Musculus masseter (pars profunda)
The masseter is the principal muscle of mastication. It originates from the zygomatic arch and facial crest of the maxilla and inserts on the lateral surface of the ramus and angle of the mandible. It has superficial and deep portions. Its main actions are to close the jaw and produce the lateral grinding motion needed to process forage. It is innervated by the masseteric nerve (from the mandibular branch of the trigeminal nerve, CN V3). Horses chew thousands of times per day; the masseter is adapted for high force over a relatively short distance.
Origin: Zygomatic arch (deep surface)
Insertion: Lateral surface and angle of the mandible (deep fibers: masseteric fossa)
Action: Closes the jaw; lateral grinding motion for mastication
Innervation: Masseteric branch of the mandibular nerve (CN V3)
Masseter myopathy or pain can cause difficulty chewing, weight loss, or head-shyness. Dental problems that alter chewing patterns can overload the muscle. Trauma (for example from pulling back while tied) can also affect it.
## Overview Masseter myositis is inflammation of the cheek muscles causing painful swelling and difficulty opening the mouth (trismus). Horses drop feed and may lose weight. Causes include dental inf
## Overview Masseter hypertrophy is enlargement of the cheek muscles from overuse. It is most often linked to cribbing or abnormal chewing from dental problems. One or both sides may be affected. A d
Difficulty chewing, dropping feed, or sensitivity over the jaw should be checked. Regular dental care helps keep the masseter working normally.
## Overview Masseter myositis is inflammation of the cheek muscles causing painful swelling and difficulty opening the mouth (trismus). Horses drop feed and may lose weight. Causes include dental infection, temporomandibular joint inflammation, trauma, or rarely immune-mediated disease. ## Causes Extension of dental or joint infection, trauma, or primary muscle inflammation. ## Diagnosis Palpation of firm, painful masseters, observation of restricted jaw opening, and investigation of the teeth and temporomandibular joints. ## Treatment Anti-inflammatories, treatment of any dental or joint infection, and soft feed. Immune-mediated cases may need corticosteroids. ## Recovery and Prognosis Good when the underlying cause is identified and treated. ## Prevention Regular dental care and prompt treatment of oral infections. Signs include: swollen firm jaw muscles; reluctance to open the mouth; dropping feed; weight loss; pain on palpation of the cheeks
## Overview Masseter hypertrophy is enlargement of the cheek muscles from overuse. It is most often linked to cribbing or abnormal chewing from dental problems. One or both sides may be affected. A dental examination is recommended to identify contributing malocclusion. ## Causes Repetitive masseter contraction from cribbing, or compensatory chewing due to dental disease. ## Diagnosis Visual and palpable enlargement, history of cribbing, and oral examination. ## Treatment Address the underlying cause (cribbing management or dental correction). The muscle enlargement itself is not harmful but indicates a problem that should be investigated. ## Recovery and Prognosis The hypertrophy may reduce if the habit or dental issue is corrected, but some enlargement can persist. ## Prevention Regular dental care and management of stereotypic behaviors. Signs include: visibly enlarged jaw muscles on one or both sides; often associated with cribbing; dental examination recommended
Masseter myopathy or pain can cause difficulty chewing, weight loss, or head-shyness. Dental problems that alter chewing patterns can overload the muscle. Trauma (for example from pulling back while tied) can also affect it.
Difficulty chewing, dropping feed, or sensitivity over the jaw should be checked. Regular dental care helps keep the masseter working normally.
See the Masseter (Deep Head) on our interactive 3D horse model. Open 3D Model