40 
03Sr THE PHYSIOaNOMY OF SERPENTS. 
which is in the centre, is slightly elevated above the other 
parts ; hut having the faculty of bending the body in a 
thousand different positions, they are often found simply 
extended on the ground with the body undulated in si- 
nuous contours. To produce progression, the serpent 
has merely to unrol his spires ; resting himself afterwards 
on his tail, rebending his body in successive lateral undu- 
lations, and, applying to the ground the numerous points 
of contact which the anterior extremities of the ribs pre- 
sent, the reptile is pushed forward and transported with a 
celerity proportional to the efforts or power of the instru- 
ments of locomotion. We have already remarked on this 
head, that the progressive movements of Ophidians are 
almost all executed in the same manner, and that the tail 
but aids the locomotion differently, according to the mo- 
dification which its form undergoes in the different tribes. 
Very often, in order to observe what passes around them, 
serpents raise themselves perpendicularly, supporting 
themselves solely on the tail, or on a part of the abdo- 
men ; their trunk is then rigid, and perfectly straight ; 
and most frequently the head is then bent and directed 
forwards : at other times they bend their bodies as an S, 
inflating their necks in this position. Suspended perpen- 
dicularly from the branch of a tree, the Boa resembles a 
stiff body without life. In descending from a tree or other 
tall object, serpents let themselves simply fall to the 
ground, — their form, and the elasticity of their parts, pre- 
venting any dangerous consequences from this fall ; on 
attaining the ground, the shock they sustain, instead of 
proving hurtful, impels them forward, and serves as a 
stimulus to their subsequent movements. 
Serpents have been repeatedly described, that can per- 
form perfectly a retrograde movement. This peculiarity 
has been especially attributed to those burrowing snakes 
that have cylindric bodies, terminated by a tail very thick 
and obtuse at the extremity ; but as this has neither been 
certified by well-informed travellers, nor by professed na- 
turalists, there is room for doubt on this point ; perhaps 
it owes its origin to the prejudices^ of the ancients, who 
* Plin. VIII. 35. ^LiAN, IX. 23. 
