MOVEMENTS OF THE WINGS OF BIRDS. 233 



these two tracings are placed over each other, they show that 

 the time of the elevation of the wing agrees with the dura- 

 lion of the undulation a, and the time of its depression coin- 

 cides with the undulation b. 



From this we may see how the undulations a and b are 

 produced in all the muscular tracings obtained from birds. 

 la fact, close by the portion of the bird's breast on which the 

 experiment is made, and near the projecting edge of the 

 sternum, there are two distinct layers of muscle; the more 

 superficial one is formed by the large pectoral, the depressor 

 of the wing ; the deeper one by the middle pectoral, or ele- 

 vator of the wing, whose tendon passes behind the forked 

 part of the sternum to attach itself to the head of the 

 humerus. 



These two muscles, being superposed, will act by their 

 dilatation on the apparatus applied to them ; the elevator of 

 the wing, swelling as it contracts, gives its signal by the un- 

 dulation a ; the great pectoral signals the depression of the 

 wing by the undulation b. 



We may verify the correctness of this explanation by means 

 of a very simple experiment. Anatomy shows us that the 

 muscle which elevates the wing is narrow, and only covers the 

 depressor in its most inward part, situated along the ridge 

 of the sternum ; so that if we displace the little apparatus 

 which shows the movement of the muscles, and remove it a 

 little outwards, it will occupy a part where the depressor of 

 the wing is not covered by the elevator, and the tracing 

 Avill only present a simple undulation, corresponding with 

 b in the curves of fig. 95. It is thus plainly shown that the 

 undulations a and b in the muscular tracings of the birds on 

 which we have experimented correspond exactly with the 

 actions of the principal muscles which elevate and depress 

 the wing ; but we cannot attach great importance to the form 

 of the tracings, in order to deduce from them the precise 

 nature of the movement performed by the muscle. These 

 movements seem, in fact, to encroach on each other ; so that 

 the relaxation of the elevator of the wing is probably not 

 completed when the depressor begins to act. 



We expect nothing more from these tracings than that 



