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ANIMAL MECHANICS. 



6. 6. The triangularis oris (depressor anguli oris) forms a 

 sheet, radiating from 68° below the major axis to a line, 

 OQ, very nearly at right angles to the same axis. 



7. 7. Quadratics labii inferioris (quadratics menti) forms a 

 radiating sheet whose fibres are perpendicular to those of the 

 orbicularis, through four-fifths of the lower elliptic quadrant, 

 from the minor axis to a direction 22 0 below the major axis, 

 near the corner of the mouth. 



To understand exactly the relative positions of the deep 

 and superficial layers, we should imagine the ellipse folded 

 on its minor axis, so as to bring all the radiating lines on the 

 same side of the mouth. 



This muscle acts alone on the orbicularis, from B to Q, 

 where the curvature of the sphincter is least, and is assisted 

 by other muscles, as it approaches the angle of the mouth 

 where the curvature is greater. It corresponds precisely 

 in its action and position with the quadratus labii superioris 

 on the upper side of the ellipse. 



8. 8. 8. 8. Buccinator. This important muscle consists of 

 a flat, thin, strong set of fibres, in contact with the mucous 

 membrane, and forming a considerable portion of the wall of 

 the mouth ; its fibres range from 34 0 above to 34 0 below the 

 major axis, the upper fibres passing into the upper lip, the 

 lower fibres into the lower lip, and the central fibres decus- 

 sating, so as to pass, as shown in the figure, into the opposite 



This whole arrangement gives a maximum of perpendicular 

 force along the major axis, at the extremity of which the 

 curvature is greatest. Its upperfibres OP' or OP, mark the 

 boundary of overlapping of the deep and superficial muscles on 

 the upper side of the mouth. Thus the whole ellipse may be 

 divided into regions as follows : — 



PAP' and QBQ' counteract orbicularis by one sheet of muscles only. 



PQ and P'Q' counteract orbicularis by two sheets of muscle. 



