404 THE MECHANICAL FUNCTIONS. 



cility; having, in fact, two thumbs, which are opposable to 

 the two fingers. They have been termed Scansores, or Zi/- • 

 godactyli. 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 pur- 

 poses 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 regarded as having this origin. 

 What confirms this view of the subject, is, that in those 

 birds which have only three toes, namely, in the Emit, the 

 Cassowary, and the Rhea, it is again the inner toe which 

 disappears, leaving only the three outer toes, namely, those 

 which have, respectively, three, four, and five phalanges. 

 The Ostrich has only two toes, one having four, and the 

 other five phalanges; here, again, it is the innermost of the 

 three former, that is, the one having three phalanges, which 

 has been suppressed.* 



A bird is capable of shifting the position of the centre of 

 gravity of its body according as circumstances require it, 

 simply by advancing or drawing back its head. While fly- 

 ing, the neck is stretched forwards to the utmost, in order 

 to bring the centre of gravity immediately under the origin 

 of the wings, by which the body is then suspended. When 

 birds stand upon their feet, they carry the head back as 

 far as possible; so as to balance the body on the base of sup- 

 port. When preparing to sleep, they bring the centre of 

 gravity still lower, by turning the head round and placing 

 it under the w^ing. These motions of the head are again re- 

 sorted to when the bird walks; and the centre of gravity is 

 thus transferred alternately from one foot to the other: hence, 



* The last bone of the outer toe of the ostrich is very small, and being 

 usually lost in preparing the skeleton, has been overlooked by naturalists; 

 but Dr. Grant has ascertained, by the careful dissection of a recent specimen, 

 the existence of tiiis fifth phalanx. 



