82 ORGANS OF LOCOMOTION. 



they may be employed. The sailing of the eagle, the bound- 

 ing of the antelope, 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 limbs 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 shall 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 to serve for running, or the fins of the fish that 

 swims. But, notwithstanding their dissimilarity, the wing of 

 the bird, the leg of the stag, and the shoulder 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 de- 

 scription of the composition of the arm or anterior extremity. 



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

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

 llade, or scapula, (a,) a broad and flat bone, applied upon 

 the bones of the trunk ; 2. The arwi, (,) formed of a single 



