ORAL MUSCLES 331 



mandible and the neck to the proximal part of the thorax and shoulder. The 

 muscles of each side interdigitate across the chin. A short distance below the 

 chin, in the neck, the ventral margins diverge (fig. 341). 



Origin. — From the tela subcutanea by somewhat scattered bundles — (1) along a line ex- 

 tending from the cartilage of the second rib to the acromion, and (2) along the dorsal margin of 

 the muscle. 



Insertion. — Into — (1) the mental protuberance of the mandible and the inferior margin 

 of the mandible; and (2) into the skin of the lower part of the cheek and at the corner of the 

 mouth, where it fuses more or less with the quadratus labii inferioris and the orbicularis oris. 



Nerve-supply. — The cervical branch (ramus colh) of the seventh cranial nerve forms beneath 

 the muscle a plexus to which the cutaneus coUi nerve contributes sensory branches. 



Relations. — The muscle is situated beneath the panniculus adiposus, to which in the neck it 

 is not very firmly attached. For the most part it is separated from the external layer of the 

 cervical fascia by loose areolar tissue. The main cutaneous rami of the cervical plexus and the 

 external jugular vein lie beneath the muscle. 



Action. — It wrinkles up the skin of the neck, depresses the corner of the mouth, and thus 

 pla5'S a part in expression of sadness, fright, and suffering. It aids the circulation by relieving 

 pressure on the underlying veins. 



Variations. — Either the facial or the distal development of the muscle may be more exten- 

 sive than that described above. On the other hand, it may be less developed than usual, and 

 rarely it is absent. Accessory shps have been seen going to the zygoma, the auricle, or the 

 mastoid process, etc., and to the clavicle and sternum. Rarely a deep transverse layer is found 

 in man. 



Fig. 342. — Diagram to Illustrate the Architecture of the Orbicularis Oris. 



(After T. D. Thane.) 



Depressor septi nasi----__^ 



~~"~~--.._^ Caninus 



Incisivus sup. --i. "• / 



Sphincter --_^^<^r^3V^ ~— -^^^T^^ZT^n.^^---'-^^ [ 



^ — ^\r^-^2^ ■ ---^ J!>"Sv ? Buccinator 



Incisivus inf. T "^———'^ \ 



Triangularis 



(6) ORAL MUSCLES 



The muscles of the mouth belonging to the facialis system include several 

 intraJabial muscles: — a sphincter, the orbicularis oris; a transverse, the com- 

 pressor labii; and four deep submucous muscles which pass from the sides of 

 the lips to the alveolar juga of the upper canine and lower lateral incisor teeth, the 

 incisivi labii superioris and inferioris. From each corner of the mouth there 

 radiate out several muscles; the caninus and zygomaticus upward to the maxilla 

 and zygomatic bone; the risorius lateralward over the cheek; the platysma and 

 the triangularis downward over the side of the jaw; and the buccinator, lateral- 

 ward over the side of the oral cavity. From each of these fibre-bundles are 

 continued into the more peripheral and superficial portions of the orbicularis. 

 In addition to these muscles there are two retractors or quadrate muscles, one of 

 which, the quadratus labii superioris, extends from the upper lip medial to the 

 angle to the bridge of the nose, the lower margin of the orbit, and the zj^go- 

 matic bone; while the other, the quadratus labii inferioris, extends from a corre- 

 sponding position in the lower lip to the side of the chin. The orbicularis oris, 

 compressor labii, and incisive muscles close the lips; the other muscles open them 

 and pull them in various directions. The buccinator, however, plays a part in the 

 closing of the mouth by offering support for the orbicularis. 



Intralabial Muscles 



The orbicularis oris (figs. 308, 341, 342, 343) is a complex muscle which surrounds the oral 

 orifice and forms the chief intrinsic musculature of the lips. Immediately about the orifice, 

 and on the deep surface of the muscle, is a fairly well-defined sphincter, although at the corners 

 of the mouth the fibre-bundles of one hp cross those of the other and are inserted into the 

 mucosa, and to a less extent into the skin. In the mid-line the fibre-bundles end partly in the 

 perimysium, partly in the skin. About this sphincter area and between its outer margin and 



