OF THE BONES OF 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 vertebrce, 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 duninish in size. 



OF THE BONES OF THE TRUNK. 



As the vertebra 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 

 w^hich are their levers, the number of the plates on the 

 lower surface of the trunk of serpents should equal that of 

 the ribs and vertebrte. 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 



