The muscles of mastication form an independent group, four in number, occupying the back part of the side of the face, and the temporal and zygomatic fossae, and consisting of the masseter, the temporal, with the external and internal pterygoid muscles. The temporal muscle is covered by a strong membrane, the temporal fascia, which, arising from the temporal ridge, is attached below to the upper border of the zygoma, after first dividing into two laminae which go to the outer and inner aspects of this border, and contain between them a small quantity of fat. From the zygoma downwards, the masseteric fascia is continued to the posterior and inferior borders of the ramus of the mandible, enveloping the masseter muscle. Closely connected with this is the parotid fascia which envelops this gland, extending backwards from the masseteric fascia to that part of the deep cervical fascia which covers the upper portion of the sterno-mastoid muscle ; covering also the deep surface of the gland, and giving off a process called the stylo-maxillary ligament, which, running from the styloid process to the angle of the mandible, separates the parotid and submaxillary glands.

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Masseter muscle

The masseter  - named from the Greek word to chew - is a strong quadrate sheet, consisting of two layers.

Origin

The superficial layer, from the lower border of the malar bone, and the lower border of the anterior two-thirds of the zygomatic arch ; the deep layer, from I the lower border of the posterior third of the zygomatic arch, and the whole of its ! inner sm'face.

Insertion

Superficial layer, into the lower half of the outer surface of the j ramus of the mandible ; the deep layer, partly with the superficial layer, and partly into the upper half of the outer surface of the ramus of the mandible.

Structure

The origin and insertion are by tendinous bands intermingled in multipenniform fashion with fleshy fibres. The fibres of the superficial sheet are 'directed obliquely downwards and backwards ; those of the deep sheet almost Svertically dow^nwards, and they are much shorter than the superficial fibres. The two sheets blend closely in front, but are separate behind, where the muscle forms ,a sort of pocket closed above at the origin and below at the insertion, as well as in I front, but open behind.

Nerve-supply

The masseteric branch of the motor division of the inferior imaxillary nerve, which enters the deep surface of the muscle just below the zygoma.

Action

To close the jaw, and by its superficial sheet to draw it slightly forwards. In closing the jaw it acts with less mechanical disadvantage than is jusual with muscles. When the pressure to be overcome is exerted upon the back teeth, the arm of the lever upon which the power acts is almost as long as that which intervenes between these teeth and the fulcrum. This fulcrum is not at the temporo-maxillary joint, but at a point below the neck of the mandible, cor- responding very nearly to the lower attachment of the internal lateral ligament. Moreover, the resultant force of the muscle, acting as it does upwards and forwards, is perpendicular to the lever, which may be roughly described as a bar extending downwards and forwards from the neck of the mandible to the point of the chin.

Relations.

Superficially, the parotid gland and its duct, the platysma tnyoides, the risorius and the masseteric fascia, the transverse facial vessels, the facial vein, the upper and lower divisions of the facial nerve ; deeply, the buccal :at-pad which separates it from the buccinator, a small part of the temporal muscle, md the ramus of the mandible.

The temporal muscle

The temporal muscle - named from its attachment to the temple (=tempus) - is a thick, somewhat triangular sheet ; more correctly it may be described as forming bhe quadrant of a circle.

Origin

1. The whole of the temporal fossa, from the lower ridge of that lame to the pterygoid ridge, with the exception of a small part close to the outer ivall of the orbit, which is occupied by fat;
2. the inner surface of the temporal fascia down to its lower attachment to the zygomatic process, from the inner surface of which some of its fibres also sometimes arise.

Insertion

The point, posterior border, and the whole of the inner surface of 'he coronoid process of the mandible, down to the last molar tooth.

Structure

The bones of the temporal fossa and the temporal fascia form a oouch, open downwards, from which the fleshy fibres of the muscle converge, the middle fibres running downwards, the anterior downwards and backwards, the posterior almost transversely forwards, to be inserted below upon both faces of a flat tendon which, becoming free of flesh on the outer surface first, embraces the point and borders of the coronoid process.

Nerve-supply

The two or three deep temporal branches of the motor portion of the inferior maxillary nerve, which enter the lower part of its deep surface.

Action

To close the jaw; its posterior fibres will also draw it backwards after the other muscles have protruded it. This muscle, like the masseter, has to contend with very little mechanical disadvantage, power being of more importance in mastication than speed.

The temporal muscle.

