590 THE MECHANICAL FUNCTIONS. 



The stork, and some other birds belonging to 

 the same order, which sleep standing on one 

 foot, have a curious mechanical contrivance for 

 locking the joint of the tarsus, and preserving 

 the leg in a state of extension without any 

 muscular effort. The mechanism is such as to 

 withstand the effect of the ordinary oscillations 

 of the body, when the bird is reposing ; but it is 

 easily unlocked by a voluntary muscular exer- 

 tion, 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 

 distinguish the different orders and families. 

 Some of the more remarkable deviations from 

 what may be considered as the standard confor- 

 mation, may, however, for a moment arrest our 

 attention. 



The Ostrich is of all birds the one that pre- 

 sents the greatest number of exceptions to the 

 general rules which appear to regulate the 



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

 actions of the Royal Irish Academy, vol. xiii, p. 20, anfl is more 

 fully described in Rees's Cyclopeedia, Art. BIRD. He observes 

 that both Cuvier and Dumeril have committed an error in re- 

 ferring this peculiarity of structure to the knee instead of the 

 tarsal joint. 



