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Face Muscles of the Head

muscle, lower, jaw, called and neck


Various thin muscles are disposed in the fore part of the head and scalp (P1. X., fig. 1), by which the brows can be "knitted," and the scalp moved to a small extent. Several small muscles are attached to the back of the ear, which are largely developed in animals with long ears, enabling the ears to be moved.

A series of thin muscles is disposed over the face, in the eyelids, over the nose, and round the mouth, by whose contractions the various movements of the face are effected. They are called muscles of expression. When one side of the face is paralysed, these muscles are quite relaxed, and so that side of the face is quite un wrinkled and expressionless. The muscles of the unaffected side, being therefore unopposed, pull the face to that side by their tonicity.

The temporal muscle is a large fan-shaped muscle passing down the side of the head in front of the ear to be inserted into the upward projecting process of the lower jaw. Its action is to raise the lower jaw.

The temporal muscle forms one of a group of muscles called muscles of mastication, be cause they are chiefly concerned in the move ments of the jaw in chewing. The other members of the group are the masseter and pterygoid muscles, passing downwards from the neighbourhood of the zygomatic arch of the temporal bone to the lower jaw. Their fixed point is above, and they therefore pull the lower jaw upwards, bringing the lower in contact with the upper teeth, while they can also produce a grinding movement of the lower on the upper teeth. Opposed to them is a

number of muscles situated in the neck, chief of which is the digastric, so called because it consists of two slender muscular bellies which are united by an interposed tendon. The ten don is attached to the hyoid bone (it, Fig. 70), situated in the neck, to which one belly (1) passes from the base of the skull behind, while the anterior belly (2) passes upwards and for wards to the middle of the lower jaw-bone. When the muscle con tracts it acts from its fixed point at the base of the skull, but the direction of action is changed by the attach ment to the hyoid bone, which acts as a pulley, the lower jaw is therefore pulled down, and the month opened.

In the neck a large muscle passes from be hind the ear down to the collar-bone in the neighbourhood of the breast-bone. When the neck is stretched, the projection of the muscle of each side can be easily made out, passing down obliquely near the middle line (st. Fig. 70). It is called the sterno-cleidomastoid, and by the contraction of the muscle of both sides the head is bent forwards. When the muscle of one side only acts, the head is inclined to that side, and slightly turned so that the face and chin look to the opposite side. It is by the spasmodic action of this muscle that wry-neck is produced.

The muscles of the eyeball are discussed in the section devoted to the consideration of the eye.