THE LIMBS 



145 



The liinbs belonging to the same region of the 

 different sides of the body resemble each other in all 

 essential particulars, being symmetrical paired organs. 

 The anterior and posterior limbs have a general resem- 

 blance in plan, although always differing in certain 

 details, these differences becoming more marked when 

 the limbs have different functions to perform. Thus, as 

 in birds, the fore limbs may be modified into wings for 

 flight, while the hind limbs are only used for walk- 

 ing on the ground ; in which case the fundamental re- 

 semblance of the two is very much masked. In the 

 horse, as we shall see, where they are all used for the 

 same purpose, standing, walking, or running on the 

 ground, the fore and hind limbs are remarkably similar 

 in construction, perhaps as much or more so than in any 

 other animal. 



The fore and hind limbs resemble each other mainly 

 in being divided primarily into three segments: one 

 proximal, or nearest the body and articulated with the 

 girdle which carries it ; one middle segment, and one 

 distal or farthest from the body. 



The proximal segment has in both cases a single 

 bone forming its supporting axis, which bone is called 

 the humerus or arm-bone in the fore limb, and the 

 femur or thigh-bone in the hind limb. The middle 

 segment of both limbs has two bones lying side by 

 side — the radius and ulna in the fore limb and the tibia 



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