586 THE MECHANICAL FUNCTIONS. 



joints, which have therefore been termed the 

 ungual bones. This remarkable numercial re- 

 lation among the several bones of the toes exists 

 quite independently of their length. 



There is one whole order of birds which are 

 particularly fitted for climbing and perching 

 upon trees, having the two middle toes parallel to 

 each other, and the inner and outer toes turned 

 back, so as to be opposed to them in their action. 

 They are thus enabled to grasp objects with the 

 greatest facility ; having, in fact, two thumbs, 

 which are opposable to the two fingers. They 

 have been termed Scansores, or Zygodactyli. 

 Almost all other birds have three toes before, 

 and one behind. 



From this enumeration it would appear as if 

 Nature, in modifying the type of vertebrated 

 animals to suit the purposes required in the bird, 

 had purposely omitted one of the toes, which are 

 usually five in number. But instances occur of 

 birds, in which we may trace the rudiment of a 

 fifth toe high upon the metatarsus, and upon its 

 inner side. The spur of the cock may be re- 

 garded as having this origin. What confirms 

 this view of the subject, is, that in those birds 

 which have only three toes, namely, in the Emu, 

 the Cassowary, and the Rhea, it is again the 

 inner toe which disappears, leaving only the 

 three outer toes, namely, those which have res- 

 pectively three, four, and five phalanges. The 



