STERNA OF THE GALLINID^E. 365 



instances of this swinging of the axis in several flying 

 birds when they walk by steps ; and it is more con- 

 spicuous in proportion as the tarsi are longer, and 

 the tibiae more moved in performing the step. But 

 such birds have generally the tarsi rather short, so 

 that on their quick marches upon the ground they 

 take short trundling steps, or they hop, as is the case 

 with most of those passeres which are habitually or 

 occasionally ground feeders. In order that the pro- 

 gress may be by steps of considerable length, it 

 becomes necessary that, if the shoulders require a 

 stiffening clavicle to fit them for flight, the posterior 

 part of the sternum and also the ribs, but especially 

 the former, should be flexible, in order that the one 

 side may contract and the other expand on the ad- 

 vance of the leg, as much as shall keep the axis of 

 the body steady to the line of motion. We have a 

 perfect confirmation of this in the sterna of gallina- 

 ceous birds, all of which are straight-forward runners, 

 and runners without hopping. Many of the long- 

 legged birds also run fast and steadily forward, and 

 never use any action of the feet for that purpose, 

 except the alternate one; but the particular struc- 

 ture of the bones, by which this is accomplished 

 in them, can be more advantageously explained 

 afterwards. 



STERNA OF THE GALLINID^E. 



The sternum of these birds has the lozenge form 

 which characterises that of the pigeon tribe, only it 

 is more elongated in proportion to its breadth. As 

 in these, the anterior part of it is the firmer, but the 

 posterior part is much more flexible, consisting of 

 five processes of bone, all narrow and slender, with 

 the notches next the middle by far the deepest, 



