REPTILES. 281 



terminate in cartilaginous processes united to the scales of 

 the belly *. 



The progressive motion of serpents is executed by the 

 assistance of the ventral scales, which serve as so many feet. 

 The body is raised up into one or more arches, and then 

 suddenly unbent, is pushed forwards, the posterior edges 

 of the scales, opposed to the ground at the hinder ex- 

 tremity of the circle, offering sufficient resistance. By 

 thus alternately elevating and unbending the body, ser- 

 pents move in their characteristic manner backwards or 

 forwards, with an astonishing degree of rapidity. When 

 at their utmost speed they appear scarcely to touch the 

 ground. In their more ordinary movements, the ventral 

 plates or scales serve as feet. These scales slide under 

 each other by a kind of inclusion, so as to permit the ven- 

 tral surface to shorten or lengthen at the will of the animal. 

 When some of the foremost scales are pressed on the 

 ground, those behind are brought forward, and in their 

 turn supporting the body, enable the fore part to advance. 

 To qualify the scales to do this with greater advantage, 

 they are connected with one another, by means of muscular 

 threads and a longitudinal band, and are likewise aided by 

 the peculiar mechanism of the ribs. These last we have 

 seen are connected with the ventral scales by a flexible car- 

 tilage. They are capable of moving on their vertebral 

 joint, either ventrally or dorsally, anteriorly or posteriorly. 

 These motions are aided by the muscles which arc inserted 

 into them, and originate in ribs contiguous to those to be 



* M. LACEPEDE has remarked, in the viper, boa, and rattlesnake, that there 

 are uniformly two ribs, and one vertebra, corresponding to each shield of 

 the belly. In one small group, some of the ribs are united by cartilaginous 

 productions on the belly, and there is even the rudiment of a sternum. 



