12 HEAD AND NECK 



oculi and to the lateral side of the zygomatic head of the 

 quadratus labii superioris. Its fibres pass downwards and 

 medially to the angle of the mouth, where some blend with 

 the orbicularis oris and others are inserted into the skin. It 

 pulls the angle of the mouth upwards and backwards. It is 

 supplied by the facial nerve. 



The Risorius. When well developed the risorius muscle 

 consists partly of some of the uppermost fibres of the platysma 

 muscle of the neck, which bend forwards and medially to the 

 angle of the mouth, and partly of additional fibres which 

 spring from the fascia over the masseter muscle and the 

 parotid gland. Both groups of fibres blend with the fibres 

 of the orbicularis oris at the angle of the mouth. The 

 risorius depresses the angle of the mouth and draws it back- 

 wards. It is supplied by the facial nerve. 



Musculus Triangularis. The triangularis (O.T. depressor 

 anguli oris) springs from the oblique line on the lateral surface 

 of the body of the mandible. Its fibres converge as they 

 pass forwards and upwards, and, at the angle of the mouth, 

 they blend with the orbicularis oris, in which some of them 

 curve past the angle and terminate in the substance of the 

 upper lip (Figs. 2, 3). It depresses the angle of the 

 mouth, and is supplied by the facial nerve. 



Musculus Quadratus Labii Inferioris (O.T. Depressor Labii 

 Inferioris). The quadrate muscle of the lower lip springs 

 from the lower part of the superficial surface of the mandible, 

 between the mental tubercle and the mental foramen, its 

 posterior border being overlapped by the triangularis. The 

 fibres pass upwards and medially, some to blend with the 

 orbicularis oris and others to gain attachment to the skin of 

 the lower lip. It depresses the lower lip, and is supplied by 

 the facial nerve. 



Platysma. Only the upper part of the broad, flat, 

 quadrangular subcutaneous muscle of the neck is at present 

 visible. The posterior fibres ascend over the lower border 

 of the ramus and the posterior part of the lower border of 

 the body of the mandible, and they have already been seen 

 taking part in the formation of the risorius. The anterior 

 fibres gain direct insertion into the anterior part of the lower 

 border of the body of the mandible. The latter attach- 

 ment is the only bony attachment which the muscle possesses, 

 all its other attachments being either to fascia or to skin. 



