THE EXTERNAL STRUCTURE OF BIRDS. 159 
leg, is called in Ornithology, as before said, the crus. 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 doa Hawk, a Swift, 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 elong gated, 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 Jong relatively, as 
in the Water-hen, Screamer, and Jacana. 
Though in descriptive Ornitholos ey the upper, single, and 
straight part of the foot above the toes is alw ays, as before 
said, ‘called the tarsus, the inexactitude of this term will be evi- 
dent when the internal skeleton of the leg and foot is under- 
stood. It is always more or less slender and cylindrical, and 
never thick like the upper part of the crus. It may be some- 
what compressed, or laterally pressed in. Rarely, as in the Pen- 
guin, it may be much flattened from behind forwards—. ¢., 
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 toes. 
Birds always walk on their toes, or are what is called dagite- 
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. 
The first toe may also be called the halluw 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 idea, 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. 
