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 muscle may be divided into an internal or labial portion, and an external or facial. The first part, which 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 Ibony attachments.
Attachments to bone
- Naso-labial slips from the back of the lower part of the cartilage of the septum of the nose;
- the incisive fossa of the maxilla just above the socket of the lateral incisor tooth;
- the incisive fossa of the mandible, below the sockets of the lateral incisor and canine teeth.
The orbicularis oris consists of three sets of fibers, 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 buccinator, and they constitute the greater part of the fine smooth bundles which He beneath the red skin of the pro- labium, and are called the labial portion of the muscle. The vertical fibres are derived from the elevators and depressors of the lips, including the zygomatici ; they form the superficial part of the facial portion, and they interlace with the trans- verse fibres. Many of them pass round the corners of the mouth and become transverse ; those arising from the maxilla and its vicinity passing to the lower lip, while those from the mandible go to the upper. The sagittal or antero- posterior fibres pass directly or somewhat obliquely from before backwards between the transverse fibres, and unite the skin to the mucous membrane. They are found chiefly in the labial portion of the muscle. The two naso-labial slips pass side by side vertically downwards to the upper part of the muscle, their position being indicated upon the surface by two small vertical ridges which descend from the columna nasi on either side of the median groove of the upper lip. The fleshy slips from the maxilla and mandible - the musculi incisivi, as they are called - pass outwards and forwards to join the deep surface of the transverse fibres near the corners of the mouth.
The buccal and supramaxillary branches of the lower division of the facial, which enter the facial part of the muscle near its outer border.
- To bring together the lips and to oppose all the other muscles which converge upon the mouth, and tend to draw it open in various directions. It thus acts negatively, and prevents the expression of any emotion, as when the lips are pursed up. If the upper fibres alone act, the upper lip will be drawn downwards ; if the lower, the lower lip will be drawn upwards. If the fibres of the labial portion contract strongly, the corners of the mouth are approximated.
- To shoot out the lips : this will chiefly depend upon the contraction of the facial portion and the musculi incisivi.
- To press the lips against the teeth : the plane of the muscle being curved, with the concavity against the arches formed by the front teeth, its contraction will carry the food backwards into the oral cavity.
In front of the labial portion is the bright red skin of the prolabium ; and in front of the facial portion lie the angular and labial series of muscles, together with some subcutaneous fat.
Upon the deep surface lies the mucous membrane of the mouth, separated from the muscular fibres by the mucous and small salivary glands, together with the superior and inferior coronary arteries.