PROGKESSION IN OR THEOUGH THE AIR. 



177 



As a consequence, the long axis of the shoulder-joint is nearly 

 at right angles to that of the elbow-joint. Similar remarks 

 may be made regarding the radius (the principal bone of 

 the forearm) {d), and the second and third metacarpal bones 

 with their phalanges {e f), all of which are greatly elongated, 

 and give strength and rigidity to the anterior or thick 

 margin of the wing. The articular surfaces of the bones 

 alluded to, as well as of the other bones of the hand, are 

 spirally disposed with reference to each other, the long axes 

 of the joints intersecting at nearly right angles. The object 

 of this arrangement is particularly evident when the wing of 

 the living bat, or of one recently dead, is extended and flexed 

 as in flight. 



In the flexed state the wing is greatly reduced in size, its 

 under surface being nearly parallel with the plane of progres- 

 sion. When the wing is fully extended its under surface 

 makes a certain angle with the horizon, the wing being then 

 in a position to give the down stroke, which is delivered 

 downwards and forwards, as in the insect. When extension 

 takes place the elbow-joint is depressed and carried forwards, 

 the wrist elevated and carried backwards, the metacarpo- 

 phalangeal joints lowered and inclined forwards, and the 

 distal phalangeal joints slightly raised and carried backwards. 

 The movement of the bat's wing in extension is consequently 

 a spiral one, the spiral running alternately from below up- 

 wards and forwards, and from above downwards and back- 

 wards (compare with fig. 79, p. 147). As the bones of the 

 arm, forearm, and hand rotate on their axes during the exten- 

 sile act, it follows that the posterior or thin margin of the 

 wing is rotated in a downward direction (the anterior or 

 thick one being rotated in an opposite direction) until the 

 wing makes an angle of something like 30° with the horizon, 

 which, as I have already endeavoured to show, is the greatest 

 angle made by the wing in flight. The action of the bat's 

 wing at the shoulder is particularly free, partly because the 

 shoulder-joint is universal in its nature, and partly because the 

 scapula participates in the movements of this region. The 

 freedom of action referred to enables the bat not only to 

 rotate and twist its wing as a whole, with a view to dimin- 

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