THE LIMBS. 155 



gether wanting, though in such cases some trace of 

 a girdle is nearly always found. 



The limbs belonging to the same region of the 

 different sides of the body resemble each other in all 

 essential particulars, being symmetrical paired or- 

 gans. The anterior and posterior limbs have a gen- 

 eral resemblance 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 walking on the ground ; in which 

 case the fundamental resemblance 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, per- 

 haps 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 seg- 

 ments : one proximal, or nearest the body and ar- 

 ticulated 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 



