2 5 2 



ANIMAL MECHANICS. 



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 

 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- 



Fig. 138. — Diagram to illustrate the method of cal- 

 culating movements around the ankle-joint. 

 Letters as hefore. 



A 



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 

 ±u n ,^i,~,. „;a~ ti ~ „, „, j- e ±u method of calculating movements 



the other side. The moments of the when the forces are oblique to the 



muscle thllS varying with position, it is axes of the body on which they 



evident that when trying to overcome a act- 

 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. lxii. 



"Arch./. Anat. u. Entwcklngsgcsch., Leipzig, 1895, S. 101. 



