PKOGliESSIOK IN OR THKOUGH THE AIR. 



191 



coracoicl and scapular bones with great facility, much in the 

 same way that the head of the radius glides upon the distal 

 end of the humerus. But the humerus has another motion ; 

 it moves like a hinge from before backwards ^ and vice versa. 

 The axis of the latter movement is almost at right angles to 

 that of the former. As, however, the shoulder-joint is con- 

 nected by long ligaments to the body, and can be drawn 

 away from it to the extent of one-eighth of an inch or more, 

 it follows that a third and twisting niovement can be performed, 

 the twisting admitting of rotation to the extent of something 

 like a quarter of a turn. In raising and extending the wing 

 preparatory to the downward stroke two opposite movements 

 are required, viz. one from before backwards, and another 

 from below upwards. As, however, the axes of these move- 

 ments are at nearly right angles to each other, a spiral or 

 twisting movement is necessary to run the one into the 

 .other — to turn the corner, in fact. 



From what has been stated it will be evident that the 

 movements of the wing, particularly at the root, are remark- 

 ably free, and very varied. A directing and restraining, as 

 well as a propelling force, is therefore necessary. 



The guiding force is to be found in the voluntary muscles 

 which connect the wing with the hoAy in the insect, and 

 which in the bat and bird, in addition to connecting the 

 wing with the body, extend along the pinion even to its tip. 

 It is also to be found in the nmsculo-elastic and other liga- 

 ments, seen to advantage in the bird. 



The Wing flexed and partly elevated by the Action of Elastic 

 Ligaments — the Nature and Position of such Ligaments in the 

 Pheasant^ Snipe, Crested Crane, Sivan, etc. — ^When the wing is 

 drawn away from the body of the bird by the hand the 

 posterior margin of the pinion formed by the primary, 

 secondary, and tertiary feathers rolls down to make a variety 

 of inclined surfaces with the horizon (cb, of fig. 63, p. 138). 

 When, however, the hand is withdrawn, even in the dead 

 bird, the wing instantly folds up ; and in doing so reduces 

 the amount of inclination in the several surfaces referred 

 to {ch,def of the same figure). The wing is folded by 

 the action of certain elastic ligaments, which are put upon 



