‘scuta (which take a variable number of fixed 
points in the surface of motion), and enable them 
to propel the body forwards and supply the 
place of so many legs. Their muscular sys- 
tem, though capable of exerting great power, 
acts at a mechanical disadvantage, and is 
quickly exhausted. Serpents move by diffe- 
rent methods, and upon different principles of 
progression. First, in a straight line, with the 
whole ventral aspect of the trunk in contact 
with the plane of motion. Secondly, in a 
curved line, with the trunk arched laterally, 
_and the ventral aspect also in contact with the 
_ ground. Thirdly, in a curved line, with the 
_ body moving by an undulatory alternate elon- 
a eg and contraction of small segments of the 
be he Fourthly, in a straight or curved line, 
with the trunk arched vertically in two or more 
curves. Fifthly, in a straight or curved line, 
_ with the trunk arched vertically in a single 
Curve, consisting of the greater part of the 
trunk. Serpents possess also the power of 
_ climbing, swimming, and springing. In the 
first order of locomotion, or that of a straight 
_ line, with the whole of the ventral aspect resting 
immediately on the ground, the serpent is urged 
_ onwards by the oscillation of the ribs acting on 
_ the abdominal scuta, at successive points of 
MOTION. 
447 
its length. The scuta of a segment or seg- 
ments having secured a fulcrum in the plane 
of motion, the ribs connected with the fixed 
seuta, acting in turns, rotate backwards; the 
next segments in advance perform a similar 
action, until the whole series have completed 
the step.” The length of the complete step 
depends on the arc through which the ribs 
oscillate, and the distances of the scuta from 
the axes of motion; and, as these are both 
small, and the motion has to be transmitted 
through the whole length of the reptile, this 
method of progression is, in consequence, very 
slow, presenting to the eye a tardy gliding 
movement. 
In the second order of motion, the progres- 
sion is performed in a curved line, with the ven- 
tral aspectalso in contact with the ground. The 
method of advancing in this order is similar to 
that in the first; but as the reptile passes 
through a larger space in order to reach a given 
point, the progression will, consequently, be 
slower. Thus, if the reptile be flexed as re- 
presented in fig. 241, its length, when extended 
being equal to the right line A 6, becomes 
by flexure only equal in the direction of 
motion to a e. 
In the third order of motion, the reptile 
‘is curved laterally, as in the second, but small 
segments of the trunk are successively flexed 
and extended, and the steps taken do not de- 
_ pend on the time or extent of the oscillations 
of the ribs, but on those of the retractions and 
___ élongations of the segments of the body; and 
_ asthe latter greatly exceed the former in celerity 
_ and amount, the progression in this order is 
vastly greater than in the preceding, which is 
Fig. 
Fig. 241. 
, IA 
the slowest. These modes of progression are 
practised by the common Snake, the collared 
Viper, and other Ophidians. 
n the fourth order, the trunk is arched into 
three or four vertical curves. This is an acce- 
lerated mode of locomotion ; first, the spaces 
taken at each step are large; and secondly, 
because the reptile moves in a straight line. 
When the head 6 is advanced from 6 to A, 
242, 
é h 
Fig.243. 
each point of application to the plane of 
motion will advance in equal spaces, either 
simultaneously or in succession ; in the former 
case, there will be a succession of leaps, with 
greater velocity and expenditure of muscular 
power than in the latter. Many serpents adopt 
this mode of progression, as the Coluber Escu- 
lapii, the Coluber Chersea, and others, (fig. 242.) 
In the fifth order, the progression is effected 
