Musculus digastricus
The digastricus connects the base of the skull to the mandible. It has two bellies separated by an intermediate tendon. The caudal belly originates from the jugular process of the occipital bone and mastoid process; the rostral belly inserts on the ventral border of the mandible. Its primary action is to depress the mandible (open the mouth). Innervation is split: the rostral belly by the mandibular branch of the trigeminal nerve (CN V3), the caudal belly by the facial nerve (CN VII). In horses the masseter and temporalis are extremely powerful closers, so the digastricus must generate significant force to open the mouth. It is relevant to temporomandibular joint function and to bit acceptance.
Origin: Jugular process of the occipital bone and mastoid process of the temporal bone
Insertion: Ventral border of the mandible (caudal to the chin)
Action: Opens the mouth (depresses the mandible); important for TMJ function and bit acceptance
Innervation: Mandibular branch of trigeminal nerve (CN V3) and facial nerve (CN VII)
Rarely injured in isolation. TMJ pain or dental disease can alter its function. The intermediate tendon relationship with the stylohyoideus is an anatomic detail of surgical interest.
## Overview Difficulty opening the mouth can result from pain or mechanical dysfunction of the jaw, temporomandibular joint, or muscles of mastication (including the digastricus). Affected horses str
Progressive fusion and thickening of the temporohyoid joint (where the stylohyoid articulates with the temporal bone). Can lead to fracture of the petrous temporal bone, damaging cranial nerves VII an
## Overview Temporomandibular joint (TMJ) dysfunction is inflammation or misalignment of the jaw joint. It causes reluctance to chew, quidding, head tossing, resistance to the bit, tenderness over th
Reluctance to open the mouth, drop feed, or accept the bit may involve the jaw muscles or TMJ; have a veterinarian examine the mouth and jaw.
## Overview Difficulty opening the mouth can result from pain or mechanical dysfunction of the jaw, temporomandibular joint, or muscles of mastication (including the digastricus). Affected horses struggle to grasp feed, quid, or accept the bit. Causes include dental disease, temporomandibular joint osteoarthritis, mandibular fracture, or muscle injury. ## Causes Dental pain, temporomandibular joint disease, fracture, or soft-tissue trauma. ## Diagnosis Oral examination, observation of eating, radiographs or computed tomography of the skull, and assessment of jaw mobility. ## Treatment Directed at the underlying cause: dental correction, anti-inflammatories, or fracture repair. Most cases improve once pain is controlled. ## Recovery and Prognosis Good when the primary problem is treatable. ## Prevention Regular dental care and prompt attention to jaw trauma. Signs include: difficulty grasping feed; reluctance to open the mouth for the bit; quidding; pain when the mouth is opened manually
Progressive fusion and thickening of the temporohyoid joint (where the stylohyoid articulates with the temporal bone). Can lead to fracture of the petrous temporal bone, damaging cranial nerves VII and VIII. A serious, potentially life-threatening condition. Signs include: Sudden onset facial nerve paralysis (drooping ear, lip, eyelid); head tilt; difficulty swallowing; loss of balance (vestibular signs); may have history of ear discharge or previous mild episodes; can cause sudden death from basilar skull fracture; diagnosed on endoscopy of the guttural pouches or CT.
## Overview Temporomandibular joint (TMJ) dysfunction is inflammation or misalignment of the jaw joint. It causes reluctance to chew, quidding, head tossing, resistance to the bit, tenderness over the temporal fossa, and visible muscle wasting above the eye. Dental problems or poor bitting often contribute. ## Causes Dental disease, trauma, or bit-related pressure. ## Diagnosis Palpation, dental examination, and sometimes radiographs or computed tomography of the TMJ. ## Treatment Correct dental issues, adjust bitting, anti-inflammatories, and physiotherapy. Injections are used in some cases. ## Recovery and Prognosis Good when the underlying cause is addressed. ## Prevention Regular dental care and appropriate bit fit. Signs include: reluctance to chew; quidding; head tossing; resistance to the bit; tenderness over the temporal fossa; muscle wasting above the eye
Rarely injured in isolation. TMJ pain or dental disease can alter its function. The intermediate tendon relationship with the stylohyoideus is an anatomic detail of surgical interest.
Reluctance to open the mouth, drop feed, or accept the bit may involve the jaw muscles or TMJ; have a veterinarian examine the mouth and jaw.
See the Digastricus on our interactive 3D horse model. Open 3D Model