332 THE MUSCULATURE 



the skin is a complex musculature comprised partly of fibre-bundles prolonged from the muscles 

 which radiate from the corners of the mouth. The more superficial portion of the muscle in 

 the upper lip is composed of fibre-bundles from the triangularis (depressor anguli oris), the more 

 superficial portion of that in the lower lip by fibre-bundles from the caninus (levator anguli oris). 

 These fibre-bundles form commissures at the angles of the mouth and extend toward the median 

 line, where many of them interdigitate with those of the opposite side, and are attached to the 

 skin of the hps. The deeper portions are partly formed by fibre-bundles prolonged from the 

 buccinator, the mandibular fibre-bundles of the latter muscle going mainly to the upper lip, 

 the maxillary fibre-bundles mainly to the lower hp. These fibre-bundles are attached chiefly 

 to the mucosa, near the corners of the mouth. 



The compressor labii, or muscle of Klein, is composed of bundles of fibres which take a 

 course transverse to those of the orbicularis, and pass obhquely from the skin surrounding the 

 oral orifice toward the mucosa which bounds its inner margin. It is said to be best marked 

 in infants. 



The incisivus labii superioris is a small muscle-bundle which passes from the alveolar jugum 

 of the upper canine tooth to the back of the orbicularis near the corner of the mouth. 



The incisivus labii inferioris passes similarly from the alveolar jugum of the lower lateral 

 incisor tooth to the back of the orbicularis in the lower hp. 



Nerve-supply. — These muscles are supplied by the buccal branches of the facial nerve which 

 enter the orbicularis on the lateral border. 



Relations. — The main mass of intrinsic musculature of the lips is placed slightly nearer the 

 mucosa than the skin. On its deep surface lie the labial arteries. 



Action. — The orbicularis draws the upper lip downward, the lower lip upward. The 

 incisive muscles draw the corners of the lips medialward, and the compressor flattens the lips. 

 Together they serve to close the mouth. Acting separately they may draw different parts of 

 it in the directions indicated by their structure. The circumferential portion of the orbicularis 

 acting with the incisive muscles makes the lips protrude. The central portion of the orbicularis 

 draws the lips together, and when the buccinator also acts, draws them against the teeth. It 

 is this portion of the muscle that has chiefly to do with nutritive functions. The more peripheral 

 parts of the muscle are chiefly utilised in the expression of the emotions. 



Retractors of the Lips or Quadrate Muscles 

 (Fig. 341) 



The quadratus labii superioris is a thin, quadrangular muscle with three heads, all of 

 which are inserted into the skin and musculature of the upper lip. 



The caput zygomaticum (zygomaticus minor) is long and slender and arises from the lower 

 part of the external surface of the zygomatic bone beneath the lower border of the palpebral 

 portion of the orbicularis oculi. It passes obliquely forward over the caninus and orbicularis 

 oris muscles, and extends to a cutaneous and muscular insertion in the upper hp medial to the 

 corner of the mouth. It lies medial to the zygomaticus. 



The caput infraorbitale (levator labii superioris), a broad, flat muscle, arises from the infra- 

 orbital margin of the maxilla, where it is concealed by the orbicularis oculi. It extends obliquely 

 forward over the caninus and beneath the caput angulare to the skin and musculature of the 

 lateral half of the upper hp. 



The caput angulare (levator labii superioris alseque nasi) arises from the root of the nose, 

 where it is fused with the frontalis. As it descends it divides into two fasciculi, one of which is 

 attached to the skin and the alar cartilage of the nose; the other passes obliquely downward 

 over the caput infraorbitale to the skin and musculature of the lateral half of the upper lip. 



Nerve-supply. — The zygomatic ramus of the seventh nerve sends branches to enter the deep 

 surface of each of the divisions of the muscle. 



Actions. — It raises the lateral half of the upper lip and the wing of the nose. It is of value 

 in inspiration, serves to express the emotion of discontent, and comes into play in violent weep- 

 ing- 



Variations. — The caput zygomaticum is often absent. It may be fused with the zygoma- 

 ticus (major). It may be doubled. Its origin may extend to neighbouring structures. The 

 other heads, though more stable, vary considerably, especially in the extent of their fusion with 

 neighbouring muscles. 



The quadratus labii inferioris (depressor labii inferioris) is a thin, rhomboid muscle which 

 arises below the canine and bicuspid teeth from the base of the mandible, between the mental 

 protuberance and the mental foramen, and extends obhquely upward in a medial direction to the 

 orbicularis oris, through which its fibre-bundles pass. Its more medial fibres cross at their 

 insertion with those of the muscle of the other side. It is attached to the skin and mucosa of 

 the lower lip. It is es.sentially a part of the platysma, and is superficially united to the skin 

 except where covered by the triangularis (depressor anguh oris). It crosses the mental vessels 

 and nerves and a part of the mentalis (levator menti). 



Nerve-supply. — The mandibular branch of the facial sends twigs into its deep surface near 

 the lateral border. 



Action. — It draws down and everts the lower hp. It is an antagonist of the mentahs 

 (levator menti). It plays a part in the expression of terror, irony, great anger, and similar 

 emotions. 



Muscles of the Angle of the Mouth 



(Fig.s. 341, 342, 344, 345) 



The caninus Hevator anguli oris) is a flat, quadrilateral muscle which arises from the canine 

 fossa of the maxilla and runs beneath the quadratus (levator) labii superioris to the corner of 



