Now, these bones and their attendant ligatures are wonderfully and wisely contrived. The humerus moves freely in its socket in the shoulder, so that it can be swung in every required direction, and yet, as should be the case, its principal movement is up and down in a vertical line--the precise movement required for the effective wingstrokes in flight. But note further. The elbow joint, unlike that of the shoulder, is a rigid hinge, permitting motion in only one plane, that of the wing itself, or nearly so. The same is true of the wrist joint, which holds the hand firmly, allowing no motion save that which opens and closes the wing. The wisdom of this arrangement will be seen at a glance. In the human arm the hand can be moved in every direction with the greatest freedom, and, moreover, the wrist may be turned and the hand laid on its back, its palm, its edge, or at almost any conceivable angle. This is a very convenient contrivance for man, but it would be a great misfortune for our avian friends if their wings would rotate so readily; for in that case they would not have sufficient rigidity to answer the purposes of flight, but would be twisted into every position by the assaults of the air currents. Besides, even in ordinary flight it would require a constant muscular effort to keep the wings in the proper position. How wisely Nature has devised the bird's flying apparatus! When outstretched, it is held firmly by the power of its own mechanism, with its broad under surface lying horizontally, and no breezy current can bend or twist it from its normal position. The set of muscles that open the wing are called the extensors, and those that close it, the flexors. The former lie upon the back of the upper arm and the front of the forearm and the hand, their tendons passing over the convexities of the elbow and wrist, while the flexors occupy the opposite sides, and their tendons run up into the concavities of the joints. There are several powerful pectoral muscles which run out from the shoulder and breast, and operate upon the upper end of the humerus, and with these the wing is lifted and the strokes are made during flight. Another mechanical contrivance deserves attention. An extremely elastic cord reaches over from the shoulder to the wrist joint, supporting a fold of skin that occupies the deep angle of the elbow, and that is covered with short, fluffy feathers. When the bird is flying, this cord is stretched and forms the front edge of that section of the wing. But, now, suppose the wing is closed, will not this cord make a cumbersome fold, flapping loosely in the angle of the elbow? Such would, indeed, be the case, did not its extreme elasticity enable it to contract to the proper length, so as to keep the wing's border straight and smooth.