Relations

Superficially, the temporal fascia which separates it from some of the auricular muscles, the sides of the epicranial aponeurosis, the auriculo-temporal nerve, and the upper branches of the facial nerve ; the zygoma and a small part of the masseter. Deeply, the temporal fossa, and the external pterygoid muscle.

At its posterior border it is crossed by the masseteric nerve and vessels.

Pterygoid externus muscle

The pterygoideus externus - named from its attachment to the pterygoid process of the sphenoid bone, and its relation to the companion muscle - consists f of two thick triangular sheets, the one lying in a horizontal, and the other in a ' vertical plane.

Origin

Upper head

The under surface of the great wing of the sphenoid bone, internal to the pterygoid ridge, and external to the foramen ovale and , foramen spinosum.

Lower head

All the outer surface of the external pterygoid | plate, except a small strip at its lower and front part.

Insertion

Upper head

1. The front of the interarticular fibrocartilage of he temporo-maxillary joint;
2. the adjacent portion of the capsular ligament;
3. the upper part of the front of the neck of the condyle of the mandible.

Lower head

The pit in the front of the neck of the condyle.

Structure

Arising by fleshy fibres, which are closely connected at the pterygoid ridge with part of the temporal muscle, the upper head forms a fan- shaped heet which passes backwards and slightly outwards to its insertion, which is by hort tendinous fibres blending below with those of the lower head. The lower nd much stronger head is at first separated from the upper by a small chink, l^hich may give passage to the internal maxillary vessels. It arises fleshy, converges iackwards, outwards, and somewhat upwards, and is inserted by short tendinous ibres.

Nerve-supply

The external pterygoid branch of the motor division of the iferior maxillary nerve, which divides into filaments entering its deep surface.

Action

1. To draw forwards the ramus of the mandible, and the interarticular fibro-cartilage;
2. to draw them inwards. The combination of these wo movements produces the oblique movement of the lower molar teeth of one ide, forwards and inwards with respect to the upper molars which are their pponents. It should be observed also that this inward movement of one side is he agent by which the ramus of the opposite side is moved outwards.
3. To issist in opening the mouth by depression of the lower jaw. As the transverse xis of this movement passes through the mandible at two points situated below he necks of the rami, it folloAvs that a forward movement of the condyles and ecks will assist in the backward movement of the angles and body which accompanies the depression of the mandible.

The Pterygoid Muscles.

Relations

Superficially, the anterior fibres of the internal pterygoid, the temporal muscle, and, at a little distance, also a small part of the masseter ; deeply, the internal pterygoid muscle, the internal maxillary vessels (unless, as sometimes, they pass across the outer surface of the lower head), the middle meningeal and inferior dental vessels, with the masseteric and posterior deep temporal nerves passing behind or through the attachment of the upper head ; the buccal and anterior deep temporal nerves running between the two heads ; the lingual gustatory and inferior dental nerves beneath the lower head.

Variations

Muscular fibres are frequently found upon the deep surface of the external pterygoid, running from the back of the external pterygoid plate to the spine of the sphenoid, or the vaginal process of the temporal bone.

Pterygoideus internus muscle

The pterygoideus internus- named from its origin and relative position - is a thick quadrilateral sheet.

Origin

1. The whole of the inner surface of the external pterygoid plate.
2. A small triangular area, consisting of the outer surface of the tuberosity of the palate bone, and a small strip in front belonging to the maxilla, and one behind belonging to the external surface of the external pterygoid plate.

Insertion

The lower half of the internal surface of the ramus of the mandible, including the adjacent parts of its lower and posterior borders, and extending as high as the mylo-hyoid ridge and inferior dental canal.

Structure

Arising fleshy, the fibres run parallel to one another, downwards, backwards, and outwards, to be inserted partly directly into the mandible, and partly in multipenniform fashion, like those of the masseter, into the tips and sides of fibrous septa, which, passing upwards from the periosteum of the mandible, separate the muscular bundles from one another.

Nerve-supply

The internal pterygoid branch of the motor division of the inferior maxillary, which enters the deep surface near its posterior border.

Action

1. To close the jaw. The same remarks which were made with respect to the very small loss of mechanical advantage in the masseter muscle apply to this muscle.
2. When closed it will draw the jaw forwards ; and also
3. it will help the external pterygoid in drawing the ramus of its own side towards the middle line.

Relations

Superficially, the external pterygoid muscle, the internal lateral ligament, the internal maxillary vessels, the inferior dental and lingual nerves; deeply, the tensor palati and superior constrictor of the pharynx, the stylo-hyoid and posterior belly of the digastric, the submaxillary gland.