ee ee 
MOTION. 
most conspicuous in some of the Natatores, as 
the Goose, Duck, &c., and in those birds 
which have the greatest space between the cotv- 
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 
mansuefacte gyros quosdam indecoro cursu 
peragunt.” 
Alarge number of birds, suchas 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- 
Tatively 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 Scanscres, 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; during which 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 
tore 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 
hosphate; 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 
451 
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 
Chelonian quadrupeds. The trunk is directed 
horizontally, resting on the four legs, which, 
like so many columns, support the centre 
of gravity. The scapule 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 and expenditure of solid materials, 
the shafts of the long bones are formed into 
hollow 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 than 
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