MASSETER
The masseter's broad origin and insertion allow it to apply chewing force over a broad area (2).  When one masseter is functioning, it's fibers shorten and help to shift the  mandible laterally to chew (1).  The masseter's form dictates its most efficient function, unilateral chewing.  When one masseter is contracting, the orientation of its fibers help to position the mandible laterally.  As the masseter elevates the mandible (2), it seats the condyle superiorly and anteriorly.  Masseters are the strongest muscles of mastication in herbivores (A) (3).  The greater the necessity for  side-to-side movements (as with a cow or horse, that is, those animals without prominent canine teeth and with large, flat molars), the greater the strength of the masseter.  In humans, the masseter is the second most efficient masticatory muscle, with a ratio of neurons to muscle fibers of 1:600.(B)

LATERAL AND MEDIAL PTERYGOIDS
At the base of the skull (4), a part of the sphenoid bone (which houses the sinuses) are the pterygoid plates. (Ptery- means wing-like, as in the winged dinosaur, Pterydactal).  The pterygoid muscles are attached to them.
The  lateral pterygoid muscles (5) originates on the lateral side of the pterygoid plate.  The superior (a) tenses during clenching, while the inferior (b) attaches on the neck of the condyle. The inferior lateral pterygoid's function follows its form. A unilateral contraction (6), will pulls the condyle in the direction of its fibers: anteriorly and medially. This excursive shift is a purposeless act.  If the teeth have been approximated following elevation of the mandible, contraction of the lateral pterygoid(s) is parafunctional.  The lateral pterygoid is responsible for the strain and load that can be exerted to the temporomandibular joint complex during parafunctional acts.  If  the teeth are not in contact, there is no resistance to lateral pterygoid contractions, therefore there is negligible strain to the TMJ.     The purpose of the  lateral pterygoids is to contract simultaneously, thereby advancing, and then opening the jaw.
     The medial pterygoid (7, in red) originates on the medical aspect of the pterygoid plate, and completes the medial half of a muscular sling around the mandible with the masseter.  It functions most efficiently when chewing is on the opposite side.  The medical pterygoid provide the slight lateral shift of the mandible during chewing.

TEMPORALIS  (better image of the temporalis)
    All of the muscles of mastication are innervated my the 3rd branch of the Trigeminal Nerve (8)
    The temporalis (9) resides within the temporal fossa, which is deepest at the anterior, and shallowest at the posterior. Therefore, the anterior temporalis is thickest and strongest and provides the force for the requirement of elevation against resistance (varying hardness of food and objects), while the posterior temporalis is thinnest and weakest.  The posterior fibers are horizontal, and fulfills the requirement of mandibular retrusion, which is only necessary after the mandible has been protruded.  (Simple and easy tasks don't require strong musculature).   The temporalis attaches to the coronoid process of the mandible, just underneath the cheekbone.  Unlike the masseter, whose attachment allows for a broad application, the temporalis' focused attachment (form) allows for a focused function:  to close the jaw until the teeth come into contact. The temporalis' evolutionary role has been to aid in grabbing prey with the canine teeth, which is why the temporalis' strength is greatest when the mandible is open ~20 mm (C). Once the teeth come together, the temporalis has built in resistance (the maxilla), and can contract at many times its necessary intensity.  In carnivores (10) (those animals with prominent canine teeth) and in humans, the temporalis is the strongest and most efficient muscle of mastication.  The ratio of neuron to muscle fiber is 1:900.

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