VII FLIGHT 211 
line with the forearm, the result will be that the 
hand will be pulled hard against the wrist. This will 
help the ligaments to resist the very great strain put 
upon them. The wonder is that, with all its strength, 
the wrist joint does not succumb. ; 
(4) The air, to a considerable extent, determines 
the movement of the wing. This will be explained 
more fully later on. 
(5) There are muscles which rotate the great 
secondary feathers and hold them in the best position 
to make the wing impervious to air. All the great 
wing feathers are so shaped that the action of the air 
upon them assists the muscular machinery. 
Such is the wing, at once strong and light ; pliable or 
stiff, according as pliability or stiffness is needed at 
different parts, or at different times ; quickly spread 
and quickly flexed, capable of the nicest adjustment 
to suit every phase of flight; worked by machinery, 
all the weightier part of which is massed upon the 
body or close to it, and turning on a pivot which 
stands firm under all pressure. 
Weight of the Breast Muscles. 
To realise how important are the muscles that 
lower the wing, it is only necessary to know what 
proportion their weight bears to that of the whole 
body. I have weighed those of two Wood-pigeons 
and two domestic pigeons, and have found in each 
case that they accounted for either just under or just 
over one fifth of the total weight. In one of the 
P2 
