ANIMAL MECHANICS. 



273 



or, 



- 2 B sin 6 cos 6d6 

 A i cos 2 6 - B % sin 2 ~0 



and each such pair of elements is zero, when B = o. 

 The geometrical condition involved in the equation 



B = o 



is shown in Fig. 73. Let the bones AB and A'B' be produced 

 to meet in the point Q. 



0 



Fig. 73- 



If the condition B = o be granted, then the point of inter- 

 section, Q, must lie somewhere on the line OQ drawn through 

 the vertex 0, perpendicular to OX, the bisector of the angle 

 made by the muscular fibres. For 



OQ = OX tan j3 = OX' tan /3 7 ; 



b tan j3 - 6' tan j3' = o, 



which is equivalent to 



B = 6 sin J3 cos - 6' sin )3' cos j3 =» o. 



When this condition is fulfilled, equation (52) becomes, 

 writing, 



