588 THE MECHANICAL FUNCTIONS. 



for walking ; because, were they to use their feet 

 for that purpose, their talons, which are required 

 to be kept sharp for seizing and tearing their 

 prey, would be blunted ; and accordingly the 

 eagle, when moving along the ground, supports 

 itself partly by the motion of its wings. 



In roosting, birds support themselves upon 

 their perch by means of one leg only, the other 

 being folded close to the body. They even 

 maintain this attitude with greater ease and se- 

 curity than if they rested upon both feet. The 

 true explanation of this curious fact was long 

 ago given by Borelli. On tracing the tendons 

 (T, T Fig. 233) of the muscles (M, M) which bend 

 the claws, and enable them to grasp an object, 

 we find them passing over the outer angles of 

 each of the intervening joints, so that whenever 

 these joints are bent, as shown in Fig. 234, those 

 tendons are put upon the stretch, and mechani- 

 cally, or without any action of the muscles, tend 

 to close the foot. When the bird is on its perch, 

 this effect is produced by the mere weight of the 

 body, which, of course, tends to bend all the 

 joints of the limb on which it rests ; so that the 

 greater that weight, the greater is the force with 

 which the toes grasp the perch. All this takes 

 place without muscular effort or volition on the 

 part of the bird. It remains in this position with 

 more security on one foot than it would have 

 done by resting upon both ; because in the latter 



