OP THE BOKES OP THE TRUNK. 
27 
The lateral undulations of the body suffice for this sort of 
locomotion, and it is only the sea-serpents which make use 
of their tail, expressly fashioned for the purpose, as an 
oar and a rudder. When the locomotion is performed on 
a solid body, the ribs, putting in motion the abdominal 
plates, thus form a series of levers, which maintain the 
impulsion produced by the undulations of the trunk, in 
alternately rising and falling, and touching with their an- 
terior extremities the plane of position. The degree of 
celerity of the locomotion depends, in a great measure, on 
the nature of the body on which the serpent moves : it draws 
itself with difficulty over a mirror, or a table with an uni- 
form and polished surface ; but it escapes with celerity on 
sandy ground, or a surface covered with a dry vegetation, 
as a heath. To climb up perpendicular objects, it knows 
how to avail itself of every little protuberance which offers 
a fulcrum to the articulations of its abdomen. 
To exercise these functions, it is requisite that the bones 
composing the trunk of serpents, as well as their muscles, 
should be properly arranged. Every one,, at the first glance, 
will be struck with the multiplicity and uniformity of these 
parts. All the vertebrse, all the ribs, are similar to each 
other, with some exceptions, as regards their shape, and it 
is only towards the extremities of the animal that these 
bones diminish in size. 
OE THE BONES OF THE TRUNK. 
As the vertebrae of the trunk support all the ribs, the 
usual distinction between cervical, dorsal, and lumbar ver- 
tebrae does not hold in serpents ; and it follows, that the 
number of the ribs on each side should always equal that 
of the vertebrae ; and also, that as the scaly abdominal 
articulations of the skin always correspond to the ribs 
which are their levers, the number of the plates on the 
lower surface of the trunk of serpents should equal that of 
the ribs and vertebrae. Every one knows that their number 
differs, not only according to the species, but also in indi- 
viduals, so that sometimes we find in serpents of the same 
species a difference of 30 or 40 vertebrae, more or less. 
The number of vertebrae of the trunk, and consequently 
