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 B 
contraction of the muscle 
will raise the hinder one, 
because that will yield the 
more easily, the muscular 
hinges at the shoulder dD < D 
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- J 
siderable arc just in front 
of the pelvis. The raising : S) 
of the hindmost ribs which Fic. 27.—B = backbone ; D, dorsal rib; 
unite with the backbone eet era 
behind the point where the 
bend takes place, will aid the vertebral muscles in 
straightening the back. 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 
vertebrze, 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. 
