FEET OF BIRDS. 521 



state of extension without any muscular effort. The 

 mechanism is such as to withstand the effect of the 

 ordinary oscillations of the body, w^hen the bird is 

 reposing ; but it is easily unlocked by a voluntary 

 muscular exertion, when the limb is to be bent for 

 progression. On these occasions the ball of the 

 metatarsal bone is driven with some force into the 

 socket of the tibia.* 



I must content myself with this general view of 

 the mechanism of birds ; as it would exceed the 

 limits within which I must confine myself, to 

 enter more fully into the peculiarities which dis- 

 tinguish the different orders and families. Some 

 of the more remarkable deviations from what may 

 be considered as the standard conformation, may, 

 however, for a moment arrest our attention. 



The Ostrich, of all birds, presents the greatest 

 number of exceptions to the general rules which 

 appear to regulate the conformation of birds, and 

 in many of its peculiarities of structure it makes 

 some approach to that which characterizes the 

 quadniped. Though this bird is provided with 

 wings, it was evidently never intended that they 

 should be used for the purposes of flight. Hence 

 the chief muscular power has been bestowed on 

 the legs, which are remarkably thick and strong, 

 and well fitted for rapid progression. The sternum 

 is flat, and does not present the keel-like projection 

 which is so remarkable in that of all other birds. 



* This mechanism is noticed by Dr. Macartney in the Transac- 

 tions of the Royal Irish Academy, vol. xiii. p. 20, and is more fully 

 described in Rees's Cyclopaedia, Art. Bird. He observes that botli 

 Cuvier and Dumeril have committed an error in referring this pecu- 

 liarity of structure to the knee instead of the tarsal joint. 



