58 ORGANS OF LOCOMOTION. 



est perfection, whatever may be the element in which they 

 are exercised. The sailing of the eagle, the bounding of 

 the antilope, the swimming of the shark, are not equalled 

 by any movements of insects. This superiority is due to 

 the internal skeleton, which, while it admits a great display 

 of force, gives to the motions, at the same time, a great 

 degree of precision. 



1. Plan of the Organs of Locomotion. 



170. The organs of progression in vertebrated animals 

 never exceed four in number, and to them the term limits is 

 more particularly applied. The study of these organs, as 

 characteristic of the different groups of vertebrate animals, is 

 most interesting, especially when prosecuted with a view to 

 trace them all back to one fundamental plan, and to ob- 

 serve the modifications, oftentimes very slight, by which 

 a very simple organ is adapted to every variety of move- 

 ment. No part of the animal structure more fully illustrates 

 the unity of design or the skill of the Intellect which has 

 so adapted a single organ to such multiplied ends. On this 

 account, we propose to illustrate this subject somewhat in 

 detail. 



171. It is easy to see that the wing which is to sustain 

 the bird in the air, must be different from the leg of the stag 

 which is made for running, or the fins of the fish that swims. 

 But, notwithstanding this diversity, the wing of the bird, the 

 leg of the stag, and the anterior fin of the fish, may 

 still be traced to the same plan of structure ; and if we 

 examine their skeletons, we find the same fundamental 

 parts. In order to show this, it is necessary to give a short 

 description of the composition of the arm or anterior ex- 

 tremity. 



172. The anterior member, in the vertebrates, is invaria- 

 bly composed of the following bones ; 1. The shoulder- 



