ANIMAL MECHANICS * 



207 



9. 9. The canine muscle {levator anguli oris) acts in a 

 manner different from all the preceding ; its fibres are tan- 

 gential to the ellipse in the neighbourhood of the extremity 

 of the axis major, and therefore aid the orbicularis rather 

 than counteract it. It may be regarded as a force in reserve 

 to aid the orbicularis where its curvature is greatest, and 

 where the most powerful apparatus of radiate muscles is 

 placed. 



If we examine the outer sheet of muscles, we find that 

 they form a continuous radiating muscle through APQ % with 

 one break between the lower border of the risorius (4) and 

 the triangularis oris (6), below the angle of the mouth ; this 

 break is marked xy on the figure, and is feebly supplied 

 by a few fibres of the platysma and triangularis. If we now 

 examine the inner sheet of muscles, we find them to form a 

 continuous sheet through BQP' (or BQP), with the ex- 

 ception of a space, xy\ through which the buccinator and 

 quadratus labii inferioris overlap each other, forming a double 

 sheet. 



It is interesting to observe that this space, x f y\ overlaps 

 the space xy, when the figure is folded on its minor axis, so 

 that the overlapping of the sheets of inner fibres compensates 

 for the deficiency of the outer sheet. 



The nineteen muscles of the mouth are all supplied with 

 motor power from the seventh pair, and their constantly varied 

 action produces the wonderful play of the lips, required 

 in articulation ; as the curvature of each portion of the or- 

 bicularis changes, corresponding changes must take place 

 in the action of the radiating fibres that counteract it, the 

 result being an endless variety in the expression of the most 

 expressive feature of the face. 



This consideration furnishes an argument against the use 

 of the beard, which not only takes away from its wearer the 



