THE HEAD AND NECK 1171 



mouth, and extend as far as the centre of the lip, but are not 

 continuous with those of the opposite half. They are inserted into 

 the skin, and are partially reinforced by fibres from the elevators 

 of the upper lip, the zygomatic muscles, the risorius, the posterior 

 fibres of the platysma myoides, and the depressor labii inferioris. 



The intermediate stratum consists of fibres which are derived from 

 the buccinator muscles. These fibres are disposed transversely, 

 and those of opposite sides are directly continuous. The fibres of 

 this stratum extend quite to the margins of the lips. 



The foregoing fibres are destitute of bony and cartilaginous 

 attachments. 



The deepest stratum consists of fibres which are so disposed as to 

 constitute specially-named muscles. These are four in number for 

 the upper lip, two for each half, namely, the musculus incisivus 

 superior and the musculus naso-labialis ; and two for the lower lip, 

 one for each half, namely, the musculus incisivus inferior. 



The musculus incisivus superior arises from the incisor fossa of 

 the superior maxilla, and turns outwards in the upper lip towards 

 the angle of the mouth, where the fibres blend with the orbicularis 

 oris. The musculus naso-labialis arises from the back part of the 

 nasal septum, and, like the preceding, turns outwards in the upper 

 lip to blend with the orbicularis oris. The musculus incisivus 

 inferior arises from the incisor fossa of the inferior maxilla, and 

 turns outwards in the lower lip towards the angle of the mouth, 

 where the fibres blend with the orbicularis oris. In addition to the 

 foregoing fibres there are oblique fibres which extend between the 

 skin and the mucous membrane. 



Nerve-supply. — ^The facial nerve. 



Action. — (i) To close the oral aperture ; and (2) to press the lips 

 against the alveolar margins. 



The muscle is not a sphincter muscle, properly so called, inasmuch 

 as the upper and lower fibres are not directly continuous at the angles 

 of the mouth. It is separated from the buccal mucous membrane 

 by the coronary arteries and the labial mucous glands. 



The labial mucous glands, which are racemose, or acino-tubular, 

 are numerous, and each is about the size of a small pea. They are 

 situated on the deep surface of the mucous membrane of the lips, 

 where they lie in the loose areolar tissue between the mucous 

 membrane and the orbicularis oris. Their ducts open into the 

 vestibule of the mouth. 



The buccal mucous glands, also racemose, or acino-tubular, are 

 situated between the mucous membrane of the cheek and the 

 buccinator. 



The molar mucous glands, also racemose, or acino-tubular, lie 

 upon the buccinator in the vicinity of the terminal part of Stensen's 

 duct. They are four or five in number, and their ducts, having 

 pierced the buccinator, open into the vestibule of the mouth 

 opposite the molar teeth. 



