212 THE STRUCTURE AND LIFE OF BIRDS chap. 



Wood-pigeons 1 the three pairs of breast muscles 

 represented three thirteenths of the whole — i.e., a little 

 less than one quarter, a very large proportion, the 

 average, among birds, being less than a fifth. The 

 Elevator is very small compared with the Depressor, 

 often so light that the most delicate scales are re- 

 quired if trustworthy results are to be obtained, The 

 differences in different species are very striking. In 

 the Wood-pigeon I have found the weight of the small 

 muscle to be a little less than one fifth of the greater, 

 in the starling just over one ninth. 



Movements of the Wing partly due to the Action 

 of the Air. 



The muscle which lowers the wing rotates the 

 humerus, as I have shown, so that the under surface 

 of the wing looks backward and downward. And 

 since the whole expanse of feathers lies to rearward 

 of the bones, the action of the air will tend to turn 

 the wing round in the same direction, just as the 

 wind swings a sign-board. When once an upward 



1 These figures do not agree with those given by two German 

 investigators, Legal and Reichel, in the Jahres-Berichte der 

 Schlesischen Gesellschaft fur Vaterlandcidtur, 1879. They 



give — ( = — = more nearly i than %) to represent the 



3'43 V 343 ' 



relative weights of the three pairs of pectoral muscles and of 

 the whole body. This seems impossible, nor is it clear why in 

 the pigeon the breast muscles should weigh so enormously 

 heavier in proportion to the weight of the body than in any 

 other bird. In the case of other species their figures are not so 

 startling, and they may be more trustworthy. 



