524 THE MECHANICAL FUNCTION. 



execute still more complex actions with their 

 fore feet, have perfect clavicles, extending from 

 the shoulder to the chest, and connecting the 

 bones of the anterior extremity with the general 

 frame-work of the skeleton. This is the case 

 in a large proportion of the family of Rodent ia, 

 such as the Squirrel, which employs its paws 

 for holding objects ; and the Beaver* which 

 likewise makes great use of its feet for moving 

 and arranging the materials of its habitation. 

 Animals that dwell in trees, and require to grasp 

 with force the branches in moving along them, 

 such as the Sloth, have also distinct clavicles. 

 Animals which rake or dig the ground, as the 

 Mole, the Ant-eater, and the Hedge-hog are all 

 provided with these bones, which, by keeping 

 the shoulders at the same constant distance from 

 the trunk, and affording a firm axis for the rota- 

 tory motions of the limb, materially assist them 

 in the performance of these actions. 



* The beaver presents a singular modification in the structure 

 of the tail, which is expanded into a flattened oval disk, covered 

 by a skin beset with scales : the whole forming a mechanical 

 instrument, which may be compared to a trowel, exceedingly 

 well adapted for the purposes to which it is applied by the animal 

 in constructing its mud habitation. 



