518 THE MECHANICAL FUNCTIONS. 



five in niunber. But instances occur of birds, in 

 Avliicli we may trace the rudiment of a fifth toe 

 high upon the metatarsus, and vipon its inner side. 

 The spur of the cock may be regarded as having 

 this origin. What contirms this view of the sub- 

 ject 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 respectively three, four, and five pha- 

 langes. 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 sup- 

 pressed. The last bone of the outer toe of the 

 ostrich is very small, and being usually lost in pre- 

 paring the skeleton, has been overlooked by natu- 

 ralists ; but Dr. Grant has ascertained, by the 

 careful dissection of a recent specimen, the exist- 

 ence of this fifth phalanx. 



A bird is capable of shifting the position of the 

 centre of gravity of its body, according as circum- 

 stances require it, simply by advancing or drawing- 

 back its head. While flying, 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 support. When preparing to 

 sleep, they bring the centre of gravity still lower, 

 by turning the head round and placing it under 

 the wing. These motions of the head are again 

 resorted to when the bird walks ; and the centre 



