MOTION. 



447 



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- 

 gation and contraction of small segments of the 

 body. 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 



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 

 scuta, 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 b, becomes 

 by flexure only equal in the direction of 

 motion to a e. 



In the third order of motion, the reptile 



Fig. 241. 



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 

 elongations of the segments of the body ; and 

 as the latter greatly exceed the former in celerity 

 and amount, the progression in this order is 

 vastly greater than in the preceding, which is 



the slowest. These modes of progression are 

 practised by the common Snake, the collared 

 Viper, and other Ophidians. 



In 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 b is advanced from b to //, 



Fig. 242. 



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, (Jig. 242.) 

 In the fifth order, the progression is effected 



