Overview
In the horse it is very large and forms the rounded surface of the cheek. It is made up of several layers separated by tendinous sheets, grouped into a superficial and a deep part. The equine mandible is narrower than the maxilla (anisognathia), so the cheek teeth meet on one side at a time. Chewing is a side-to-side grinding movement in which the masseter plays the main role.
Origin
Facial crest (crista facialis) of the maxilla and zygomatic arch.
Insertion
Lateral surface of the mandibular ramus, as far as its ventral border and the angle.
Action
Raises the mandible and closes the mouth. Contraction on one side shifts the mandible towards that side and produces the grinding stroke.
Innervation
Masseteric nerve, a branch of the mandibular nerve (cranial nerve V).
Clinical relevance
- Because of anisognathia and the lateral stroke, wear leaves sharp enamel points on the buccal edge of the upper cheek teeth and the lingual edge of the lower ones, which injure the cheek and tongue. They are rasped ("floated") at routine dental checks.
- The facial artery, with the facial vein and parotid duct, crosses the ventral border of the mandible at the rostral edge of the masseter. This is the usual place to take the pulse in the horse.
- Atrophy limited to one side suggests a lesion of the mandibular nerve on that side.