MOTION. 
land and water. The outline of the Sauria 
presents a lengthened form, having the atlanto- 
sacral axis much greater than the transverse ; 
they partake more of the figure of the Ophidian 
than of the Chelonian reptiles, but differ from 
the former in having legs for the support of 
the trunk and for locomotion, and also in having 
amore complex skeleton, including besides the 
vertebral and costal bones, scapule, clavicles,* 
Sternoid, pelvic, and sacral elements, as in 
‘Mammiferous quadrupeds. The acetabula of the 
Scapulz and ossa ilii are inclined horizontally 
Outwards; the humerus and femur, which are 
short, take the same direction; the ginglymoid 
articulations of the elbow and knee joints are 
inclined backwards eccentrically to those of the 
Shoulder and thigh. The effect of this is that 
the extension and flexion of the fore-arm and 
leg, being made in the plane of the trans- 
verse horizontal section of the body, are at the 
Same time movements of abduction and ad- 
duction; an arrangement which renders the ex- 
tremities ill adapted for rapid progression on 
land. Those Sauria which have the four legs 
nearly of an equal length may be considered 
the best adapted for locomotion; the vertebral 
column being in this case parallel to the plane 
of motion. When the legs are nearly of equal 
length, the bones of the anterior and posterior ex- 
tremities bear the following proportions : in the 
arm of the Crocodilus acutus the humerus is 
found to be 4.334, the ulna 3.083; and in the 
leg, the femur 4.666, the tibia 3.5 inches, so 
that 4.334 + 3.083 — 4.666 — 3.5 =—0.75 
inch for the difference of the length of these 
bones. The metatarsus and toes are longer 
and broader than the carpus and phalanges of 
the fingers, and present a large surface to strike 
the water in swimming.t The posterior extre- 
mities of the Biporcatus are palmated,{ those 
of the Cayman semipalmated. The legs of 
the Sauria are very short compared with the 
length of the animal, (which, in the Bipor- 
catus, is more than 10 feet,) and their hori- 
zontal inclination tends still more to depress 
the centre of gravity towards the plane of mo- 
tion. From the same cause the legs act on 
land at a great mechanical disadvantage. 
In the Crocodilean Sauria, the cervical ver- 
tebre have but a limited lateral motion owing 
to the projection and interposition of the false 
ribs; the dorsal vertebre have their transverse 
rocesses elongated and fixed to the ribs, which 
ve no tubercles, consequently there is but 
little lateral motion of the back ; the lengthened 
tail, however, admits of considerable lateral 
pray and is of great use in swimming. The 
rocodile cannot curve its trunk abruptly or 
turn it at an acute angle with facility; it runs, 
however, with considerable agility in a right 
line. The bones of the skeleton are of a 
fibrous spongy character, which diminishes the 
Specific gravity of the animal, and is a great 
* The Crocodile has no clavicle. 
+ Hunterian Museum. 
¢ The same remarks ap ly generally to the Alli- 
rs of America as to the Crocodiles of the Old 
orld, 
VOL, III. 
449 
advantage in its swiftest mode of progression, 
which is in a fluid medium. 
Lacert@.—The Lacertine Saurians are smaller 
in dimensions than the Crocodilean, and pos- 
sess much greater mobility of the vertebral 
column ; the prolongation of the animal in the 
axis of its mesial section is much greater than 
in its transverse section. As in the Cioco- 
dilean species, most of the Lizards are pro- 
vided with four legs, but a very few species 
have two only. In a few forms, such as the 
Chirotes and Bipes, the animal cannot support 
the body above the surface of motion, and 
consequently drags the thoracic and abdominal 
segments along upon the ground. In con- 
sequence of the construction of the hands, 
claws, and prehensile tail, many Lizards climb 
with facility. The Gecko is provided with a 
pneumatic apparatus which it employs in a 
Tanner similar to that of the house-fly; the 
under surface of each of the five toes, which 
(with the exception of the thumb) terminate 
in a sharp claw, is furnished both in the fore 
and hind feet with as many as sixteen trans- 
verse plicw, which open into as many cavities 
or sacs (fig. 244). The contraction of the 
muscles, acting upon these plice and sacs, 
erects the former and dilates the cavities of the 
latter ; the serrated edges being at the same 
time accurately applied to any smooth surface, 
a vacuum is produced, and by this structure 
the animal is enabled to climb up the vertical 
planes of walls, and to walk in an inverted 
position on the ceilings of rooms. The Anolis 
and Tupenambis, as well as the Gecko, run 
with considerable speed, and have the power 
of leaping a great distance; others propel them- 
selves either backwards or forwards by applying 
two or more parts to the 
ground as a fulcrum, and 
by the alternate flexion 
and extension of the body, 
aided by the long and 
flexible tail. 
In the Thecadactylus 
the toes are expanded and 
furnished at their lower ex- 
tremities with transverse 
folds; these folds are di- 
vided by a deep longitu- 
dinal groove, in which the 
claw can be entirely con- 
cealed ; by this provision 
the claws are preserved 
sharp for climbing. 
In the Ptyodactylus, 
the toes are flattened into 
So! plates, the lower parts of 
ie which are striated like a 
fan; the middle of the fold is cleft, and the 
claw is fixed in the fissure. All the toes have 
the claws very much curved. This peculiar 
organization of the feet enables the animal to 
climb with great facility. 
In order to allow of the more secure pre- 
hension of its insect food on the agitated 
branches of trees, the Cameleon has short, 
strong, muscular limbs ; a strong, flexible, and 
prehensile tail; two thumbs ad ot to three 
G 
Fig. 244. 
WAL 
lima cyst 
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