SEC. Ill EXTERIOR PARTS OF BIRDS i8i 



use of their webbed toes, must present a broad surface to the water 

 in giving the backward stroke, and bring the foot forward with the 

 toes closed, presenting only an edge to the water, — on the principle 

 of the feathering of oars in rowing. It is carried to an extreme in 

 a loon, where, when the foot is closed, the digit marked It in the 

 figure lies below and behind Zt. It is probably least marked in 

 birds of prey, which give the clutch with their talons spread. The 

 jointings of the individual phalanges of the toes upon one another 

 are simple hinges, permitting motion of extension to a right line or 

 a little beyond in some cases, with very free flexion in the opposite 

 direction. On the whole, the mechanics of a bird's foot are less 

 peculiar than those of the wing, and quite like those of the limbs 

 of a quadruped. 



In ordinary hopping, walking, and running, and in perching as 

 well, only the toes rest upon or grasp the support, from B to be- 

 yond, being more or less vertically over Z>. Such resting of the 

 toes is complete for %, Zt, M in the figure, or for all the anterior 

 toes ; but for the hind toe it varies according to the length and 

 position of that digit, from complete incumbency, like that of the 

 front toes, to mere touching of the tip of that toe, or not even this : 

 the hind toe is then sure to be functionless. But many of the 

 lower birds, such as loons and grebes, cannot stand at all upright on 

 their toes, but rest with the heel (C) touching the ground ; and in 

 many such cases the tail furnishes additional support, making a 

 tripod with the feet, as in the kangaroo. Such birds might be 

 called plantigrade (Lat. planta, the sole ; gradus, a step) in strict 

 anatomical conformity with the quadrupeds so designated. The 

 others are all digitigrade, standing or walking on their toes alone. 

 But no birds progress on the ends of their toes, or toe-nails, as 

 hoofed quadrupeds do. A bird's walking or running is the same as 

 ours, so far as the ordinary mechanics of the motions are concerned ; 

 but its so-called " hopping " is really leaping, both legs moving at 

 once. Most birds, down to Columbce, leap , when on the ground, a 

 mode of progression characteristic of the higher orders ; but many 

 of the more terrestrial Passeres and Accipitres progress by ordinary 

 walking when on the ground, as is invariably the case with parrots, 

 pigeons, gallinaceous birds, and all waders and swimmers. 



The student need scarcely be reassured that, whatever their modi- 

 fications, their relative development, motions, and postures, the 

 several segments of both fore and hind limbs of any vertebrate, 

 quadruped or biped, feathered or featherless, are fixed in one mor- 

 phologically identical series, thus : 1, shoulder or hip-joint ; 2, upper 

 arm or thigh, humerus or femur ; 3, elbow or knee-joint ; 4, fore- 

 arm or leg proper, radius and ulna or tibia and fibula ; 5, wrist, 

 bend of wing, carpus, or heel, ankle, tarsus ; 6, hand proper, meta- 



