18 BRITISH BIRDS 



where it is often found, it holds out both wings, which are compared 

 to sails ; they possibly serve rather as the pole of the tight-rope 

 walker, to preserve the balance of the bird when hurrying along at 

 full speed. In the Secretary Vulture of Africa the wings can be 

 used for flying, but they are also used as weapons wherewith to 

 combat the poisonous snakes upon which the bird so usefully feeds. 

 It strikes down the venomous serpent when the latter is attempting 

 to strike the bird. The Chauna of South America has strong spurs 

 upon its wings, which are used for fighting as well as for flying. 

 But the most curious use to which wings are put is afforded by the 

 Penguin. If the reader has never seen the ' diving birds ' fed at the 

 Zoological Gardens, let him go there on the first opportunity, and 

 see how rapidly and gracefully the Penguin ' flies ' under water by 

 the flapping of its wings. They are shorter than those of most 

 birds, and the feathers have become flattened and almost scale-like, 

 so as to offer no resistance to the water ; at the same time the 

 bones of the wing are flattened, BO that a broad surface is provided, 

 which of course acts like an oar. With this oar-like wing the 

 Penguin can outswim a small fish. 



Sternum and Ribs. 



The breast-bone or sternum (fig. 8, p. 13) of birds shows the samo 

 relation to the power of flight that is shown by so many, if not by all, 

 parts of the skeleton. It is relatively a very large bone, and is in all 

 perfectly flying birds furnished in the middle line, below, with a 

 strongly marked keel, the presence of which has given its name to 

 the great group of birds called carinates. The ostrich tribe, from 

 whose sterna the keel is absent, are termed ' ratite,' or * raftlike.' 

 The reason for the keel is the attachment of the great pectoral 

 muscle, which is the most important muscle of flight. The sternum 

 often offers useful characters to the systematist. The surface of the 

 bone is sometimes in various degrees fenestrate, or more or less 

 deeply incised, the one condition being an exaggeration of the other, 

 and both the conditions being due to defective ossification. The 

 sternum is attached to the vertebral column by the ribs, which are 

 well developed in all birds, but vary very much in number. A 

 highly characteristic feature of the ribs of birds is a small bony pro- 

 jection of the hinder margin of a certain number of them, called 

 the uncinate processes. These are present in all birds, with the 



