ORGANS OF LOCOMOTION. 63 



creasing the weight of the body. The more firm and solid 

 arm of the hird indicates a more sustained flight. The 

 short arm of the whale, with his spreading fingers, resem- 

 bles a strong oar. The enormous hand of the mole, with 

 its long elbow, is made for the difficult and long-continued 

 efforts requisite in burrowing. The twisted arm of the tor- 

 toise can be applied to no other purpose than creeping. 

 And finally, the arm of the fish, completely enveloped in 

 the mass of the flesh, presents, externally, a mere delicate 

 balancer, the pectoral fin. 



179. The posterior members are closely analogous in 

 structure to the anterior. The bones of which they are 

 composed, are, 1. The pelvis, (Fig. 46), which corresponds 

 to the shoulder blade ; 2. The thigh bone or femur, which is 

 a simple bone like the humerus ; 3. The bones of the leg, the 

 tibia andjibula, which, like the radius and ulna, sometimes 

 coalesce into one bone ; and lastly, the bones of the foot, 

 which are divided, like those of the hand, into three parts, 

 the tarsus, metatarsus, and toes. The modifications are 

 generally less marked than in the arm, inasmuch as there is 

 less diversity of function ; for in all animals, without excep- 

 tion, the posterior extremities are used exclusively for walk- 

 ing or swimming. 



180. The anterior extremity of the vertebrates, however 



varied in form, whether it be an 

 arm, a wing, or a fin, is thus 

 shown to be composed of essen- 

 tially the same parts, and con- 

 structed upon the same general 

 plan. This affinity does not extend 

 to the invertebrates, although in 

 Fig. 44. Fig. 45. many instances their limbs bear 



certain resemblance to those of the vertebrates, and are 

 even used for similar purposes, yet they have no real 



