268 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



What is the signification of these two muscular actions ? It is readily- 

 seen that the undulation a corresponds to the action of the muscle which 

 elevates the wing, and h to that of the muscle which depresses it. 

 This can readily be proved by comparing the trace of the, muscular 

 action in the electric trace of the elevation and depression of the 

 wang. These two tracings, placed one under the other, show that 

 the period of elevation of the wing agrees with the extent of the 

 unduhitiou a, and the period of depression with the undulation h. 



But to establish this agreement we must take the unequal rapidity of 

 the transmission of the electric and aerial signals into account. We 

 may consider the electric transmission as instantaneous, while the aerial 

 transmission is at the same rate as the rapidity of sound through the 

 air, that is, 334 metres per second. If the points of the two sujles are 

 placed vertically one above another, the tracings will not be exactly 

 superposed, but the electric signal will precede the other by a distance 

 corresponding to a certain fraction of a second, according to the length 

 of the tube which has been employed. We can even compute, from the 

 length of the air-tube, the amount of retardation, but it is more certainly 

 ascertained by a special determination for the particular tube which 

 may be in use. In a previous experiment, motions were simultauf ously 

 transmitted by the tube and by electricity, and the discrepancy de- 

 termined. In the apparatus which I am using, the constant discrepancy 

 is .01: of a second. I should therefore set back the electric signals by 

 a corresponding distance, in order that they may agree with the signals 

 transmitted by the air-tube. Fig. 19 shows the superposed tracing 

 from a harrier after correction. 



It is easy to understand how the undulations a and & are produced in 

 all the tracings of the muscles of birds. In fact there exist two distinct 

 planes of muscles, in the upper part of the region investigated, near the 

 end of the sternum. The most superficial is formed by the great i)ec- 

 toral which lowers the wing, the decider by the median pectoral or eleva- 

 tor of the wing, the tendon of which passes behind the bifurcation of 

 the sternum to attach itself to the head of the humerus. The two sup- 

 perposed muscles act by their swelling upon the apparatus applied to 

 them. The median j^ectoral swells when it contracts, signalizing the 

 undulation a by its action ; the great i)ectoral signalizes the lowering of 

 the wing in the undulation & in a similar manner. 



We can verify the correctness uf this explanation by a very simple 

 experiment. Anatomy shows us that the median pectoral is narrow, and 

 only covers the inner i^ortion of the great pectoral along the keel of the 

 sternum. So if we displace the little apparatus which reveals the motion 

 of these muscles, and carry it further outward, it will occupy a region 

 where the median pectoral does not cover the great pectoral, and the 

 tracing only presents a simple undulation, which corresponds to h in the 

 figures. 



It is, therefore, sufficiently demonstrated that the undulations a and 

 &, in the muscular tracings of the birds upon which I have experimented, 

 correspond exactly to the jirincipal elevating and depressing muscles of 

 the wing; but we cannot attach much importance to the form of these 

 tracings for deducing the precise nature of the motion effected by the 

 muscle. In fact, these motions appear to override one another. So the 

 relaxation of the median pectoral is probably incomplete when the great 

 I)ectoral commences to act. We should expect no more from these tracings 

 than they naturally furnish, that is to say, the number of vibrations of 

 the wing, the greater or less regularity of its movements, the equality, 

 inequality, and energy of each of them. Eestricting the inquiry within 



