4 Part 3: Lateral Head Dissection

IMPORTANT NOTE: For this part of the chapter, the Guide directions will refer to either “ALL specimens” (meaning dissect pony and calf specimens in the same way) or to one species in particular, i.e., “ALL (PONY) specimens” or “ALL (CALF) specimens.” This added instructional note is needed because the dissection may differ slightly according to species.

1. ALL specimens: After the head has been split, continue with the dissection on the lateral side of both halves of the head (right and left) simultaneously.
   • Dissection Note: As you proceed with the dissection, some structures may be better seen on one half of the head versus the other. Choose the better half showing the indicated structure.
2. ALL specimens: Identify the large masseter muscle on the lateral side of the mandible. At this point, do not clear away any fascia or cut any structures – simply identify the masseter m. and note the location.
3. ALL (PONY) specimens: Observe/identify the large nerves crossing the lateral surface of the masseter m. These are the dorsal and ventral buccal branches of the facial n. (Fig. 7-3A)
   • These nerves are motor to muscles of facial expression [the mandibular n. of the trigeminal (V) n. supplies the muscles of mastication].
   • Identify the dorsal and ventral buccal branches of the facial n.
   • Note that this arrangement of nerve branches is slightly different than in the calf. In the calf, only the dorsal buccal branch crosses the lateral surface of the masseter m., while in the pony both the dorsal and ventral buccal branches cross the lateral surface of the masseter m.
4. ALL (CALF) specimens: Observe/identify the singular large nerve crossing the lateral surface of the masseter m. This is the dorsal buccal branch of the facial n. Next, find/identify the ventral buccal branch of the facial n. which courses distally around the ventral aspect of the masseter m. (Fig. 7-3B).
   • These nerves are motor to muscles of facial expression [the mandibular n. of the trigeminal (V) n. supplies the muscles of mastication].
   • Identify the dorsal and ventral buccal branches of the facial n. as described.
   • Note that this arrangement of nerve branches is slightly different than in the pony. In the calf, only the dorsal buccal branch crosses the lateral surface of the masseter m., while in the pony both the dorsal and ventral buccal branches cross the lateral surface of the masseter m.
5. ALL specimens: In the lower lip, identify the mental nerve emerging from the mental foramen of the mandible (Fig. 7-3).
   • This nerve supplies the lower lip and is large due to the importance of the lips in prehension of food. Note that the mental nerve is a branch of the inferior alveolar nerve (which supplies innervation to the mandibular teeth).
   • Use the spreading technique and/or the pointed end of your small iris scissors to help clear away fascia in the region of the mental foramen to identify the mental nerve emerging from the foramen.
6. ALL (PONY) specimens: Identify the parotid salivary gland and parotid duct. (Fig. 7-3)
   • (Re-) Identify the parotid salivary gland surrounding the base of the ear and extending toward the mandible.
     • • Note that the parotid salivary gland is large and covers the region of Viborg’s triangle.
       • Terminology Note: The name parotid = para + otic because it is adjacent to the cartilaginous ear canal, and otic refers to the ear.
   • Identify the parotid duct extending from the ventral surface of the parotid salivary gland. Trace the parotid duct as it curves down around the ventral edge of the mandible.
     • • Comparative Species Note: Recall that in the dog, the parotid duct runs across the lateral aspect of the masseter m. so its path is more direct than it is in the horse.
7. ALL (CALF) specimens: Identify the parotid salivary gland and parotid duct.
   • Identify the parotid salivary gland near the base of the ear and extending ventrally along the caudal aspect of the mandible.
     • • In the calf, the parotid salivary gland is more long and narrow in shape as compared to the pony. 
       • Terminology Note: The name parotid = para + otic because it is adjacent to the cartilaginous ear canal, and otic refers to the ear.
   • Identify the parotid duct extending from the ventral surface of the parotid salivary gland. Trace the parotid duct as it curves down around the ventral edge of the mandible.
     • • Comparative Species Note: Recall that in the dog, the parotid duct runs across the lateral aspect of the masseter m. so its path is more direct than it is in the horse.
8. ALL (PONY) specimens: Identify the maxillary vein, linguofacial vein, facial vein and facial artery. 
   • Dissection Note: The linguofacial and maxillary veins were previously identified where they joined together to create the external jugular vein in the neck.
   • (Re-) Identify the maxillary vein, which is partially hidden from view by the parotid salivary gland. Reflect the parotid salivary gland as needed to see the maxillary v.
     • • The maxillary v. drains the more dorsal structures of the head and then runs ventrally to join the linguofacial vein to form the external jugular vein.
   • (Re-) Identify the linguofacial vein which drains the tongue and other ventral structures of the head. It then joins the maxillary v. to form the external jugular vein.
     • • Identify the facial vein at the point where it curves around the ventral edge of the mandible, running alongside the parotid duct that was previously identified.
   • Identify the facial artery (Fig.7-3) that is also found curving around the ventral edge of the mandible, alongside the facial v.
     • • Observe that the facial vein and artery are closely associated with the parotid duct, which follows the same ventral path (TVA, 529). 
       • The facial artery holds clinical significance, as it is an area where you can palpate a pulse on a live animal.
9. ALL (PONY) specimens: On one half of the head, attempt to identify the venous sinuses and the transverse facial artery by reflecting and removing portions of the masseter m. 
   • Choose ONE HALF of the head (either right or left) and attempt to identify the large venous sinuses and transverse facial artery that lie ventral to the facial crest of the horse. (TVA, 507).
     • • Beginning ventrally, return to the facial vein and trace it as it curves up around the masseter m.
       • Look for venous branches that connect with the facial vein, coming from underneath the masseter m.
       • Very carefully, reflect and remove portions of the masseter muscle, peeling the muscle away from the facial vein.
   • After removing portions of the masseter, attempt to identify where the buccal, deep facial and transverse veins (with dilations known as the venous sinuses) join the facial vein. (Fig. 7-6)
   • Carefully cut the attachment of the masseter m. to the facial crest (as close to the bone as possible) from the facial tuberosity to a point caudal to the lateral canthus of the eye.
   • Carefully remove a large piece of the masseter m. as shown in Figure 7-7, being very cautious so as to preserve the buccal nerves (if possible).
   • Just ventral to the facial crest, identify the transverse facial artery. Note that the transverse facial vein satellites this artery.
     • • Note that the transverse facial artery is palpable in a live horse and can be used for taking a pulse.
       • Also note that the corresponding transverse facial vein that satellites the artery is an alternative location for blood collection (venipuncture) in the horse.
10. ALL (CALF) specimens: Identify the maxillary vein, linguofacial vein, facial vein and facial artery.

