INFERIOR LABIAL GROUP. 175 



superioris alaeque nasi lifts the upper lip with the ala of the nose, and 

 expands the opening of the nares. The depressor labii superioris 

 alaeque nasi is the antagonist to this muscle, drawing the upper lip and 

 ala of the nose downwards, and contracting 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 integument 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 le- 

 vator 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 

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

 ridge of the lower jaw, and inserted by its apex into the angle of the 

 mouth, where it is continuous with the levator anguli oris and zygo- 

 maticus major. 



Relations. By its superficial surface with the integument ; and by 

 its deep surface with the depressor labii inferioris, the buccinator and 

 the branches of the mental nerve and artery. 



The LEVATOR LABII INFERIORIS (levator menti) is a small conical 

 slip of muscle arising from the incisive fossa of the lower jaw, and in- 

 serted into the integument of the chin. It is in relation with 

 the mucous membrane of the mouth, with its fellow, and with the de- 

 pressor labii inferioris. 



Actions. The depressor labii inferioris draws the lower lip directly 

 downwards, and at the same time a little outwards. The depressor 

 anguli oris, from the radiate direction of its fibres, will pull the angle 

 of the mouth either downwards and inwards, or downwards and out- 

 wards, and be expressive of grief ; or acting with the levator anguli 

 oris and zygomaticus major, it will draw the angle of the mouth 

 directly backwards. The levator labii inferioris raises and protrudes 

 the integument of the chin. 



