406 THE MECHANICAL FUNCTIONS. 



the force with which the toes grasp the perch. All this 

 takes place without muscular effort or volition on the part 

 of the bird. It remains in this position with more security 

 on one foot than it would have done by resting upon both; 

 because, in the latter case, the weight of the body being di- 

 vided between them, does not stretch the tendons sufficient- 

 ly. In this position, the bird not only sleeps in perfect se- 

 curity, but resists the impulse of the wind and the shaking 

 of the bough. 



The great length of the toes of birds enables them to stand 

 steadily on one leg: and in this attitude many employ the 

 other foot as a hand; especially parrots, whose head is too 

 heavy to be readily brought to the ground. Some birds, 

 which frequent the banks of rivers, are in the practice of 

 holding a stone in one foot, while they rest upon the other: 

 this contributes to increase their stability in two ways; first, 

 it adds to the weight of the body, which is the force that 

 stretches the tendons, and causes them to grasp the bough; 

 and, secondly, it also lowers the centre of gravity. 



The stork, and some other birds belonging to the same 

 order, which sleep standing on one foot, have a curious me- 

 chanical contrivance for locking the joint of the tarsus, and 

 preserving the leg in a state of extension without any mus- 

 cular effort. The mechanism is such as to withstand the ef- 

 fect of the ordinary oscillations of the body, when the bird 

 is reposing; but it is easily unlocked by a voluntary muscu- 

 lar 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 me- 

 chanism of birds; as it would exceed the limits within which 

 I must confine myself, to enter more fully into the peculi- 

 arities which distinguish the different orders and families. 



* This mechanism is noticed by Dr. Macartney, in the Transactions of the 

 Royal Irish Academy, vol. xiii. p. 20, and is more fully described in Rees's 

 Cyclopjedia, Art. Bird. He observes that both Cavier and Dumeril have 

 committed an en-or in referring this peculiarity of structure to the knee in- 

 stead of the tarsal joint. 



