62 



ANATOMY OF THE RABBIT 



coracoid process, and the clavicle in either perfect or less perfect 

 development. 



There is no more striking feature of homology than that shown 

 by the free extremities in the different forms of vertebrates. This 

 is true homology because it concerns the resemblances, part for 

 part, in the anterior or in the posterior limb of any one vertebrate 

 as compared with the corresponding elements in the same position 

 in other forms. The front limb of the rabbit (Fig. 35, A) is slightly 



Fig. 35. Homologies of the mammalian limb. A, forefoot, rabbit. 

 B, forefoot, horse. C, human hand, r, radius; u, ulna; I-V, metacarpa bones. 



elongated and semi-digitigrade, the weight being supported on the 

 tips of the bones of the palm (metacarpals), as shown in Fig. 23. 

 These modifications make it more efficient for running than a more 

 primitive limb, which is shorter and plantigrade (having the palm 

 or sole applied to the ground), though it is less specialized and less 

 efficient as a running organ than the limb of the horse (Fig. 35, B). 

 The human hand retains a fairly primitive form as to its general 

 proportions, but is modified into a seizing or grasping type, the 

 thumb being opposable to the remaining digits. The limbs of the 

 rabbit, of the horse, and of man, however, are all modifications of a 

 primitive, five-toed limb, sometimes termed the ideal pentadactyl 

 plantigrade type, in which the palm of the hand or sole of the 

 foot is placed flat on the ground. The composition of this primitive 



