THE TAIL. 55 



importance of these uses, the sternum of birds is of 

 no small value in the distinction of the different 

 tribes ; and on that account we shall have to revert 

 to the notice of it in another section. 



The furcal bone, or " merry-thought," formed by 

 the union of the clavicles, and attached to the two 

 projecting processes of the coracoid bones, keeps the 

 shoulders apart from each other, and in their proper 

 places during the powerful action of the wings in 

 flight. The more powerful the flight of the bird, the 

 more completely does this bone form an arch, so that 

 it resists that compression which might otherwise 

 take place in the throat and chest. Its form and 

 strength vary much with the habits of the diff'erent 

 tribes. 



The bones of the pelvis advance considerably 

 forwards ; but they are open in the rear for the pas- 

 sage of the eggs. They and the bones of the rump 

 are closely united together. The ribs, too, have 

 very little motion, so that the operation of breathing 

 alters the form and capacity of the body much less 

 than in the mammalia. The greater part of the 

 body may be regarded as a sort of box formed of 

 bones, not breaking b}"- slight pressure (as a box 

 composed of one bone would be apt to do), but still 

 very firm and stiff, stiffer anteriorly in those birds 

 which fly much and powerfully, and more produced, 

 particularly both in the sternum and the ribs, in 

 birds which swim and dive. As a whole, it may be 

 said to admit of no lateral bending ; and the little 

 that it admits of in the vertical direction is confined 

 to the lumbar vertebrae. 



THE TAIL. 



The bony part of the tail of birds is generally 



