Part 25

The =superficial layer=. The temporal fascia stretches over the temporal fossa, being attached to its borders; that is, to the sagittal and lambdoidal crests, to the curved ridge which connects the sagittal crest with the zygomatic process of the frontal bone, to the caudal border of this process, to the caudal border of the malar bone, to the caudal border of the ligament connecting the malar bone and the zygomatic process of the frontal, and to the dorsal border of the zygomatic process of the temporal bone and its dorsal root. The craniolateral part of this fascia is much stronger than the remainder.

The muscle-fibres take origin from the strong craniolateral part of the inner surface of the temporal fascia, from the groove on the temporal bone dorsad of the dorsal root of its zygomatic process, from the whole inner surface of this process and of the zygomatic process of the malar bone, and sometimes also from that part of the frontal bone which lies within the temporal fossa.

Insertion.--The outer surface of the coronoid process of the mandible dorsad of the coronoid fossa, and both borders of the coronoid process. The cranial fibres may be inserted craniad of the coronoid process onto the aponeurosis covering the deep portion of the muscle. The caudal part of this portion is more or less distinct and is sometimes described as a separate head.

The =deep portion=.

Origin by fleshy fibres from the whole surface of the temporal fossa, except in cases where the cranial part of the fossa is occupied by the superficial portion. The fibres converge toward the coronoid process, and the ventral third of the outer surface is covered by strong fascia.

Insertion.--The whole inner surface of the coronoid process of the mandible.

Relations.--Outer surface with the external ear, the ear-muscles (Fig. 63), the epicranius (Fig. 63, h), the corrugatores supercilii lateralis (d) and medialis (b). Inner surface with the skull, the pterygoideus externus (Fig. 66, b), and the structures within the orbit.

Action.--Elevator of the lower jaw.

=M. pterygoideus externus= (Fig. 66, b).--This lies on the medial side of the mandible, ventrad of the ventral portion of the temporal.

Origin.--The external pterygoid fossa (Fig. 40, h), which lies on the lateral surface of the perpendicular plate of the palatine bone, and on the surface of the pterygoid process of the sphenoid bone dorsad of the internal pterygoid fossa. The fossa extends from the foramen rotundum to the sphenopalatine foramen. The muscle passes with parallel fibres laterad, ventrad, and caudad and has its

Insertion by a strong flat tendon into the medial surface of the mandible near its ventral border between the opening of the mandibular canal and the base of the angular process.

Relations.--Ventral surface with the masseter (a), the digastric, and the soft palate with its muscles. Dorsal surface with the temporal, and near its origin with the muscles of the eye, and other structures in the orbit.

Action.--Elevator of the lower jaw.

=M. pterygoideus internus= (Fig. 66, c).--This lies caudad of the pterygoideus externus, nearly parallel to it, and is connected with it at its lateral end.

Origin from the whole surface of the internal pterygoid fossa (Fig. 40, s).

Insertion by fleshy fibres into the ventral surface of the external pterygoid and its tendon, into the medial surface of the angular process of the mandible and caudad of the angular process for about one centimeter into a fascia common to it and the masseter muscle, and into the stylomandibular ligament.

The tongue and other structures forming the floor of the mouth have been removed, showing the roof of the mouth. On the right side the masseter and pterygoideus internus have been removed, a, a′, M. masseter; b, M. pterygoideus externus; c, M. pterygoideus internus; d, d′, M. tensor veli palatini; e, e′, M. levator veli palatini; f, M. constrictor pharyngis superior. 1, aponeurosis of the velum palatinum; 2, ridges formed by the palatines and pterygoid portions of the sphenoid; 3, hamular processes; 4, free caudal border of the velum palatinum; 5, bulla tympani.]

Relations.--Ventral surface with the styloglossus and the soft palate. Dorsal surface with the bulla tympani (5) and base of the skull. Cranial surface with the pterygoideus externus (b), with which it is partly united.

Action.--Assists the pterygoideus externus.

(b) Muscles of the Hyoid Bone.

=M. stylohyoideus= (Fig. 65, d).--The stylohyoid is a ribbon-like muscle about three millimeters wide. It lies on the superficial surface of the digastric, caudad and ventrad of the angle of the jaw.

Origin by fleshy fibres from the outer surface of the stylohyal bone near its middle. The muscle curves over the outer surface of the digastric (b), and ends about one centimeter from the middle line in a slender flat tendon.

Insertion into the middle of the ventral surface of the body of the hyoid bone. Some of the fibres of the mylohyoid (c) are inserted into its tendon.

Relations.--Outer surface with the submaxillary gland (2) and the submentalis muscle. Inner surface with the digastric (b), the hyoglossus (Fig. 67, h), and the geniohyoid (Fig. 67, g).

Action.--Raises the hyoid.

=M. geniohyoideus= (Fig. 67, g).--The geniohyoid is a long slender muscle which lies next to the median line between the symphysis menti and the hyoid bone, closely applied to its fellow of the opposite side and within (dorsad of) the mylohyoid (Fig. 65, c).

Origin from the ventral half of the inner surface of the mandibula for about one millimeter from the symphysis.

Insertion by muscle-fibres into the lateral half of the ventral surface of the body of the hyoid bone.