460 THE MECHANICAL FUNCTIONS. 



body, apparently for the purpose of supporting 

 the abdominal viscera.* The bones of the ex- 

 tremities are very perfectly formed, approaching 

 in their shape and arrangement very nearly to 

 the corresponding parts of the skeleton of the 

 higher orders of quadrupeds. The toes are 

 usually provided with membranes spread be- 

 tween them, to assist in swimming. The form 

 of the tail, which is generally compressed ver- 

 tically, like that of fishes, though perhaps not to 

 an equal degree, is another indication of their 

 being formed for an aquatic life : for where the 

 tail has this shape, we always find that the 

 chief muscular power is bestowed upon it as an 

 instrument of aquatic progression, producing, by 

 its lateral flexions, a horizontal movement of the 

 body. Crocodiles and alligators, for instance, 

 which have this conformation, are comparatively 

 weak when on land, and as soon as they have 

 seized their prey, their efforts are always directed 

 to drag it with them into the water; knowing 

 that when in their own element they can readily 

 master its struggles, and dispose of it as they 

 please. 



In the Gecko tribe, we find a particular me- 

 chanism provided for effecting the adhesion of 

 the feet to the objects to which they are applied. 



* They appear to be analogous to the marsupial bones peculiar 

 to a family of mammalia. 



