MUSCULAR SYSTEM OF MAMMALIA. 



61 



It consists of two semi- elliptic planes of muscular fibres which 

 surround the mouth and interlace on either side with those of 

 the ( buccinator ' and other dilators of the oral orifice. The ex- 

 ternal or peripheral surface adheres to the skin, the internal or 

 posterior surface is covered by the mucous membrane of the 

 mouth. Acting as a whole it closes the mouth, bringing the lips 



28 



29 



nuiccles of the head and neck. 



Muscles of the face. 



in contact and pressing them firmly together, but the upper and 

 lower halves can act separately, or the fibres of one side may 

 contract while the others are quiescent, so that different parts of 

 the lips may be moved by different portions of the muscle, which 

 may be regulated or antagonised by the muscles which con- 

 verge to the mouth. A pair of accessory strips to the orbi- 

 cularis, ' accessorii orbicularis superioris,' rise from the alveolar 

 border of the premaxillary, and arching outward on each side are 

 continuous at the angles of the mouth with the other muscles 

 there inserted. A second pair, ' naso-labiales,' descend from the 

 septum of the nose to the upper lip, but with an interval, cor- 

 responding with the depression on the skin beneath that septum. 



