VI 



FORM AND FUNCTION 



91 



connects that one will move which yields most easily. 

 Let the accompanying diagram represent two ribs, 

 the left-hand one being the 

 anterior of the two. The 

 contraction of the muscle 

 will raise the hinder one, 

 because that will yield the 

 more easily, the muscular 

 hinges at the shoulder 

 joints allowing the hinder 

 part of the back to rise. 

 Moreover the backbone of 

 most birds that I have ex- 

 amined bends downwards 

 easily, and through a con- 

 siderable arc just in front 

 of the pelvis. The raising 

 of the hindmost ribs which 

 unite with the backbone 

 behind the point where the 

 bend takes place, will aid the vertebral muscles in 

 straightening the back. 1 Wishing to test these conclu- 

 sions by experiment, I suspended a freshly-killed pigeon 

 by its wings, and inflated the air-sacks by means of a 

 blowing-tube. The backbone a little in front of the 



1 Other muscles assist. The levatores costarum, which I have 

 found highly developed in the domestic pigeon, arising from the 

 vertebrae, then passing backwards and attaching to the ribs some 

 way down, tend to make the upper part of the rib horizontal, 

 thus broadening the chamber beneath. The triangularis sterni, 

 which arises from the inside of the sternum, from its anterior 

 lateral end, attaches to the sternal rib-pieces, and tends to make 

 them perpendicular. 



27. 



-B = backbone 

 E.I., ext. intercostal; 

 piece ; ST, sternum. 



T), dorsal rib ; 

 S, sternal rib- 



