32 COMrARISONS OF STRUCTURE IN AKIMALS. 



fingers contributes, no less than to nicety of 

 perception in testing the quality of the surface 

 of objects, and which, had the cuticle been 

 smoothly polished, would have been at a much 

 lower ratio. To persons accustomed to heavy 

 labour — as the blacksmith and ploughman — the 

 cuticle becomes much thickened, and operates 

 as a defence to the nervous tissue, though at 

 the same time it interferes with delicacy of tact. 

 " The hand of little labour has the nicer sense." 

 Such is a general sketch of the human hand 

 considered restrictively ; and conformable to it 

 is the arm, according to that principle of 

 harmony which ever prevails in organic 

 bodies. The peculiar flexibility of the wrist 

 joint, the power of flexure, and of pronation 

 and supination enjoyed by the fore-arm, and 

 the amazing freedom of the shoulder -joint, 

 which permits the arms to be whirled about, 

 are all essential to the efficacy of the hand, and 

 the multitude of piu-poses to which it is ap- 

 plied. It is in a shallow socket of the blade- 

 bone (scapula) that the shoulder-bone (humerus) 

 moves freely ; and the blades are kept in 

 their position by two clavicles, or collar-bones, 

 which extend between them and the top of the 

 breast-bone. Thus are the shoulders kept 



