ON THE MECHANISM OF THE BIRD’S WING. 277 
43. ON A POINT IN THE MECHANISM OF THE BIRD'S 
WING.* 
Te beautiful investigations of Borelli, together with those of Page 82. 
- M. Marey, make it certain that in any organ which is employed as a 
flapping wing there must be a stiff or rigid anterior margin. In the 
insect the stout anterior nervure performs this function; in the bird 
the bones of the arm, forearm, and manus do the same. How, in the 
latter, this necessary rigidity is developed, considering the presence of 
the elbow- and wrist-joints, must be, at first sight, a matter of surprise. 
It depends on the mechanical arrangement by which, when, in the 
wing, the arm is bent on the forearm, the manus is always similarly 
bent on the forearm; and when extension of the forearm is made, 
extension of the manus equally certainly follows. This occurs when 
all the muscles and tendons are removed, and the ligaments binding 
the bones together are alone left. 
The explanation of this mechanism is not difficult. The arm con- 
sists of one bone only, the humerus; the forearm of two, the ulna and 
radius ; the manus of the two carpals together with the metacarpals and 
phalanges. The mutual relations of these two bones are such that the 
radius and ulna move one above another like the two limbs of a pair 
of drawing-parallels, each being fixed proximally to the humerus and 
distally to the carpus. The plane common to the radius and ulna is 
the same as that in which flexion and extension of the elbow is per- 
formed, so that one of the two bones of the forearm, the radius, 
articulates with the humerus at a point nearer the shoulder, or further ~ 
from the elbow, than its companion, the ulna. At the wrist the radius 
. Is consequently superior, articulating with the carpal bone on the 
pollex side: whilst the ulna articulates with the other element of the 
carpus. This condition maintaining the parallel movements of the 
radius on the ulna must necessarily be attended by a parallel move- 
ment of the humerus on the manus. When the humerus bends upon 
the ulna, the manus therefore similarly bends upon the forearm; and 
the triceps muscle is able, unassisted, to maintain the whole limb in a 
rigid state during extension. 
In making a wooden model of these bones to illustrate the above 
described mechanism, one or two points of mechanical detail suggested 
a reference to the shape of the distal end of the humerus. The wing 
= « Proceedings of the Zoological Society,” 1875, pp. 82, 3. Read, Feb. 16, 
1875. 
