LOCOMOTION OF ANIMALS ON SOLIDS. 181 



and two behind, an arrangement which gives great holding power. Many 

 birds habitually perch upon boughs or branches, and in them the foot possesses 

 a very perfect prehensile power ; and, by a peculiar arrangement of the tendon of 

 the iiexors of the toes, when the weight of the animal bends the tarsus upon the 

 leg, the tendon is stretched mechanically over the heel, and so serves, without 

 further muscular effort, to tighten the whole of the toes upon the object which 

 they grasp ; moreover, on simply raising its weight, by extending the hinder 

 limbs, the hold of the toes is simultaneously loosened, independently of any 

 proper extending effort. In birds which sleep whilst perching, this mechanism 

 is constantly employed, in certain instances, the animal sleeping securely by 

 resting on, and grasping with, one leg only. The folding back of the head 

 under the wing, an attitude so suggestive of repose, retirement, and reliant 

 security, fulfils the further purpose of aiding in the easy .preservation of the 

 equilibrium of the body, by removing backwards the centre of gravity more 

 completely over the now single base of support. There are certain birds of the 

 crane-tribe, which have very long legs, and moderately prehensile toes, which 

 still balance themselves in " the daytime habitually on one leg, whether they 

 sleep or not ; in these cases, too, the head is drawn back under the wing, the 

 centre of gravity is placed probably over, or nearly over, the column of support, 

 and this is kept vertical by the extreme extension of the tarsal joint, and its 

 stiffening by ligamentous connections. 



A certain number of small birds seem to have no power of walking, but in- 

 variably hop over the ground, a movement which may be compared, in regard 

 to its mechanism, with the leap in man ; but most birds whether large or small, 

 can either hop or walk ; the latter motion is much more frequently employed, 

 and resembles the walking mode of progression in man, the action of the bird 

 being truly biped, although the weight is supported on the phalanges and meta- 

 tarsus, and not on the tarsal bones also. The running birds perform that 

 movement on the two feet, alternately, as in man ; and, in the case of the 

 larger cursorial birds, as the ostrich, for example, the rate and endurance of 

 the motion are very great ; the speed of the ostrich is, indeed, said to be as high 

 as that of the race-horse, and the great length and size of its lower extremities, 

 and the diminution in the number of its toes, which, in some species, are only 

 two in number, add, as similar arrangements do in the horse, to the solidity 

 and security of the foot, as an organ of support and locomotion. The peculi- 

 arities of the lower limbs in climbing birds, have already been noticed ; in that 

 action, the limbs are moved alternately, as in running. 



In the four-footed Saurian Keptiles, the crocodiles, alligators, lizards, and 

 others, the movement is essentially quadruped in its character and mechanism ; 

 but, with the exception of certain active lizards, and even they cannot long 

 maintain their celerity, the motion of these animals is heavy and awkward ; 

 this is owing to the shortness of their limbs, the imperfect modelling of the 

 articular surfaces of the bones, and the comparative want of energy in their 

 muscular system ; and something is also due to the length of their unwieldy 

 body between the anterior and posterior limbs, and to the lateral position of 

 the points of attachment of the limbs. In the apodous or footless reptiles, the 

 Ophidia, or snakes, the body is no longer raised from the ground upon limbs, 

 but its weight is supported on the under, or abdominal, surface of the trunk, 

 and though the centre of gravity here occupies, as in all masses of matter, a 

 single point, the weight is supported on an enormously long base, correspond- 

 ing with almost the width and length of the animal, the head and adjoining 

 part of the neck, however, being, as a rule, lifted from the ground. The man- 

 ner in which serpents move over solid surfaces is threefold. First, they may 

 fix the anterior part of the body, and drag the trunk after it, and then, again, 

 project and fix the fore-part, following this by a second advancement of the 

 hinder portion ; this is called the geometric method of progression. Secondly, 

 the animal holds upon the ground by successive portions of its abdominal sur- 

 face, and throws the intermediate parts into slight vertical undulations. 

 Lastly, and much more habitually, holding in the same way by its under sur- 

 face, it throws the body into lateral undulations, and so advances over the 

 ground. The mechanism concerned in these movements consists, first, of the 

 extremely movable and flexuous vertebral column, in which the lateral play is 



