2i 4 THE STRUCTURE AND LIFE OF BIRDS chap. 



base they are bound together by a strong elastic band, 

 the outer ends of which pass into muscles which arise 

 from the vertebrae 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 vertebrae 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 consequently 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— -(i) that of unassisted 



