AlK-VrsSEKS OF HIKBS. 19 



" Thus, that the bird may niisc and direct itself in the air, all the muscles nmst con- 

 tract themselves in the followiug manner : the clavicle and the omoplate being fixed by 

 tlie trapezium, the rhombtiid, the upper pari of the great dorsal, the eosto-seapular and 

 the short chnieular, and the -whig being purth' imfolded, brought forward and raised by 

 the action of the middle pectoral of the internal subclavian, the elevators of the hmnerus, 

 of tlie eoraco-braehial, and of the extensors of the anterior membrane of the wing, the 

 bird spi'ings into the air, completely expanding his wings. At the same time the gri'at 

 pectorals, the primary agents of the wings, of which the point is fixed in the humerus by 

 the insertion of their respecti^•e tendon, conti-act suddenly ; and, in conseqxience of the 

 resistance ^\'hich the air opposes to the movement of the M-ings, carry all their po\\-er to 

 the stermun ; by this intervention they cause the trimk to rise, and the wings, whose 

 immediate depression is resisted by the atmospheric air, as we have just said, are never- 

 theless depressed by these indirect means. 



""\ATiile all this is performed -with extreme quiclaiess, several muscles of the wing, 

 besides, among others, the extensors of the tail, stri^-e to extend the wing ; but as the 

 resistance of the air on the extremities of the pluuiage is very great, and this fluid opj)os- 

 ing all rapid movement on their part, these muscles tlicn direct their power against the 

 sides of the trimk. Taking then their position on the bone of the wing, on the external 

 side of the ^\-ing, and acting by their upper extremity, they extend the main wing-bone 

 over the fore ^ving-bone ; and as this action and that of the great pectorals occm- at the 

 same instant and in concert on each side of the tnmk, the latter is forced up in a middle 

 direction. 



"Thus the combination of these various eftbrts impart to the trunk a force of projecting 

 forward and ascending, by ^\-hich it is propelled ^^-ith the wings. This projection is 

 evidently similar to the leap of other animals. The great pectorals then relax, and the 

 wings immediately reascend, partly by the reaction of the air on their lower surface, and 

 the descent of trunk, and partly by the action of the middle pectorals and their congeners, 

 whose contraetion, so to sjjeak, continues dmiug the fiight. 



" After having darted forwards, the bird remains for an instant imsupported by the 

 air ; this fluid then, by its reaction, rei)els and tends to raise it still higher than the leap 

 alone could ha\e done ; and afterwards pre\-ents it again falling as low as the point of 

 dei^arture. 



•' The asc(>nsion of the trunk is doubtless favoured by the internal air, which insinuates 

 itself into every part of the animal, and A\-hich the latter has the factilty of retaining. 



" This air, which is perhaps a light gas, being dilated and rarefied by great heat, not 

 only is its specific gra^'ity proliably diminished, btit it must also contribute to diminish 

 that of the bird, by inflatmg it and supplyuig all vacancies during the flight. 



" If the bird which descends precipitately fears to hurt itself on approaching the earth, 

 it opens its wings and its tail, and takes several little leaps, which, diminishing the rapidity 

 of the descent, permit it to alight gently on the earth. 



" It is by the assistance of the tail that certain bii'ds are enabled to descend with precipi- 

 tation from a great height : by spreading the tail and closing the ^^•ings, they cause the 

 action of the air to predominate on the hind-part of the body, which directs the fore-part 

 downwards, and leaves it entirely to the influence of gravitation. The tail may strengthen 

 the action of the wing by mo^•ing towards the same side." 



To give some idea of the diu-ation and continuity of motion in birds, and likewise of the 

 proportion of time and space which their courses occupy, their swiftness has been often 

 compared with that of quadrupeds' in their greatest progressions, whether natural or 

 forced. The stag, the rein-deer, and the elk can traverse forty leagues in a single day. 

 The rein-deer, harnessed to a sledge, can mixke a journey of. thirty, and continue this 

 during manv davs in succession. The camel can travel 300 leagues in eight days. Thg 