• • Identify the maxillary vein, which is partially hidden from view by the parotid salivary gland. Reflect the parotid salivary gland as needed to see the maxillary v.
    • • The maxillary v. drains the more dorsal structures of the head and then runs ventrally to join the linguofacial vein to form the external jugular vein.
  • Identify the linguofacial vein which drains the tongue and other ventral structures of the head. It then joins the maxillary v. to form the external jugular vein.
  • Identify the facial vein at the point where it curves around the ventral edge of the mandible, running alongside the parotid duct that was previously identified.
  • Identify the facial artery (Fig.7-3) that is also found curving around the ventral edge of the mandible, alongside the facial v.
    • • Observe that the facial vein and artery are closely associated with the parotid duct, which follows the same ventral path (TVA, 529). 
      • The facial artery holds clinical significance, as it is an area where you can palpate a pulse on a live animal.
  • Note that in the calf specimen, the arrangement of the facial vein and artery, as well as the parotid duct, is similar to the arrangement in the pony.

Figure 7-6. Venous sinuses in the horse. 1, Facial v.; 2, transverse facial vein; 3, deep facial vein; 4, buccal vein. Dilations of 2, 3 and 4 form venous sinuses. (Modified from Walesby et. al. The transverse facial venous sinus: an alternative location for blood collection in the horse. Equine Veterinary Education. 19(2). 100-102, 2007.)

Figure 7-7. Image with masseter muscle cut to reveal venous sinuses. (TVA Figure 18-9)

11. ALL (PONY) specimens: Identify the post-orbital fat pad and the temporalis m.

• • Caudal to the eye, identify the post-orbital fat pad overlying the temporalis muscle (Fig.
    7-3/14).
  • Reflect the thin auricular muscles to expose and identify the temporalis muscle, which is less
    developed in the horse than in the dog. Much of the temporalis muscle inserts on the coronoid
    process of the mandible.
    • Reflect (or remove) the post-orbital fat pad and other soft tissue to better expose the
      temporalis muscle on the cranial edge of the ramus of the mandible.

12. ALL (CALF) specimens: Identify the temporalis m.

• • Reflect the thin auricular muscles to expose and identify the temporalis muscle which fills
    the temporal fossa ventral to the temporal line.
    • Note that in the calf, the temporalis m. is located entirely on the lateral side (it does not extend dorsally as it does in the pony).