136 



ANIMAL MECHANICS. 



have for the strength of the tendon y'x\ the equation (vide 

 P- 83). 



F=F+P. (ic) 



The tendon x'm! above the loop must be made sufficiently 

 strong to resist the force of the muscle F ; while the tendon 

 y'x below the loop need only have a strength sufficient to re- 

 sist the force 



P = P -F; 



or, in other words, the cross section of the tendons of the 

 fingers of a hand, will fall short of the cross section of the 

 tendons at the wrist, by a quantity that represents the friction 

 of these tendons in passing through their various sheaths to 

 be distributed to the ends of the fingers. 



The mechanism of a true foot, or walking organ, is the 

 reverse of that of a true hand. When the toes of the foot 

 reach the ground, they are partly flexed, as at OA'B'C, and 

 when the point C touches the ground, the weight of the body 

 unbends the toes, changing them from the position OA'B'C 

 to the position OABC. The muscle Om' resists the unbend- 

 ing of the toes, and is forcibly extended to the length Om, 

 acting thus as a sort of spring, and preventing the sudden 

 shock that would otherwise be given to the body each time 

 that the foot reaches the ground. Hence it is evident, for 

 the reasons already given, that the tendon y'x'm' (which is in- 

 extensible), must be drawn through the loop sc, in a direction 

 opposite to that of the arrow, and that the friction of the loop 

 acts towards 0, helping the portion of the tendon xm situated 

 above the loop ; hence in case we have 



P + F=P; (i 5 ) 



from which it appears that the cross section of the tendons of 

 the toes must exceed the cross section of the flexor tendons 

 at the heel, by an amount (F) representing the friction of the 

 sheaths. 



