42 INTRODUCTION. - 



28. Abductor primi digiti arises from the insertion of the 

 tendon of the supinator radii longus, 15, and draws the first 

 finger outwards from the second. 



29. Abductor digiti arises from the whole length of the outer 

 edge of the radial metacarpal bone, and is inserted into the 

 base of the first phalanx, which it draws forward. 



80. Adductor digiti arises from the ulnar carpal bone, and 

 the whole length of the ulnar metacarpal, and is inserted into 

 the edge of the third or little finger, which is so firmly attached 

 to the second as to have no motion of its own. The action of 

 this muscle therefore is to draw the fingers outwards. 



31. Supinator or extensor digiti fills up the space between 

 the two metacarpal bones, and is inserted fleshy into the base 

 of the first phalanx, and by a tendon into that of the second. 

 It pulls the second finger upwards and backwards. 



By this complex apparatus, then, the wings are made to 

 perform all those powerful, delicate, and varied motions neces- 

 sary for ordinary flight, for escape, pursuit, and the numberless 

 inflexions used every day by birds in their usual avocations. 

 These motions will be better understood by inspecting the 

 figure than by following a laboured description, and still better 

 by dissecting the wing of a Pigeon, or any other bird of mode- 

 rate or large size. 



The adaptation of the form and structure of birds to flying 

 or progression in the air, is obvious and intelligible. Their 

 body is oval, with the larger end forwards, and the more power- 

 ful muscles placed on the breast, so that when the horizontal 

 direction is assumed, the centre of gravity comes between the 

 wings, and is kept near the lower part by the weight of the 

 pectoral muscles. The length and flexibility of the neck en- 

 able the bird to make the necessary changes in the centre of 

 gravity, w^hile the solidity of the dorsal spine gives advantage 

 to the action of the muscles ; the head is terminated by a 

 pointed bill, which aids in cleaving the air ; the feet, when 

 short, are draA\^i up and concealed under the feathers ; w^hen 

 long, stretched out beneath or beyond the tail, which is more 

 or less expanded, and helps to support the body in the air, as 

 well as, by acting in the manner of a rudder, to change its 



