File:Animal and vegetable physiology, considered with reference to natural theology (1836) (14784640383).jpg

Identifier: animalvegetable01roge (find matches)
Title: Animal and vegetable physiology, considered with reference to natural theology
Year: 1836 (1830s)
Authors: Roget, Peter Mark, 1779-1869
Subjects: Biology Physiology Plant physiology Natural theology
Publisher: Philadelphia, Carey, Lea & Blanchard
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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other birds, whose legs are very short, have awaddling gait. It may be observed that the more perfect-ly predaceous birds are not the best formed for walking; be-cause where they use their feet for that purpose, their talons,which are required to be kept sharp for seizing and tearingtheir prey, would be blunted; and accordingly the eagle,when moving along the ground, supports itself partly by themotion of its win^s. In roosting, birds support themselves upon their perchby means of one leg only, the other being folded close tothe body. They even maintain this attitude with greaterease and security than if they rested upon both feet. Thetrue explanation of this curious fact was long ago given byBorelli. On tracing the tendons (t, t Fig. 233) of the mus-cles (m, m) which bend the claws, and enable them to graspan object, we find them passing over the outer angles of eachof the intervening joints, so that whenever these joints arebent, as shown in Fig. 234, those tendons are put upon the
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stretch, and mechanically, or without any action of the inus-cles, tend to close the foot. When the bird is on its perch,this effect is produced by the mere weight of the body,which of course, tends to bend all the joints of the limb onwhich it rests; so that the greater that weight, the greater is 406 THE MECHANICAL FUNCTIONS. the force with which the toes grasp the perch. All thistakes place without muscular effort or volition on the partof the bird. It remains in this position with more securityon one foot than it would have done by resting upon both;because, in the latter case, the weight of the body being di-vided between them, does not stretch the tendons sufficient-ly. In this position, the bird not only sleeps in perfect se-curity, but resists the impulse of the wind and the shakingof the bough. The great length of the toes of birds enables them to standsteadily on one leg: and in this attitude many employ theother foot as a hand; especially parrots, whose head is tooheavy to

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