450 



MOTION. 



fingers on the anterior extremity, and three 

 thumbs opposite to two ringers on the posterior. 

 Many of the climbing Lacertine Sauria have 

 very elongated and flexible fingers, which give 

 them great power of prehension and rapidity of 

 motion, so that they are enabled to climb ra- 

 pidly up the vertical surface of walls and 

 trunks of trees. 



Chelonia. The Chelonian reptiles are en- 

 closed in a ponderous case formed by the cara- 

 pace and plastrum, which they are destined to 

 drag along with them in all their movements. 

 In the terrestrial species, the dorsal, costal, 

 sternal, and pelvic osseous elements are all 

 fixed, leaving the neck and caudal extremities 

 of the body alone free. The legs, which are 

 short and curved, act in consequence of their 

 horizontal inclination at a great mechanical 

 disadvantage, rendering the progression of the 

 Chelonia proverbially slow. The humerus is 

 bent, and in pronation is locked against the 

 plastrum ; the latter tends to assist it in sup- 

 porting the animal. The same body also pre- 

 vents the femur from exercising any great degree 

 of flexion, but allows it more freedom of action 

 in extension; from the relative position of the 

 cotyloid articulation and the absence of that 

 impediment in the direction of extension, the 

 hinder extremities give altogether a more 

 effective impulse in walking. The length of 

 the arm and fore-arm equals that of the thigh 

 and leg. 



In the Testudo elephantopus, the femur mea- 

 sures five inches, the tibia five, the humerus 

 six, the radius four ; then as 5 + 5 = 6 -f- 4, 

 the sum of the lengths of the anterior and 

 posterior extremities is equal. The feet and 

 hands of the Turtle are furnished with a mem- 

 branous expansion between the toes and fin- 

 gers, which enables them to act as fins. The 

 anterior extremities in this species are much 

 more developed than the posterior. The 

 body is remarkably flattened and depressed 

 so as to present the smallest amount of re- 

 sistance in cleaving the water in quest of their 

 vegetable food. In the Emydes, or fresh- 

 water Chelonia, the feet are palmated,by which 

 means they are enabled to move with greater 

 facility and expedition on soft and "yield- 

 ing surfaces, such as muddy banks and 

 rivers, as well as to swim in pursuit of their 

 prey in the water. The Chelonia are re- 

 markable for the great transverse extension of 

 the trunk compared with their length; they 

 differ in their mode of progression from the 

 Ophidian reptiles in having legs, and from the 

 Saurians in the immobility of the ribs. The 

 Aquatic Chelonia are rendered of less specific 

 gravity than their bulk would indicate, both 

 by the spongy texture of their organs of support 

 and by the great extent of their respiratory 

 apparatus, which reduce their specific gravity 

 to that of the medium in which they move, and 

 admit of their sleeping motionless on the sur- 

 face of the water. 



The structure of the arch-formed carapace 

 and dense plastrum, and the more solid union 

 of all the osseous elements surrounding the 

 trunk of the Terrestrial Cheloniu, enable them 



to resist the external pressure to which, from 

 their partially burrowing habits, they are sub- 

 jected, and also to endure the trampling of 

 large quadrupeds. 



Birds. The feathered tribe traverse the sur- 

 face of the earth as digitigrade bipeds. In 

 standing, the trunk is elevated and supported at 

 various heights above the plane of position, by 

 the legs through which its weight is transmitted 

 to the ground. It is balanced and kept in equi- 

 libno on an axis passing through the centres of 

 the heads of the femurs perpendicularly to the 

 plane of the mesial section. 



The inclination of the trunk lies between the 

 vertical and horizontal planes ; but its angle of 

 elevation depends on the position and weight 

 of its various elements and appendages, such 

 as the head, neck, and anterior extremities, 

 which determine the distance of the centre of 

 gravity to the cotyloid articulation. The ilio- 

 femoral articulations being placed more for- 

 wards in the ossa innominata than in quadru- 

 peds, enables them to bring the centre of 

 gravity within the base of support on their 

 two feet with little elevation of the trunk. 

 Several methods are employed by birds to 

 alter the relative position of their centre of 

 gravity in standing, namely, first, by curva- 

 ture of the neck; secondly, by folding the 

 wing on each side ; thirdly, by the elevation 

 or depression of the trunk above or below the 

 horizontal plane in which the cotyloid joints 

 are situated. All these different positions of 

 the trunk and its appendages throw the centre 

 of gravity backwards towards the vertical line 

 passing through the base of support, which may 

 also be changed and thrown forwards by bend- 

 ing the joints of the legs. 



The areas of the bases of support vary in 

 different orders of birds according to the num- 

 ber, length, and direction of their toes. The 

 action of the gracilis muscle, which enables 

 birds to stand on one leg in repose, was de- 

 monstrated by Borelli, and though his views 

 were opposed by Vicq D'Azyr and Barthez, 

 they have been confirmed by Monro, Cuvier, 

 Miiller, Roget, and Owen. In walking slowly, 

 the body rests a long time on both legs and a 

 short time on one ; during the former period the 

 motion of the trunk is retarded, but during the 

 latter it is accelerated. In these movements, one 

 leg is flexed, raised from the ground, and swung 

 forwards to take a new position in advance, whilst 

 the other supports the trunk and propels it for- 

 wards ; and as soon as the foot of the raised leg 

 arrives in a position perpendicular to the head of 

 the femur, the hind leg is lifted and repeats the 

 1 ike movements. The time during'which the body 

 is supported on one leg, in proportion to that 

 when it is resting on both, depends on the 

 celerity of progression. The time of the oscil- 

 lation of the swinging leg is governed by the 

 length of the leg and the arc through which it 

 is suffered to oscillate.* In walking, the centre 

 of gravity oscillates laterally ; this motion is 



* For further details on walking upon two legs 

 see those on human progression, many of which 

 are applicable to birds. 



