the eyelids and the edge of the lips.

• Skin only the base of the ear – leave the skin on the rest of the pinna.

Identify the philtrum, which is the vertical midline groove separating right and left parts of the nose and upper lip.

Bring the skinned head to an instructor to be split in half with the band saw.

Using one half of your specimen head, on the lateral side, note some of the superficial muscles (if present after skinning).

• If still present and not reflected with the skin, note the layer of cutaneous muscle covering the lateral aspect of the neck and face; the main portion of this cutaneous muscle is called the platysma m. (See DG Fig. 5-21A)

  • • If needed, carefully reflect the platysma m. rostrally (toward the nose) to uncover deeper structures.

• Comment: There are numerous facial muscles that will not be dissected, but will be described here for your reference. These muscles will be available to view on demonstration specimens.

  • • Note that the orbicularis oris m. is the muscle seen curving around the edges of the mouth. (Platysma m. usually attaches to/integrates into the orbicularis oris m.)

    • Note that there is a thicker muscle, the buccinator m., that forms the foundation of the cheek; placing one finger inside the left cheek and pushing outward will help you see the placement of this muscle. (See DG Fig. 5-21B)

    • Note the levator nasolabialis m., which is a flat muscle coursing rostroventrally across the snout (lateral surface of the maxillary bone). This muscle is responsible for dilating the nostrils and raising the upper (superior) lip. (See DG Fig. 5-21B)

      Related Learning Objective

      • D7.1

       

Identify the superior & inferior palpebrae (the upper and lower eyelids), which surround the palpebral fissure (the opening between the eyelids). The two eyelids come together/join at the ‘corners’ of the eye on medial and lateral sides at the medial & lateral palpebral commissures respectively.

• Comments:

  • • You may also hear the medial & lateral palpebral commissures referred to as the medial & lateral canthus.

    • Note that the orbicularis oculi m. is the muscle seen surrounding the eyelid region. (It may or may not be present after removing the skin.) (See DG Fig. 21A, B, C, and D)

    • Adjacent to the lateral palpebral commissure, you may also see the small retractor anguli oculi lateralis m. extending caudally from the orbicularis oculi m.

Identify the conjunctival sac which is the ‘cavity’ formed by palpebral and bulbar conjunctiva.

• The palpebral conjunctiva is the mucous membrane lining the inner surface of each eyelid.

• The bulbar conjunctiva is the continuation of the mucous membrane lining onto the globe of the eye.

• The point where the palpebral conjunctiva reflects onto the globe to become the bulbar conjunctiva is known as the fornix (the angle formed between palpebral and bulbar conjunctiva).

• When you close the eyelids (bring the superior and inferior palpebrae together to close the palpebral fissure) you form the complete conjunctival sac made up of the mucous membrane lining.

• Dissection Note: Cutting the lateral palpebral commissure will make it easier to observe the palpebral and bulbar conjunctiva and identify the fornix.

At the medial commissure, attempt to observe the lacrimal caruncle and the dorsal & ventral lacrimal puncta. (Note that these may be difficult to see on preserved specimens and are easier to see in live animals; you could look at your own lacrimal puncta in a mirror later to visualize them.)

• The lacrimal puncta are the small openings that serve as the beginning of the lacrimal duct which then leads to the lacrimal sac – both of which will not be visible on gross specimens. The lacrimal sac then leads to the nasolacrimal duct that then empties into the rostral part of the nasal cavity.

• Attempt to identify the external opening of the nasolacrimal duct inside the nose. If you are not able to visualize this opening on your specimen, be sure to look at a demonstration specimen or ask an instructor.

  Related Learning Objective

  • D7.1

   

Near the medial palpebral commissure of the eye, identify the plica semilunaris (aka third eyelid, nictitans, or nictitating membrane).

• Comment: This ‘third eyelid’ is a structure composed of a T-shaped cartilage and a fold of palpebral conjunctiva that protrudes from the medial angle of the eye. It also has a superficial gland of the third eyelid associated with it, but you do not need to dissect it.

Examine the external ear and identify the auricle (pinna) and note that this part of the ear is made up of auricular cartilage. The auricular cartilage is funnel-shaped and forms the external ear canal. (See DG Fig. 5-31)

• Comment: The external ear canal also includes the small annular cartilage adjacent to the skull, but you need not dissect/identify it.

• On the lateral aspect of the pinna, identify the small pouch formed in the skin, which is called the marginal cutaneous sac.

Dorsally, identify the auricular muscles attaching to the ear. These muscles include a group of rostral auricular muscles that extend from the forehead toward the ear (auricular cartilage) and a group of caudal auricular muscles that arise from the median raphe of the neck and attach directly to the auricular cartilage. (See DG Fig. 5-21C)

• Transect the auricular muscles near the dorsal midline and reflect them laterally toward the ear.

• Comment: Note that you may see a small boot-shaped cartilaginous plate in the muscles rostral and medial to the external ear. This is called the scutiform cartilage and need not be dissected/identified.

  Related Learning Objective

  • D7.5

   

After reflecting the auricular muscles toward the ear, identify the temporalis m. (aka temporal m.) which is found just deep to the auricular muscles. The temporalis m. is a large muscle of mastication that fills the temporal fossa and inserts on the coronoid process of the mandible. The temporalis m. is involved with the action of closing the jaw. (See DG Figs 5-24, 5-26A, and 5-32)

• Dissection Note: The temporalis muscle fuses with another large muscle of mastication, the masseter muscle, underneath the zygomatic arch. This will be discussed next.

On the lateral aspect of the head, identify the masseter m. The masseter muscle is the large rounded muscle of the lateral cheek. The masseter covers the caudal portion of the mandible. The masseter m. is involved with the action of closing the jaw. (See DG Figs. 5-21D, 5-26A, and 5-32)