617 



LOCOMOTION OF ANIMALS. 



LOCOMOTION OF ANIMALS. 



618 



The head, neck, and body of the bird are poised upon its legs in 

 such a manner that the head may be elevated or depressed without 

 danger of its falling. Those portions of the body situated in front of 

 the legs tend to depress the head towards the ground, and those 

 situated behind the legs tend to raise the head in the opposite 

 direction ; the weight of these parts being equal keeps the body 

 balanced on the heads of the thigh-bones, which are not only the 

 points of support of the body, but likewise the axis of motion on 

 which the body turns. 



We observe in most birds which move entirely on the land, such as 

 the turkey, pheasant, partridge, and numerous others, that the axis of 

 the body is inclined to the horizon, the head being the most elevated 

 portion of the whole animal. As a line passing through the heads of 

 the thigh-bones is the axis on which the body moves and is supported, 

 the centre of gravity of the body must pass through that axis, or the 

 body of the animal would rotate on it. We know, by daily experience, 

 that the beam of a weighing-machine is made to turn on a pivot ; and 

 that if unequal weights be attached at the extremities, as loaded 

 scales, for instance, the arm will be depressed in the direction of the 

 greatest weight, and the arm on the opposite side of the beam will be 

 elevated ; but if the weights be equal the beam will remain stationary. 

 Now, the body of the bird is poised similarly to the beam and weights; 

 and it is curious to observe the methods by which the equilibrium of 

 the body on the legs is maintained. 



The head of the bird, being placed at the extremity of the neck, 

 which in many species is very long and mobile, acts as a weight 

 placed at the eud of a long lever, whose length is the distance of the 

 head from the axis already mentioned, about which the body of the 

 animal turns. Now, as the distance of the bead is varied by every 

 change in the direction of the neck, the mechanical effect is the same 

 as the shortening or lengthening of one arm of the beam of a weighing- 

 machine would produce ; that is, the force increases in proportion to 

 the increased length, and vice versa. The vertcbrte of birds are 

 firmly joined together, and do not move on each other as they do in 

 man and in quadrupeds, so that the animal cannot twist and turn its 

 body on itself to adjust the trunk on its legs, and it is only at the 

 extremities of the trunk that the parts are moveable. The tail when 

 spread out like a fan furnishes a large surface to strike the air, and in 

 this manner its action is important to some birds when walking, as is 

 very conspicuous in the moorhen and others. The head, neck, and 

 tail then are the parts employed in keeping the trunk of the bird 

 poised on its legs in such a manner that it can stand very steadily, and 

 walk and run without falling ; thus we observe that whilst the animal 

 moves the head, neck, and tail are in constant motion. The length of 

 the legs varies considerably in different kinds of birds, and controls 

 the velocity of their movements very sensibly, as does also the mode 

 in which the legs are employed. Cranes and herons whilst walking 

 swing one leg partly round the other, that is, one leg in swinging 

 describes a portion of a circle around the other which is resting on 

 the ground. The Oi-aUatora are furnished with long leg*, which 

 being destitute of feathers are well calculated for walking in water and 

 for making long strides in search of food. There are many other 

 birds which move one leg at a time, whilst the other supports the 

 body, the two legs interchanging their offices alternately, as in human 

 progression. Thus, when the bird walks slowly the body is supported 



finches, and others, do not move the legs alternately, but simulta- 

 neously, performing a succession of small leaps. This mode of 



i progression is common to birds which are furnished with very short 

 legs, a circumstance which would make the step excessively short if 

 performed by moving the legs alternately in the ordinary manner. 

 Even this method however does not enable them to advance with 

 much celerity, and on the least alarm they adopt the more ready and 

 effectual mode of escape by taking wing. 



In the parrot, cuckoo, and woodpecker the internal toes and thumb 



I are turned backwards (fig. 38), 



j a structure which enables these 

 birds to grasp objects in the 

 manner of a hand. By this 

 means also they have the 

 power of climbing trees with 

 great facility, during which 

 action the tail is frequently Flg ' M - ??* of Woodpecker, 



employed to aid in keeping-the axis of the body in a vertical position 

 as in liy. 39. 



\WiFl <: *".- 



Fig. 39. Green Woodpecker. 



Some birds, such as the ostrich, emu, or cassowary, are not eudowed 

 with the power of flight, the weight of the body, compared with the 

 size of the wings, being too great to enable them to sustain themselves 

 in the air; but in walking and running they far surpass all other 

 birds. Their legs are of enormous length, a construction which 

 enables them to take steps of great magnitude with considerable 

 rapidity. The ostrich (jig. 40) runs with amazing fpced, and can 

 outstrip the fleetest Arabian horse. 



Fig. 10. Hunting the Ostilch. 



(luring u short period by both legs, and during a longer period by one; 

 but if the bird adopts a very rapid pace the body is supported by one 

 leg only at a time, as one leg rises the instant the other reaches the 

 ground ; but at no period of walking are both legs raised at the 

 name moment from the ground, as that would constitute running or 

 hopping. 



Many birds, such as blackt'irdo, sparrows, the various kinds of 



Next in size and speed to the ostrich are the cassowaries. Of those 

 in Australia the speed exceeds that of the swiftest grayhound, and 

 the length of their legs is very considerable. 



In fast walking the hinder leg, having been extended to its greatest 

 length, is raised the instant the head of the thigh-bone in the forward 

 leg comes vertically over the foot, and the position of the two legs at 

 the moment the hind leg is about to be raised is such that they form 



