50 COIIPARISONS OF STRUCTURE IN ANIMALS, 



by which these alternate movements is accom- 

 plished is very beautiful yet very simple. The 

 claw or talon, we may observe, encases the last 

 or terminal phalangal bone of the toe; now thifj 

 bone is placed lateral to the one preceding, and 

 is articulated to it by a rolling hinge-hke joint; 

 when the talon is draAvn back, it passes down 

 the outer side of the preceding bone, which 

 side is flattened off, so as to ofler no impediment. 

 Thus the claw may be rolled back into a sheath 

 of skin, and again rolled forward and unsheathed 

 when required for use. In the latter case, the 

 action is volimtary, in the former, involuntary; 

 for the rolling back into the sheath is effected 

 by the agency of an elastic spring-ligament 

 which passes from the upper arch of the talon- 

 bone obliquely down to the next bone, where it 

 is firmly secured near the joint. In rolling 

 forward the talon-bone, this hgament is strained 

 on the stretch, and ready to act, the moment 

 the muscular power which brought the talon 

 forward ceases. We see, then, that it is by the 

 contraction of the powerful flexor muscles 

 voluntarily exerted, and which are connected, 

 by means of a very strong tendon, with the 

 base of the talon-bone at its a;aterior part, that 

 the formidable claw is unsheathed ; and when 



