450 
fingers on the anterior extremity, and three 
thumbs opposite to two fingers 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- 
— and plastrum, which they are destined to 
rag 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 + 4, 
the sum of the lengths of the anterior and 
sterior extremities is equal. The feet and 
ands 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 fiom 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 Chelonia, enable them 
MOTION. 
to resist the external pressure to which, rom 
their partially burrowing habits, they are su < 
jected, and also to endure the trampling of 
lai uadrupeds. 
As qpa aey ia 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, k 
the legs through which its weight is transmitted 
to the ground. It is balanced and kept in equi- — 
librio on an axis passing through the centres of 
the heads of the femurs perpendicularly to the 
plane of the mesial section. i 
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. ae 
The areas of the bases of support vary in 
different orders of birds according to the num-— 
ber, length, and direction of their toes. e 
action of the gracilis muscle, which enables 
birds to stand on one leg in repose, was 
monstrated by Borelli, and though his views 
were opposed by Vieq D’Azyr and Barthez 
they have been confirmed by Monro, Cuvier 
Miller, Roget, and Owen. In walking slowly 
the body rests a long time on both legs and 
short time on one ; during the former period t 
motion of the trunk is retarded, but during th 
latter itis accelerated. In these movements, 0 
leg is flexed, raised from the ground, and swut 
forwards to take a new position in advance, whil 
the other supports the trunk and propels it fe 
wards; and as soon as the foot of the raised 
arrives in a position 
rpendicular to the head 
the femur, the hind leg is lifted and repe ats 1 
like movements. The time during which the bo 
is supported on one leg, in proportion to th 
when it is resting on both, depends on- 
celerity of progression. The time of the 
lation of the swinging leg is governed by t 
length of the leg and the are through whie 
is suffered to oscillate.* In walking, the ee 
of gravity oscillates laterally; this motic 
* For further details on walking upon two! 
see those on human progression, many of W. 
are applicable to birds. 
