278 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



of the great pectoral is inserted, and to consider that this crest is situ- 

 ated on the anterior edge of the humerus, to comprehend that the action 

 of the great pectoral, whose hbers are carried backward and downward, 

 should produce a rotary motion of the humerus around its longitudinal 

 axis. The conformation of the humeral articulation is perfectly adapted 

 to this motion. Finally, the existence of this rotation is rendered still 

 more necessary by the resistance which the air presents to the back of 

 the wing and opposes to the descent of its feathered portion. We can 

 demonstrate the existence of this motion and measure its extent by 

 means of the registering apparatus. But I have thought it best to defer 

 these researches, especially as they necessitate the construction of spe- 

 cial apparatus, which would require numerous experiments, and would 

 produce, after all, results of very slight importance. In fact, we are 

 enabled to deduce from the attachment of the muscles the nature of 

 the motion which they produce, and this deduction is especially easy. 



1 have always sought to verify the existence of this rotary motion of 

 the humerus, and to measure its extent, by the application of electricity 

 to the muscles of the bird. In the experiment for measuring the static 

 power developed by the contraction of the great pectoral muscle, previ- 

 ously described, I noticed that at each excitement of this muscle the 

 humerus executed a rotary motion upon its axis. I fixed in the humerus 

 a rod, perpendicular to its axis, and was enabled, by the angle formed 

 by the two positions of this rod, to demonstrate that the rotation in the 

 harrier corresponded to an angle of thirty -five or forty degrees. It 

 seemed that the limits of this angle were fixed by the attachments of 

 the median and great pectoral muscles. If traction be exerted upon 

 the two antagonistic muscles of a newly dissected bird, it will be seen 

 that the median pectoral raises this member so that its upper face is 

 turned somewhat backward. The action of the great pectoral changes 

 this position of the wing comjjletely, and carries its upper face strongly 

 upward and even a little forward. These expressions, upward and 

 downward, are relative to a plane cutting the bird into a dorsal and a 

 ventral half; but this plane, doubtless, is not entirely parallel with the 

 horizon during flight. But it is certain that the resistance of the air 

 should give a much more pronounced deflection to the feathers during 

 the more rapid descent of the wing. 



The most diflicult to measure of the influences which change the plane 

 of the bird's wing is that which relates to the pressure of the air on the 

 feathers. Perhaps it may not be impossible to devise an apparatus 

 capable of measuring it, but it so varies with the variations of the 

 velocity with which the wing is lowered, that any measurement which 

 might be obtained would be only the expression of a particular case. 

 It is very probable, on the contrary, that the change of plane due to the 

 action of the pectoral muscles is a much more constant phenomenon. 

 We can infer the action of the two motions of the bird's wing from what 

 has been said of the mechanism of the flight of insects. It is evident 

 that the descent of the wing will have the double effect of raising the 

 bird and of imparting to it a horizontal motion. As to the ascent of the 

 wing, its office cannot be the same, because the imbrication of the 

 feathers does not offer a resistant surface to the air. 



Everything tends to show that the ascending wing cuts the air with 

 its anterior edge, but, as we shall see, another phenomenon occurs which 

 uplifts the body of the bird during the elevation of the wing; this is 

 the transformation of the impulse which the bird has acquired during 

 the lowering of the wing. This impulse is changed in rising, by a mech- 

 anism analogous to that which raises the toy kite. 



