320 THE MECHANICAL FUNCTIONS. 



as to be put upon the stretch, when the former are in action. 

 By the contractions of these muscles, the orifices of the cavi- 

 ties, or sacs to which they belong, are opened, and the serrated 

 edges applied accurately to the surfaces with which the feet 

 are in contact. Sir Everard Home, in his account of this 

 structure, compares it to the sucking disk of the Remora.^ 

 By its means the animal is enabled to walk securely upon 

 the smoothest surfaces, even in opposition to the tendency 

 of gravity. It can run very quickly along the walls or ceil- 

 ing of a building, in situations where it cannot be supported 

 by the feet, but must depend altogether upon the suspension 

 derived from a succession of rapid and momentary adhe- 

 sions. 



Although the Sauria are better formed for progressive 

 motion than any of the other orders of reptiles, yet the 

 greater shortness and oblique position of their limbs, com- 

 pared with those of mamrniferous quadrupeds, obliges them 

 in o-eneral to rest the weight of the trunk of the body on 

 the ground, when they are not actually moving. None of 

 these reptiles have any other kind of pace than that of walk- 

 ing, or jumping; being incapable of performing either a trot 

 or a gallop, in consequence of the obliquity of the plane in 

 which their limbs move. The Chameleon walks with great 

 slowness and apparent difficulty; and we have seen that, in 

 consequence of the structure of the bones of its neck, the 

 Crocodile^ though capable of swift motion in a straight line, 

 is unable to turn itself round quickly. The general type 

 of these reptiles, having reference to an amphibious life, has 

 not attained that exclusive adaptation to a terrestrial exist- 

 ence, which we find in the higher orders of the Mammalia. 

 But before proceeding to consider these, we have to notice 

 a sino-ular group of animals, whose conformation appears to 

 be exceedingly anomalous, and as if it interrupted the regu- 

 larity of the ascending series, of which it seems to be a col- 

 lateral ramification. 



* Philosophical Transactions for 1816, p. 151, and 323. 



