vii FLIGHT 205 



so rotates in the course of the stroke th.it further 

 bewilderment is apt to arise. When the wing is 

 folded, what is really the upper side of the humerus 

 looks partly upward, but mainly towards the body. 

 It may be easily recognised by an unmistakable 

 landmark, the foramen or aperture through which the 

 bone is aerated (see Fig. 56). In the primitive an- 

 cestors of birds, with their forelimbs not yet adjusted 

 for flight, this would have been on the upper surface, 

 and postaxial. On what is really the under surface, 

 but in existing birds looks forward instead of down- 

 ward when the wing is at rest, there is another land- 

 mark. Where the broad expansion at the rear end 

 of the bone begins to narrow down, there may be seen 

 a rather long mark on the bone (GP). This is 

 where the Great Pectoral muscle attaches — on what is 

 properly the under side near its prseaxial edge. In 

 describing what we may call the geography of the 

 bone, we must always state what would be the case 

 if it were set as it is in other animals, and as it is in 

 the bird itself during the down stroke. 



At its near end it is broad, and, according as a 

 muscle attaches at one margin or the other, can be 

 made to rotate either way. A pull from below on the 

 prseaxial margin will make it turn its under surface 

 backward. A pull on the prseaxial margin of the 

 upper side will make it face forwards. And so forth. 



The Great Pectoral muscle springs from the lower 

 part of the keel (supposing the bird is placed breast 

 downwards), partly also from the sides of the breast- 

 bone above the keel and from the ribs where they 

 join the breastbone. From the breast it passes forward, 



