THE EXTEBlfAIi STErCTUEB OF BIEDS. 159 



leg, is called in Ornithology, as before said, the cms. It is also 

 thick at its upper part, but tapers downwards, becoming much 

 contracted towards the ankle-joint. 



The pelvic limbs vary much in structure, in harmony with the 

 very different uses to which they may be put in creatures which 

 differ so much in modes of life as do a Hawk, a Svsift, a Heron, 

 and a Penguin. 



The entire length of the whole limb varies greatly, being rela- 

 tively very short in the Swift and frigate-bird, and relatively 

 extremely long in the Stork, the Flamingo, and the Stilt. When 

 the limbs are very elongated, it is the leg and upper part of the 

 foot which are mainly lengthened. The thigh is always rela- 

 tively short. The toes may be exceptionally long relatively, as 

 in the Water-hen, Screamer, and Jacana. 



Though in descriptive Ornithology the upper, single, and 

 straight part of the foot above the toes is always, as before 

 said, called, the tarsus, the inexactitude of this term wiU be evi- 

 dent when the internal skeleton of the leg and foot is under- 

 stood. It is always more or less slender a ad cylindrical, and 

 never thick like the upper part of the crus. It may be some- 

 what compressed, or laterally pressed in. Earely, as in the Pen- 

 guin, it may be much flattened from behind forwards — i.e., 

 decidedly widest in the transverse diameter. 



The most variable part of the leg is, as might be expected, 

 the foot, and especially the number, shape, and disposition of 

 the digits or tdes. 



Birds always walk on their toes, or are what is called digiti- 

 grade, with the exception of the Penguin, which applies its 

 tarsus also to the ground, and is thus the only plantigrade kind 

 of Bird. 



The number of the digits varies from four, which are present 

 in the immense majority of birds, to two — a number present 

 only in the Ostrich. 



Thej/?rst toe may also be called the TiaUuce and compared with 

 the great toe in ourselves. It has almost always two joints, 

 not including that at its base. 



The second toe may also be called the index, and compared 

 with that which is next to our great toe. It has always, with 

 two exceptions *, three joints. 



The third toe may also be Called the medius and compared 



* The genera Ceyx and Alcyone. 



