📝 STANDARD DESCRIPTION:
4 Pairs — INNERVATED BY MANDIBULAR DIVISION OF CN V (Trigeminal):
1. Masseter — large, on lateral cheek; closes jaw (most powerful in herbivores).
2. Temporalis — fan-shaped, in temporal fossa; closes jaw.
3. Medial Pterygoid — deep, medial side of mandible; closes jaw + side-to-side grinding.
4. Lateral Pterygoid — protracts and shifts the mandible side-to-side (key for grinding cud).
Digastricus — small; OPENS the jaw.
In herbivores (ox, horse): masseter + medial pterygoid form a sling around the mandible for the side-to-side grinding motion of cud-chewing — much larger than in carnivores.
📚 DETAILED DESCRIPTION:
MUSCLES OF MASTICATION — CN V (mandibular branch) TERRITORY
Mastication = chewing. The herbivore design includes a side-to-side grinding action made possible by enlarged pterygoid + masseter muscles forming a 'sling' around the mandible.
1. MASSETER:
• Origin: zygomatic arch + facial crest.
• Insertion: lateral surface of mandibular ramus.
• Action: Closes jaw (powerful in herbivores; thick & meaty).
2. TEMPORALIS:
• Origin: temporal fossa.
• Insertion: coronoid process of mandible.
• Action: Closes jaw. Larger in carnivores than herbivores.
3. MEDIAL PTERYGOID:
• Origin: pterygoid bone + palatine.
• Insertion: medial mandible.
• Action: Closes jaw + medial-lateral grinding.
4. LATERAL PTERYGOID:
• Origin: pterygoid crest.
• Insertion: condylar process of mandible.
• Action: Protracts mandible (forward) and side-to-side. Critical for cud grinding.
5. DIGASTRICUS:
• 2 bellies (rostral + caudal) joined by tendon at hyoid.
• Action: OPENS the jaw (depresses mandible).
• Also assists in swallowing.
FUNCTIONAL SYNTHESIS:
• Closing jaw: Masseter + Temporalis + Medial Pterygoid (massive in herbivores).
• Opening jaw: Digastricus (small, gravity does most work).
• Side-to-side grinding: Asymmetric action of medial + lateral pterygoids (one side at a time → 'one-sided chewing' in cud-chewers).
• Protraction: Lateral pterygoids together.
CN V (TRIGEMINAL) — MANDIBULAR DIVISION: Provides motor to all masticatory muscles. Sensory to the lower jaw, lower lip, lower teeth, anterior 2/3 tongue (general sensation).
// COMPARATIVE ANALYSIS
| OX/HORSE (HERBIVORES) |
Massive masseter + medial pterygoid — visible bulge on cheek. Side-to-side cud-grinding chew. |
| DOG/CAT (CARNIVORES) |
Massive temporalis for vertical bone-crushing bite. Pterygoids small (no grinding). |
| FOWL |
Lacks teeth → mastication done by gizzard. Jaw muscles small. |
CLINICAL RELEVANCE
Masseter Myositis (Dog): Auto-immune, German Shepherd predisposed; jaw cannot open / bilateral atrophy. Lockjaw (Tetanus): Tonic spasm of masticatory muscles → trismus. Trigeminal Neuritis: Idiopathic; bilateral mandibular paralysis ('dropped jaw' in dogs).
