432 THE LIFE OF A BIRD. 



be compared to that of a boat, with a deep pro- 

 jecting keel. Upon it are arranged those thick 

 and powerful muscles which act upon the wings, 

 and one of which is in general the largest muscle 

 in the body ; in birds of flight often equalling in 

 weight all the other muscles of the body put 

 together. With the upper part of this bone the 

 well-known " merry-thought " bone is connected. 

 Two other bones are also attached to it, which are 

 of great importance to the bird, and are exceed- 

 ingly characteristic of its structure. These bones 

 are all very different in shape in some birds from 

 those of others, varying according to the varying 

 nature of the habits of birds ; those which are 

 endowed with the powers of flight having a struc- 

 ture proportionately strong and firm, and those 

 which live in the waters having a lesser degree of 

 strength about the breast-bone, &c., and a greater 

 degree about the lower parts of the body. 



Before concluding this sketch of the external 

 structure of a bird, and our little volume together, 

 a remarkable fact about the bones remains to be 

 mentioned. The hard material of which bones 

 are formed consists of earthy substances and of 

 animal matter united, and so forming a light but 



