440 THE MUSCLES 



Insertion. — The deep surface of the skin and the subjacent muscular mass 

 along the lower i.^d^e of the prolabium of the lower lij). 



Structure. — It consists of parallel muscular fibres, many of them continuous 

 with tliose of the platysma myoides, which pass u])wardsand inwards to interlace 

 with the fibres of the orbicularis oris along the line aljove mentioned. The inner 

 margins of the muscles of the two sides meet above in the middle line. 



Nerve-supply. — The supramandilnilar branch of the lower division of the 

 facial nerve, which sends filaments to its deep surface near its outer Ijorder. 



Action. — To draw down and somewhat evert the lower lip. 



Relations. — Superficially, the skin and the depressor anguli oris; deeply, the 

 facial portion of the orbicularis oris, the mental vessels, and nerve. 



Muscles eadiating from the ISIouth 



The two muscles which radiate from the mouth are the depressor alse nasi and 

 the levator menti. 



1. DEPRESSOR AL^ NASI 



The depressor alee nasi has lieen described with the muscles of the nose. 



2. LEVATOR MEXTI 



The levator menti, or levator labii inferioris — named from its action — is a 

 short, thick, and somewhat fan-shaped muscle, belonging to the second layer. 



Origin. — The incisive fossa of the mandible, below and a little internal to the 

 attachment of the incisivus inferior of the orbicularis oris. 



Insertion. — The subcutaneous tissue just above the point of the chin. 



Structure. — Arising fleshy, its fibres diverge slightly as they pass downwards 

 and inwards to meet in the middle line Avith those of the opposite side just below 

 a pellet of fat which lies beneath the skin at a short distance above the point of 

 the chin. 



Nerve-supply. — The supramandibular l^raneh of the lower division of the 

 facial nerve, which terminates in this muscle. 



Action. — To draw upwards the skin covering the prominence of the chin, and 

 to elevate and shoot out the lower lip. It has been sometimes called the mnsculus 

 superbus, from the haughty and contemptuous expression which it produces when 

 it acts at the same time with the depressor anguli oris. A slighter contraction, 

 however, gives the mouth an expression of firmness and decision. 



Relations. — Superficially, the mucous membrane of the mouth and the facial 

 portion of the orl)icularis oris al)ove, the subcutaneous fat below; deeply, the 

 mandible. 



MUSCLES OF MASTICATION 



The muscles of mastication form an independent group, four in number, 

 occupying the l)ack part of the side of the face, and the temporal and zygomatic 

 fossae, and consisting of the masseter, the temporal, with the external and internal 

 pterygoid muscles. The temporal muscle is covered by a strong membrane, the 

 temporal fascia, which, arising from the temporal ridge, is attached below to the 

 upper border of tlie zygoma, after first dividing into two laminte which go to the 

 outer and inner aspects of this border, and contain between them a small (luantity 

 of fat. From the zygoma downwards, the masseteric fascia is continued to the 

 ])osterior and inferior borders of the ramus of the inandi])lc. enveloping the mas- 

 seter muscle. Closely connected with this is the parotid fascia which envelops 

 the parotid gland, extending l)ackwards from the masseteric fascia to that part of 

 the deep cervical fascia which covers the upper portion of the sterno-mastoid 



