PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 271 



the bird. I have said that the exact commencemeut of this motion is 

 undetermined, the elevator of the wing not having fallen into repose 

 before the depressor commences to act, and if we would represent the 

 probable curve of the action of these two muscles from that which the 

 myrograph obtains for us, it will be necessary for us to complete the 

 tracing by means of dotted lines as in Fig. 22. 



Fig. 22. 



Trace of the action of a harrier during fliglat : a, action of the elevating mnscle ; h, of 

 the depressing muscle. The dotted lines which descend to the axis of the curve com- 

 plete the probable form of the motions of the two nuiscles of the wing. 



Thus reconstructed, the form of the curves of the elevator and de- 

 pressor reveals the nature of the resistance which each of these mus- 

 cles has encountered. The curve a of the median i^ectoral is that of a 

 muscle acting on a weight; it seems to indicate that the inertia of the 

 wing is the only obstacle which the elevator muscle has to overcome. 

 The curve h shows us a deflection, during part of which the contraction 

 of the muscle takes a slower motion ; it is here that the resistance of 

 the air is interposed. These things happen, then, exactly as in the 

 experiments which I have made upon my own muscles and those of the 

 frog. But you may ask why the deflection of the curve is not produced 

 sooner ; and if the depressor muscle can rapidly contract for a certain 

 period before encountering sufficient resistance from the air to imi)ede 

 its motion. This is just what happens ; we have the proof of it in the 

 anatomical disposition of the attachments of the great pectoral muscle. 

 We shall see hereafter how the motion of the humerus around its articula- 

 tion is produced ; at present I will only say that in the first i)art of its 

 action the great pectoral in contracting produces a pivot-like motion of 

 the wing upon the head of the humerus, and that in this first motion 

 the muscle does not experience the resistance of the air which retards 

 its contraction an instant later. 



The reader will perhaps consider that an inordiimte number of de- 

 ductions are made from the forms of the curves of the muscles ; but 

 those who will familiarize themselves with the use of the registering 

 apparatus, and in particular with the myograph, will soon be convinced 

 that chance does not enter into the formation of the curves, but that the 

 details should find their explanation in the dynamic conditions of the 

 production of muscular power. 



Motions executed by the icing of a hird during flight. — We have seen, 

 in regard to the mechanism of the flight of insects, that the fundamental 

 experiment has been that which has shown the trajectory of the point of 

 the wing in each of its evolutions. The knowledge of the mechanism of 

 flight flows, so to speak, naturally from this first idea. The same deter- 

 mination is equally indispensable for the flight of birds, but the optic 



