434 THE MUSCLES 



Insertion. — Tlie skin over the posterior and lower part of the alar cartilage. 



Structure. — A thin stratum of parallel muscular iilircs running forwards and 

 downwards. 



Nerve-supply. — The infraorl)ital l)ranch of the upper division of the facial 

 nerve. 



Action. — To dilate the nostril. 



Relations. — Superficially, the skin and levator labii superioris akeque nasi; 

 deeply, the sesamoid cartilages. 



MUSCLES OF THE MOUTH 



The muscles of the mouth consist, first, of the orl)icularis oris, which is the 

 sphincter surrounding the aperture; and secondly, of the l)ands which radiate from 

 this, as a centre, to their origin upon the adjacent facial bones. These may be 

 grouped into three sets: viz. (1) the transverse series, Avhich radiate transversely. 

 This consists of only one pair of muscles, tlie buccinators. (2) The angular series, 

 which pass from above or below to the corner of the mouth. (3) The labial series, 

 which pass nearly vertically either downwards or upwards to the lips, and chiefly 

 to their inner half. 



A fourth series may be added, Avhich have no special connection with the mouth, 

 viz. two nearly vertical muscles close to the middle line, which radiate from an 

 origin close to the mouth. Of tliese, one has already been descrilied, the depressor 

 alse nasi; the other arises from the loAver Jaw, the levator menti. 



The muscles miglit also be arranged according to their stratification, for they 

 form two, and, in some parts, three layers over the greater part of this region. 



1. ORBICULARIS ORIS 



The orbicularis oris — an unpaired muscle, named from its shape and situation 

 — is an oval sheet with the long axis placed transversely, and its fibres arranged 

 round a transverse central aperture. Like the orbicularis oculi, the mviscle may 

 be divided into an internal or labial portion, and an external ov facial. The first part, 

 Avhich is superficial, has no bony connection, except through the medium of the 

 adjacent muscles with which it is closely blended. The second, which is deep and 

 which forms part of the third layer of the facial muscles, has the following small 

 attachments to bone and cartilage. 



Attachments to bone and cartilage. — (1) Naso-labial slips from the back of 

 the lower })art of the cartilage of the septum of the nose; (2) the incisive fossa 

 of the maxilla just above the socket of the lateral incisor tooth ; (3) the incisive 

 fossa of the mandible, below the sockets of the lateral incisor and canine teeth. 



Structure. — The orbicularis oris consists of three sets of fibres, which are 

 in their direction approximately transverse, vertical, and sagittal or antero- 

 posterior. The transverse set form the most conspicuous part of the muscle; 

 they are continuous on either side with the fibres of the l)Viccinator, and they con- 

 stitute the greater part of the fine smooth bundles which lie beneath the red skin of 

 the prolal)ium, and are called the labial portion of the muscle. The vertical 

 fibres are derived from the elevators and depressors of the lips, including the zygo- 

 matici; they form the superficial part of the facial portion, and they interlace with 

 the transverse fil)res. ]\lany of them pass round tlie corners of the mouth and 

 become transverse; those arising from the maxilla and its vicinity ]iassing to the 

 lower li]), Avhile those from the mandil)le go to the upper. The sagittal or antero- 

 posterior fibres ])ass directly or somewhat oldiquely from ])efore backwards 

 betwc'i'ii the transverse fil)r('s. and unite the skin to the mucous membrane. They 

 are found chiefly in the labial portion of the muscle. The two naso-labinl slips 

 pass side by side vertically downwards to the upper part of the muscle, their ])osi- 

 tion being indicated upon the surface l)y two small vertical ridges which descend 

 from the columna nasi on either side of the median groove of the upj^er lip. The 



