70 ANATOMY OF THE RABBIT 



an imaginary extension of the axis of the limb through the hand or 

 foot. Adduction is the reverse movement. The muscles bringing 

 about such movements are then known as abductor or adductor 

 muscles.) 



In mammals the limbs are set in rather close to and more nearly 

 underneath the body, a position better fitted for complete and 

 permanent support. As compared with the neutral type, the elbow, 

 as indicated in profile in (B), is rotated backward through ninety 

 degrees, the hand forward through ninety degrees. The radius 

 and ulna are crossed upon each other, the radial side of the limb 

 being lateral at the elbow and medial at the hand. The extension 

 angle h, at the elbow, is now posterior, that of the wrist, c, anterior. 

 Many interesting observations may be made by placing the human 

 arm in the corresponding positions. In most mammals, for ex- 

 ample, the hand is fixed in a prone position with the radius and 

 ulna crossed. In man and to a certain extent in some mammals 

 the hand may be placed in a supine position or the limb held as 

 in the neutral type. In either case the bones of the forearm are 

 parallel. The human condition with respect to this character there- 

 fore appears to be rather primitive, and is in great contrast to 

 that shown in specialized running animals such as the horse, where 

 the radius takes over almost the entire support of the forearm and 

 the ulna becomes simply an accessory of the elbow-joint. 



The hind limb of a mammal (C) is rotated in its entirety forward 

 through ninety degrees. The tibia and fibula retain their parallel 

 position. The extension and flexion angles retain their mutual 

 positions but the former become anterior and the latter posterior. 

 The foot, in its more usual plantigrade condition, presents one of 

 the striking cases of muscle arrangements in the limbs, in that, for 

 example, as was pointed out above, an extensor muscle, originating 

 on the front of the leg and terminating on the dorsum of the toes, 

 will extend the toes and will bend the foot on the leg at the ankle 

 joint. In a morphological sense, the foot is not thereby flexed and 

 the muscles are named and classified accordingly. An attempt has 

 been made in preceding paragraphs to clarify the apparent contra- 

 dictions in terminology thus introduced. 



From the mechanical point of view, each limb can act both as a 

 strut and as a lever. When acting as a strut, it exerts forces along 



