214. THE STRUCTURE AND LIFE OF BIRDS cuwap, 
base they are bound together by a strong elastic band, 
the outer ends of which pass into muscles which arise 
from the vertebrz of the tail. When these muscles 
contract, the fan opens, the distance between the quills 
being increased where the elastic band holds them, 
their bases meanwhile being held firm. The tail itself 
can be moved in any direction, having several pairs of 
muscles attached to the vertebrze and to the pelvis at 
different levels so that it can be raised or lowered, and 
also (since one member of a pair of muscles can con- 
tract without the other) moved to right or left. It is 
often important that the tail should form a hollow on 
its under side. This is ensured by the form of the 
feathers which is similar to that of the primaries and 
secondaries in the wing. The outer webs are very 
narrow in all except the one or two central pairs ; the 
broad inner webs offer more resistance to the air and 
are forced upward, each pressing hard against the 
outer web of the feather next to it on the inner side. 
The result is that the tail, besides being almost im- 
pervious to air, presents a curved surface with the 
concavity underneath. The Woodpecker uses his 
strong tail feathers to support him as he clings to a 
tree ; and consequeritly one is not surprised to find 
that in him the muscles which lower the tail are very 
highly developed. 
Rate of Stroke. 
There are three ways of estimating this, one of 
which has been tried, I believe, only in the case of 
insects. These three ways are—(1) that of unassisted 
