306 STECCTURE OF THE 



the head of the humeral bone, farther without the 

 centre of motion than the others are within it, and 

 therefore it acts upon a longer lever of power, and 

 has a shorter lever of resistance to overcome than 

 the muscles which elevate the wing-. 



All the round bones of the wing are hollow, or 

 tubular, which makes them much lighter than if they 

 had been solid and of the same dimensions, and much 

 stronger and stiffer than if they were solid with the 

 same quantity of bone. But at all places where 

 there is either a muscle or a tendon inserted, the 

 action of which would be upon one side only of the 

 tube, there is a tie, or septum of bone extending 

 from the one side to the other, so that it may throw 

 the strain equally upon both sides of the tube. If 

 this strain is great, these 'ties are branched, so that 

 they throw it upon a considerable portion of the 

 bone, and as they are ties in the one direction and 

 struts in the cross one, and all gradually run into the 

 bone by heads enlarging in curves, they give the 

 same strength as if the bone were entirely solid 

 throughout. Indeed, for maximum of strength and 

 stiffness, with minimum of weight, these bones are 

 quite a study. 



The elbow joint, I, is very beautifully constructed. 

 It is a sort of triple hi^ge ; and when the humerus 

 and radius are brought into the same line in the 

 stretching of the wing, the processes, as may be seen 

 Irom their form, stop it from having the least flexure 

 in the other direction, while the head of the ulna 

 comes over the centre, between the double processes 

 of the two principal bones, and wedges it against all 

 bending upwards, something in the same manner as 

 the keystone of an arch. 



The wrist-joint at II, is also both ver}^ peculiar 



