PROGKESSION IN OR THROUGH THE AIR. 



193 



does, elevate its wings prior to flight, quite independently of 

 the air. When the bird is fairly launched in space the 

 elevator muscles are assisted by the tendency which the body 

 has to fall downwards and forwards : by the reaction of the 

 air; and by the contraction of the elastic ligaments. The 

 air and the elastic ligaments contribute to ■ the elevation of 

 the wing, but both are obviously under control — they, in fact, 

 form links in a chain of motion which at once begins and 

 terminates in the muscular system. 



That the elastic ligaments are subsidiary and to a certain 

 extent under the control of the muscular system in the same 

 sense that the air is, is evident from the fact that voluntary 

 muscular fibres run into the ligaments in question at various 

 points {a, b of fig. 98, p. 181). The ligaments and muscular 

 fibres act in conjunction, and fold or flex the forearm on the 

 arm. There are others which flex the hand upon the forearm. 

 Others draw the wing towards the body. 



The elastic ligaments, while occupying a similar position in 

 the wings of all birds, are variously constructed and variously 

 combined with voluntary muscles in the several species. 



The Elastic Ligaments more highly differentiated in Wings 

 which vibrate ra;pidly. — The elastic ligaments of the swan are 

 more complicated and more liberally supplied with voluntary 

 muscle than those of the crane, and this is no doubt owing to 

 the fact that the wings of the swan are driven at a much 

 higher speed than those of the crane. In the snipe the wings 

 are made to vibrate very much more rapidly than in the swan, 

 and, as a consequence, we find that the fibro-elastic bands are 

 not only greatly increased, but they are also geared to a much 

 greater number of voluntary muscles, all which seems to 

 prove that the musculo-elastic apparatus employed for recover- 

 ing or flexing the wing towards the end of the down stroke, 

 becomes more and more highly difl"erentiated in proportion to 

 the rapidity with which the wing is moved. ^ The reason for 

 this is obvious. If the wing is to be worked at a higher 

 ST)eed, it must, as a consequence, be more rapidly flexed and 



^ A careful account of the musculo-elastic structures occurring in the wing 

 of the pigeon is given by Mr. Macgillivray in his History of British Birds, 

 pp. 37, 38. 



