MOTION. 



451 



most conspicuous in some of the Natatores, as 

 the Goose, Duck, &c., and in those birds 

 which have the greatest space between the coty- 

 loid cavities and the least length of legs. Many 

 birds depress and elevate the head, and ex- 

 tend and retract the neck at each step, in order 

 to preserve in equilibrio the forces acting on 

 the centre of gravity ; the Moor-hen and some 

 others are also observed to spread out the tail 

 like a fan, which is alternately elevated and de- 

 pressed at each step ; and these successive 

 actions of the tail taking place at the extremity 

 of that portion of the lever opposite to the 

 head, tend to produce an equilibrium between 

 those portions of the trunk lying on each side 

 of the cotyloid articulations. 



In the progression of some of the long-legged 

 Grallatores, such as the Crane and Stork, the 

 swinging leg describes a portion of a circular 

 curve round the standing leg. Pliny appears 

 to have observed this kind of movement, for, 

 in his Hist. Nat., lib. x. c. 23, he says, " Grues 

 mansuefactaa gyros quosdam indecoro cursu 

 peragunt." 



Alarge number of birds, such as the Sparrows, 

 Canaries, Blackbirds, &c., instead of moving the 

 two feet alternately, move them simultaneously : 

 in these movements the legs are first flexed and 

 then suddenly extended in succession, by which 

 means the trunk and both legs are elevated 

 from the ground, and progression is effected 

 by a series of short leaps. In running the 

 Cursores outstrip all other birds, and per- 

 haps all other animals. The osseous columns 

 which support the trunk are of great length 

 and size, and are acted on by powerful muscles. 

 The femurs, though short, have a considerable 

 diameter ; the tibia is long, but still longer is 

 the tarso-metatarsal bone. The bones of the 

 legs elevate the trunk to a great height, by 

 which they are enabled to stride over a large 

 space at each step. Aided by its compa- 

 ratively diminutive wings, the Ostrich will out- 

 strip the fleetest Arabian horse in his flight 

 across the desert, as will the Cassowary the 

 swiftest Greyhound. Thus, though deprived by 

 the comparative smallness of their wings of the 

 power of aerial progression, these birds are fully 

 compensated by the velocity of the terrestrial 

 movements. 



The Scansores, such as the Woodpecker, 

 Parrot, Cuckoo, &.c have the internal toes and 

 thumbs turned backwards, which enables them, 

 with the assistance of the tail, to climb and to 

 suspend themselves to the upright trunks of 

 trees ; duringwhich action, the legs, tail, and that 

 portion of the tree against which they rest form 

 the three sides of a triangle. 



Mammiferous Quadrupeds. The locomotive 

 organs of the mammiferous quadrupeds are 

 more highly organized than those of the Ba- 

 trachia and Chelonia. The bones of the skele- 

 ton are more compact, hard, and dense, and 

 contain a greater proportion of the calcareous 

 phosphate; they are, therefore, better calcu- 

 lated to resist the shocks incidental to terres- 

 trial progression: the vertebral column, which 

 is directed horizontally, is convex at its dorsal 

 and concave at its ventral aspect. It forms a 



single arch, extending from the pelvis to the 

 last cervical vertebra, as it is kept bent by 

 strong ligaments. It constitutes a powerful 

 elastic column, well adapted to support the 

 weight of the abdominal viscera as also of extra- 

 neous burdens which these animals are destined 

 to bear. The spinal column of mammiferous 

 quadrupeds is endowed with much greater 

 mobility and elasticity than in the Saurian and 

 Cheloniati quadrupeds. The trunk is directed 

 horizontally, resting on the four legs, which, 

 like so many columns, support the centre 

 of gravity. The scapulce and pelvis have 

 the power of rotating in a vertical plane 

 through a large arc; the axes of the acetabula 

 of the shoulder and hip-joints are directed 

 vertically downwards to receive the heads 

 of the ossa humeri and femoris, the shafts 

 of which are directed vertically upwards. 

 These ball-and-socket joints permit the several 

 motions of flexion and extension, abduction 

 and adduction, pronation and supination : 

 the rest of the joints of each extremity are 

 ginglymoid, a construction which, although 

 it restricts the limbs thus articulated to move- 

 ments in one plane, yet secures to these 

 movements greater precision. The joints are 

 lined and lubricated by synovial membranes, 

 which, throughout the life of the animal, effec- 

 tually secure them from injurious friction not- 

 withstanding their varied and long-continued 

 exertions. The elastic ligaments permit great 

 freedom of action under all ordinary circum- 

 stances without rupture. The osseous columns, 

 which enter into the composition of the extre- 

 mities, are piled upon each other endways, 

 with their long axes either vertical or inclined; 

 and, in order to give them the power of sus- 

 taining the greatest possible pressure with the 

 least weight arid expenditure of solid materials, 

 the shafts of the long bones are formed into 

 hollosv cylinders, of which the height and base 

 are adjusted to each other with the greatest 

 mechanical precision. The bones of the extre- 

 mities in most mammiferous quadrupeds are 

 inclined to each other's axes at a greater or 

 less angle, the magnitude of which is in pro- 

 portion to the bulk and speed of the animal. 

 In those quadrupeds which have the greatest 

 bulk and least velocity of locomotion, the bones 

 approach nearest the vertical direction : such is 

 the case with the elephant. On the contrary, 

 in those animals which are remarkable for the 

 greatest speed, the axes of the bones are in- 

 clined to each other at the greatest angle. 



Although the angular disposition of the 

 bones diminishes their power of sustaining 

 great weight, and increases the expenditure of 

 muscular effort, yet it confers on the legs 

 greater elasticity, and enables them, from the 

 oblique transmission of the impulse, to sustain 

 sudden shocks without fracture of the bones ; it 

 also enlarges the range of motion, and gives 

 the posterior extremities greater power of pro- 

 jecting the body forwards during rapid loco- 

 motion. The bones of the anterior extremity 

 having to support the weight of the head and 

 neck, as well as a large proportion of the trunk, 

 are inclined nearer to a vertical direction thaa 



2 e 2 



