ORGANS OF LOCOMOTION. 87 



trived for the spread of a wing, without increasing the 

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

 bird indicates a more sustained flight. The short arm of the 

 whale, with his spreading fingers, resembles a strong oar. 

 The enormous hand of the mole, with its long elbow, is con- 

 structed for the difficult and prolonged efforts requisite in bur- 

 rowing. The twisted arm of the tortoise can be applied to no 

 other movement 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 identical in their structure 

 with the anterior ones. 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 single bone, like the humerus ; 3. The bones of the leg, 

 the tibia and fibula, which, like the radius and ulna, some- 

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

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

 parts, the tarsus or ankle, the metatarsus or instep, and the 

 toes. The modifications are generally less marked than in 

 the arm, inasmuch as there is less diversity of function ; for 

 in all animals, without exception, the posterior extremities 

 are used exclusively for support or locomotion. 



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 ex- 

 tend to the invertebrates ; for al- 

 though in many instances their Fig. 44. Fig. 45. 

 limbs bear a certain resemblance to those of the vertebrates, 

 and are even used for similar purposes, yet they have no real 



