ANIMAL MECHANICS. 



205 



(to avoid confusion) the deep layer of radiating fibres is shown 

 on the right-hand side of the diagram. 



1. i.i. Quadratics labii superior is. The fibres of this sheet 

 of muscle act perpendicularly to the orbicularis om, from the 

 extremity of the minor axis to a line passing through the 

 focus, and making an angle of 40 0 with that axis. This sheet 

 of muscle acts upon three-fourths of the elliptic quadrant, and 

 is the only radiate muscle that counteracts the orbicularis in 

 this part of its extent. 



2. Orbicularis malaris. This little muscular slip is a portion 

 of the orbicularis oculi, on the outer side, and passes into the 

 upper lip, with the fibres of the quadratus labii superioris. 

 Its mechanical action is feeble, but identical with that of the 

 fibres of the latter muscle. Its chief use is not mechanical 

 but expressional, for it correlates the action of the sphincter 

 of the eye with that of the lifter of the upper lip ; as in the 

 highly natural gesture of winking with one eye, and lifting 

 at the same time the corner of the corresponding lip. It was 

 fully described by Santorini, who calls it 'perpetuus lacertulus 

 ab imo orbiculari. 



3. The Zygomatic muscular slips occupy the position 

 shown in the diagram, and require no further description, as 

 they are w r ell known. 



4. 4. Risorius. This muscle forms a thin sheet of radiating 

 fibres, extending from 35 0 above the axis major to 40 0 below 

 the same ; in conjunction with the deeper seated buccinator, it 

 is intended to control the action of the orbicularis at the 

 extremity of its major axis, where its curvature is greatest, 

 and consequently the perpendicular force also required to be 

 greatest. 



5. The Platysma of the neck (subcutaneus colli) sends a 

 sheet of fibres to the angle of the mouth, which act in a di- 

 rection 30 0 below the major axis. 



