12 



ANIMAL LOCOMOTIOK. 



extremity of the biped or the tail of the fish. It is con- 

 structed on a similar plan, and acts on the same principle. 

 The tail of the fish, the wing of the bird, and the extremity 

 of the biped and quadruped, are screws structurally and 

 functionally. In proof of this, compare the bones of the wing 

 of a bird with the bones of the arm of a man, or those of the 

 fore-leg of an elephant, or any other quadruped. In either 

 case the bones are twisted upon themselves like the screw of 

 an augur. The tail of the fish, the extremities of the biped and 

 quadruped, and the wing of the bird, when moving, describe 

 waved tracks. Thus the wing of the bird, when it is made 

 to oscillate, is thrown into double or figure-of-8 curves, like 

 the body of the fish. When, moreover, the wing ascends and 

 descends to make the up and down strokes, it rotates within 

 the facettes or depressions situated on the scapula and coracoid 

 bones, precisely in the same way that the arm of a man rotates 

 in the glenoid cavity, or the leg in the acetabular cavity in 

 the act of walking. The ascent and descent of the wing in 

 flying correspond to the steps made by the extremities in 

 walking ; the wing rotating upon the body of the bird during 

 the down stroke, the body of the bird rotating on the wing 

 during the up stroke. When the wing descends it describes 

 a downward and forward curve, and elevates the body in an 

 upward and forward curve. When the body descends, it 

 describes a downward and forward curve, the wing being 

 elevated in an upward and forward curve. The curves 

 made by the wing and body in flight form, when united, 

 waved lines, which intersect each other at every beat of 

 the wing. The wing and the body act upon each other 

 alternately (the one being active when the other is passive), 

 and the descent of the wing is not more necessary to the 

 elevation of the body than the descent of the body is to 

 the elevation of the wing. It is thus that the weight of the 

 flying animal is utilized, slip avoided, and continuity of move- 

 ment secured. 



As to the actual waste of tissue involved in walking, swim- 

 ming, an(^ flying, there is much discrepancy of opinion. It is 



tusk of a sword-fisli imbedded in it, is to be seen in the Hunteriaii Museum 

 of the Royal College of Surgeons of England. 



