SHORT MUSCLES. 347 



form its extent of motion more rapidly than a long 

 one ; but it will not move so heavy a member, or 

 move it over the same effective extent at one contrac- 

 tion. Hence we find that birds which rush upon their 

 prey on the wing, have the sternum broader at those 

 parts to which the grand and middle pectoral, which 

 depress and raise the wing in flight, are applied; that 

 (though its tendon is longer, from passing through 

 the pulley) the body of the middle pectoral is shorter 

 than that of the grand, having a lighter labour to 

 perform, but having to perform it more quickly ; and 

 that birds which have this habit have the wings, and 

 indeed the whole frame, more solid and heavy than 

 other birds. But we find that in those birds which 

 make no rush, but capture their food in the air at the 

 speed of ordinary flight, as in the swallows and bee- 

 eaters, or which flit about, not in chace of their food, 

 but simply in quest of it, as the humming-birds, the 

 sternum is narrowed in these parts so as to adapt it to 

 the action of shorter muscles, without the load of an 

 additional portion of tendon ; and that, in proportion 

 as the habit of these birds requires the action of the 

 wing to be more powerful, the keel is deepened to 

 admit the insertion of a greater number of muscular 

 fibres, or fasciculi. 



In birds which fly with equal speed on a level, or 

 an ascent or a descent, the raising of the wing with 

 great rapidity and considerable force is much more 

 necessary than in those which get their most rapid 

 motions upon a descent ; and it is especially necessary 

 in the group of birds under consideration, which 

 always have to hold the wings in readiness for being 

 expanded, while they are adhering by the feet to 

 upright or sloping surfaces. Accordingly, they have 

 the middle pectoral larger in its volume, and also in 



