INFERIOR LABIAL GROUP. 181 



its insertion with the levator labii superioris proprius ; it is not unfre- 

 quently wanting. 



Relations. The zygomaticus major muscle is in relation by its superficial 

 surface with the lower segment of the orbicularis palpebrarum above, and 

 the fat of the cheek and integument for the rest of its extent. By its deep 

 surface with the malar bone, the masseter, and buccinator muscle, and 

 the facial vessels. The zygomaticus minor, being in front of the major, 

 has no relation with the masseter muscle, while interiorly it rests upon the 

 evator anguli oris. 



The DEPRESSOR LABII SUPERIORIS ALJEQUE NASI (myrtiformis) is seen 

 by drawing upwards the upper lip, and raising the mucous membrane. 

 It is a small oval slip of muscle, situated on each side of the fraenum, 

 arising from the incisive fossa, and passing upwards to be inserted into 

 the upper lip and into the ala and columna of the nose. This muscle is 

 continuous by its outer border with the edge of the compressor nasr. 



Relations. By its superficial surface with the mucous membrane of the 

 mouth, the orbicularis oris and levator labii superioris alaBque nasi muscle; 

 and by its deep surface with the superior maxillary bone. 



Actions. The orbicularis oris produces the direct closure of the lips 

 by means of its continuity at the angles of the mouth with the fibres of the 

 buccinator. When acting singly in the forcible closure of the mouth, the 

 integument is thrown into wrinkles in consequence of its firm connexion 

 with the surface of the muscle. The levator labii superioris alaBque nasi 

 lifts the upper lip with the ala of the nose, and expands the opening of the 

 nares. The depressor labii superioris alaBque nasi is the. antagonist to this 

 muscle, drawing the upper lip and ala of the nose downwards, and con- 

 tracting the opening of the nares. The levator labii superioris proprius 

 is the proper elevator of the upper lip ; acting singly it draws the lip a 

 little to one side. The levator anguli oris lifts the angle of the mouth and 

 draws it inwards, while the zygomatic pull it upwards and outwards, as 

 in laughing. 



6. Inferior Labial Group. Depressor labii inferioris, 



Depressor anguli oris, 

 Levator labii inferioris. 



Dissection. To dissect the inferior labial region continue the vertical 

 section from the margin of the lower lip to the point of the chin. Then 

 carry an incision along the margin of the lower jaw to its angle. Dissect 

 off the integument and superficial fascia from the whole of this surface, 

 and the muscles of the inferior labial region will be exposed. 



The DEPRESSOR LABII INFERIORIS (quadratus menti) arises from the 

 oblique line by the side of the symphysis of the lower jaw, and passing 

 upwards and inwards is inserted into the orbicularis muscle and integu- 

 ment of the lower lip. 



Relations. By its superficial surface with the platysma myoides, part 

 of the depressor anguli oris, and with the integument of the chin, with 

 which it is closely connected. By the deep surface with the levator labii 

 inferioris, the labial glands and mucous membrane of the lower lip. and 

 with the mental nerve and artery. 



The DEPRESSOR ANGULI ORIS (triangularis oris) is a triangular p*ane of 

 muscle arising by a broad base from the external oblique ridge of the 

 16 



