ANIMAL MECHANICS. 



373 



8. Glutceus quintus. — This muscle is not found in the 

 Lion. 



9. Obturator interims. — The tendon of this muscle, as in 

 all animals, passes over a pulley formed by the posterior edge 

 of the ischium, and is inserted into the upper portion of the 

 pit of the great trochanter. If we measure the distances 

 from the centre of the socket to the insertion of the muscle 

 and to the centre of the tendon where it passes over the pul- 

 ley, we find — 



Distance from insertion to centre of socket = 2.40 in. 

 Distance from pulley to centre of socket =2.30 ,, 



From these measurements we readily find 



n Work done 



sin 20 = ^ — : ttj — r - 09.91 per cent. 



Maximum Work 1 



We have now seen that in the case of all the muscles used 

 in abduction, the centre of the acetabulum is nearly pierced 

 by the axis of maximum work, proper to each one of the 

 seven muscles ; and it is to be remembered that the head 

 of the femur does not fit the acetabulum exactly, so that there 

 is a play permitted to the axis of rotation, which does not ne- 

 cessarily pass accurately through the centre of the socket in 

 every motion. The diameter of the acetabulum is 1.42 in., 

 and hence .the rotation may take place, in some instances, 

 round a tangent to the acetabulum sphere, at a distance of 

 0.71 in. from the centre. 



Collecting together the seven muscles of abduction, we 

 find :•— 



