90 ANATOMY AND PHYSIOLOGY 



lids like a broad flat ribbon, forming the circular or orbital portion, 

 and bearing the eyebrows (Fig. 76). 



Action. When the palpebral fibers contract the lids cover the 

 eyeballs lightly; when the circular fibers contract the lids are 

 pressed against the ball. 



Nerve. Seventh cranial. 



Orbicularis oris (ring muscle of the mouth). Surrounds the 

 opening of the mouth, constituting the larger portion of the lips. 

 The fibers have only one bony attachment on the maxilla, below 

 the septum of the nose. 



Action. It closes the mouth. 



The lips themselves are moved in various ways by muscles 

 above and below them the elevators and depressors of the lips 

 (all supplied by the seventh cranial nerve}. 



Special points. Most of the changes in the expression of the face are 

 caused by the action of the ring muscles and of those which are attached to 

 them. For example, the lifting of the eyelids by the frontalis expresses sur- 

 prise. The wrinkling of the brows by the corrugators speaks disapproval or 

 bewilderment. The risorius, or grinning muscle, draws the corners of the 

 mouth outward. The sneering muscle lifts the nostril and lip together. 

 Pleasure is expressed by the lifting of the angles of the lips upward and 

 outward, while grief depresses them. (There are but three of the depress- 

 ors, or grieving muscles, on each side, and six for the manifestation of hap- 

 pier feelings.) 



MUSCLES OF MASTICATION, FIVE IN NUMBER 



The temporal muscle. Occupying the entire temporal fossa. 

 Origin. The floor of the fossa, and the temporal fascia covering 

 it. Insertion. The coronoid process of the mandible. 



Action. It closes the mouth and draws the mandible or lower 

 jaw-bone backward. 



Nerve. Fifth cranial (or tri-facial). 



The masseter. At the side of the face (Fig. 78). Origin. 

 The zygomatic arch. Insertion. The lateral surface of the ramus 

 of the mandible. 



Action. It closes the mouth and moves the jaw forward 

 slightly. 



Nerve. Fifth cranial. 



