PETT1GREW ON THE MECHANISM OF FLIGHT. 239 



rule, deeply concave on its under or ventral surface (Plate XI IT. fig. 15), and in this re pect 

 resembles the wing of the Hasores or heavy-bodied birds ( Plato X IV. fii^s. 31 and 32). It 

 differs from the wing of the bird in being supported, in addition to the bones of the arm 

 and hand, by the bones of the feet and tail, and likewise by the back and side of the 

 body, and in presenting an uninterrupted or continuous membrane both in flexion and 



in extension. If the tail is long, the area of the wing is increased, this with the Legs 



distending and supporting the membrane in a backward direction. Tins arrangement 

 facilitates the evolutions of the bat on the wing, the elevation or depression of the feet 

 and tail, and the spreading thereof, assisting it in ascending, descending, and tanking. 



The Bones of the Wing of the Bat— the spiral configuration of their articular surfaces. 



The bones of the arm and hand are especially deserving of attention. The humerus 

 (Plate XIII. fig. 15 r) is short and powerful, and twisted upon itself to the extent of 

 something less than a quarter of a turn. As a consequence, the 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 me- 

 tacarpal bones with their phalanges (*/), all of which are greatly elongated, and give 

 strength and rigidity to the anterior or thick margin of the wing. The articular surface^ 

 of the bones alluded to, as well as of the other bones of the hand, are spirally dispos< I 

 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 * . 



Extension and Flexion in the Wing of the Bat.-In the flexed condition the hu- 



and the second and third metacarpal bones, with their phalanges, are parallel 

 to each other, the radius and ulna being raised a little above the others, and the 

 fourth and fifth metacarpal bones, with their phalanges, a little below-the whole 

 being parallel, or nearly parallel, with the long axis of the body. In the flexed state 

 the surface of the win? is diminished to its utmost, so that it presents its narrow 



merus 



and 



edge to the wind. When extension takes place the elbow-jomt is depressed 



ried forwards, the wrist elevated and earned backwards, the metacarpo phatoaj al joints 



lowered and inelined forwards, and the distal phalangeal joints sightly rawed t and ear- 



ried backwards. The movement of the hat's wing in extension is consequent ,y a spiral 



_, lllc ™ .—s alternately from below upwards and forwards, and from above 

 downwards and backwards. As the bones of the arm, forearm, and hand rotate on thou- 



the spiral 



dui 



the extensile act, it follows that the posterior or thin margin of th< 



rotated in a downward direction (the anterior or thick one being rotated m an opposite 



direction) until the wing makes an angle of something luke 30 with t he horizon^ ,M 



as I have already endeavoured to show, is the greatest angle made by the , ,n S m fhght. 



nave cuieauy en --,.. M x B . fo on-feee f neatest resistance, and is then m 

 When the wing is fully extended it presents its sui face 01 rcai 



j *v* ™rt nf mv subiect, to obtain tbe body of one of the larger Bats (a 

 • I had the good fortune when engaged on ,„« part my ^ ^^ J ^ ^ ^ 



species of Pteropus) immediately after death. i5y its am 1 was 



me in previous examinations. . of ^ ^ the back 



t The raising of the distal phalanges enables the Bat to arran D e ais | 



during extreme flexion when the animal is reposing. 



