108 STERNAL MUSCLES. 



we may further remark, that there are three muscles 

 on each side of the keel of the sternum, in all flying 

 birds ; and that the principal action of the wing in 

 flight is produced by those muscles. There is an 

 advantage in this arrangement ; and so also is there 

 an advantage in that arrangement in the legs of birds, 

 by which the muscles of these are placed near the 

 body, and the tarsi and part of the tibiae formed chiefly 

 of bones, tendons, and ligaments. By the arrange- 

 ment in the wings, the wing itself is rendered lighter 

 than if muscles had been distributed over its whole 

 length, to any greater extent than is necessary for 

 stretching and bending the local joints. Not only 

 this, but the circulation of blood necessary for the 

 support of much muscular energy, and the supply of 

 the waste which that would occasion, could not be 

 carried on without an extraordinary propelling power, 

 in organs which move so rapidly as the extremities of 

 the wings. Here, again, we may perceive a remark- 

 able coincidence of advantages. That structure of 

 the wing which adapts it best for being an organ of 

 support and progressive motion jointly in the air, is 

 also the one which renders it the most easily move- 

 able and the most easily nourished, so that the labour, 

 whether mechanical in the muscles, or physiological 

 in the energy of life by which the muscles are worked, 

 is always the least possible in proportion to the motion 

 produced ; and thus while every bird has enough and 

 to spare of energy for the average performance of 

 those functions required for its place, in nature, it 

 is at the same time an instance of the most perfect 

 economy in the whole of its furnishings and arrange- 

 ments. 



Of the three muscles on each side of the sternum, 

 by far the largest one is that which depresses the 



