of the Leg of the Ostrich. 269 



made by the bones meeting at the joint, so that the effect of the 

 whole is to unbend the limb and give it a maximum of extension 

 at the moment of leaving the ground. During the spring, the 

 antagonist muscles again bend the joints, so that on next touch- 

 ing the ground it is at its maximum of flexion, again waiting 

 to be unbent by the muscles that open the angles of the joint, 

 and so on ; as long as the animal runs, it is thrown alternately 

 from each foot in contact with the ground as from a catapult, 

 and advances by successive leaps or springs from foot to foot. 



To take mathematical account of the moments of the muscular 

 forces round each joint, it would be necessary to make the fol- 

 lowing measurements : — 



Let the points and I be the origin and insertion of any 

 muscle, and A the centre of motion of the joint. The effect of 

 the contraction of the muscle is to develope a moment or couple 

 round the point A, acting in the plane A I, and proportional 

 to the weight of the given muscle. If it be required to find 

 the total effect of the muscles acting on a given joint with 

 respect to any line A X, we should have to find the angle made 

 by each plane of moment A I with the line A X, and so 

 calculate, by the usual rule of composition of moments, the 

 total effect of all the forces acting and tending to produce rota- 

 tion. Without going into minute detail, and taking only the 

 muscles that act perpendicularly, or nearly so, to the transverse 

 horizontal axis, we find the following muscles to represent the 

 moment of the spring forwards : — 



1. Muscles extending the Femur on the Axis of the Body. 



1 . Tensor vaginae femoris ] oz. 



2. Glutseus maximus .... V 46 - 75 



3. Agitator caudae J 



4. Biceps femoris 41-25 



5. Semimembranosus 1 7' 00 



0, Semitendinosus 14-00 



7. Gracilis , 4-50 



8. Adductor magnus 4*75 



9. Pectinaeus 4-00 



132-25 

 2. Muscles extending the Leg on the Femur. 



10. Rectus femoris 7*25 



11. Vastus externus (1) 25-25 



12. „ „ (2) 5-00 



13. Vastus interims (1) 13 - 50 



14. „ „ (2) 3-75 



1 5. Cruraeus 7'25 



62-00 



