438 THE MUSCLES 



strained painful expression to the features, sueh as is seen in tetanus, and ealled 

 the ' risus sardonicus. ' 



Relations. — Su]>erficially, the skin and sul)eutaneous fat; deeply, the masseter 

 and bueeinator, the faeial artery and vein, and branehes of the facial nerve. 



4. DEPRESSOR ANGULI ORIS 



The depressor anguli oris — named from its action upon the corner of the 



mouth — is a triangular sheet, and belongs to the superficial layer of facial muscles. 



Origin. — The outer aspect of the loAver border of the body of the mandil)le and 

 the external oblique line below the canine, bicuspid, and first molar teeth. 



Insertion. — The subcutaneous connective tissue, and the muscular mass ex- 

 ternal to the corner of the mouth. 



Structure. — Its fibres, arising fleshy, converge upwards and inwards, and, 

 having diminished to a narrow band, join the general muscular mass at the corner 

 of the mouth, some of them being continued upwards into the levator anguli oris, 

 and the most external into the zygomaticus major. 



The fibres which form the inner border of the muscle are often continued 

 downwards below the mandible, and form wdth those of the other side a l^and, 

 partly muscular and partly fibrous, which, lying beneath the platysma myoides, 

 supports and compresses the sulicutaneous fat below and behind the prominence of 

 the chin. When there is much subcutaneous fat, the absence of this support gives 

 rise to a consideral^le prominence behind this band, producing the so-called 

 ' double chin. ' 



Nerve-supply. — The supramandibular Ijranch of the lower division of the 

 facial nerve, which sends filaments to the back part of its deep surface. 



Action. — To draw downwards and somewhat outwards the angle of the mouth, 

 giving an expression of sorrow to the face, and making the individual look ' down 

 in the mouth.' 



Relations. — The skin superficially; and deeply, the depressor labii inferioris 

 and inferior coronary artery. 



Labial Group of Muscles 



The labial group consists of three muscles which pass downwards from the 

 maxilla, and form a continuous sheet which might fairly be included under one 

 name; and a fourth muscle which passes upwards from the mandible. All the 

 muscles are inserted into the lips, and especially into that part of them which inter- 

 venes between the middle line and a point half way between this and the corner of 

 the mouth. 



The upper set is formed by the levator labii superioris ahieque nasi, the levator 

 labii superioris, and the zygomaticus minor; the lower muscle is the depressor 

 labii inferioris. 



1. LEVATOR LABII SUPERIORIS AL.EQUE NASI 



The levator labii superioris alaeque nasi — named from its action as an 

 elevator of the U])i)er li]) and the nostril — is a somewliat triangular sheet, bifur- 

 cating below at the base which corresponds to the insertion of the muscle. It 

 belongs to the superficial layer of facial muscles. 



Origin.— The anterior half of the outer surface of tlie nasal process of the 

 maxilla. 



Insertion. — (1) The deep surface of the skin Avhich covers the lower part of the 

 ala nasi; (2) the deep surface of the skin and the general muscular mass, of the 

 inner half of the u]iper lip. 



Structure. — Arising lleshy, its fibres diverge somewhat as they pass down- 

 wards; they tlien decussate with those of the orbicularis oris close to the prolabium. 



