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ANIMAL MECHANICS. 



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FIG. 138. Diagram to illustrate the method of cal- 

 culating movements around the ankle-joint. 

 Letters as before. 



man of his own weight, then his ankle-joints would be fulcra fixed in 



space, and would be situated in the middle points of the levers ; the 



feet would be levers of the first order, or two-armed levers. If now 



he were to stand on his own feet, and raise himself on his own toes, 



he would be doing just the 

 same mechanical act as be- 

 fore ; therefore the foot is a 

 lever of the first order. 



L. Hermann, 1 who has 

 C constructed a most ingeni- 



ous model to illustrate the 



mechanism of the ankle-joint, 



discusses with Ewald at some 

 length the question, as to 

 whether the foot is a lever 

 of the second order, as Weber 

 thought, or a lever of the 

 first order, as Ewald main- 

 tains. The question is, how- 

 ever, merely one of words and definitions. 



Fischer 2 has given the subject as a general problem an exact and 



comprehensive study. It is evident that the same reasoning will apply 



to many different movements, as for instance raising the body by 



extending the arms on parallel bars. 



Forces acting obliquely upon a bone. 



So far, we have studied forces such as 



those of gravity or muscular contraction, 



acting in a direction at right angles to 



the bone, and in calculating their moments 



we have measured the distance along the 



bone from their point of action to the 



fixed point. We cannot do this if they 



act obliquely, the moments being in this 



case calculated from the nearest distance 



from the fixed point to the line of direction 



of the force. This nearest distance will 



be that of a straight line passing from the 



fixed point, and falling vertically upon 



the line of direction of the forces. Thus, 



in Fig. 139, gravity x gr will give the 



moments on one side of r, and muscular 



action X rm will give the moments upon FIG. 139. Diagram to represent the 



the other side. The moments of the 



muscle thus varying with position, it is 



evident that when trying to overcome a 



constant resistance a flexor muscle exerts 



least traction when the limb is fully extended, but that the moment will 



increase as flexion takes place. If the moment of the resisting force does 



not, as is the case with gravity, augment in the same proportion, it will 



therefore become less, compared to the moment of the muscular force, 



and the muscle will do its work under the most favourable conditions, i.e. 



1 Arch. f. d. ges. Physiol., Bonn, Bd. Ixii. 



2 Arch. f. Anat. u. Entwcklngsgexch., Leipzig, 1895, S. 101. 



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method of calculating movements 

 when the forces are oblique to the 

 axes of the body on which they 

 act. 



