BIRDS 141 



connect upper arm and fore-arm in front, and upper arm and trunk 

 behind, there is a certain resemblance to the bat's wing, but we 

 do not find in the bird long slender fingers supporting a well- 

 developed membrane, but a reduced hand possessing only three 

 modified digits. The efficient part of the wing consists of feathers, 

 and the limb is specialized so as to serve as a firm support for 

 these. 



When a bird is not flying it is supported by the hind -limbs, 

 but the problem of bipedal progression is not solved here in the 

 same way as in a human being (see p. 66), where the trunk is 

 raised into a vertical position and brought into the same line 

 as the legs. The hind-limbs of a bird are, as it were, shifted 

 forwards so that the body is balanced somewhat obliquely between 

 them. There are, however, some forms, such as Penguins (fig. 

 127), in which the trunk is raised into an approximately upright 

 position, but even then the resemblance to the attitude of an erect 

 human being is remote, for not only is the thigh attached as in 

 other birds, but it points slantingly upwards instead of being 

 directed downwards. It would not indeed be possible for a bird 

 to completely straighten out its leg, for the thigh is more or less 

 enclosed in the boundaries of the trunk, and is not entirely free, 

 as in ourselves. 



It is also noticeable that the regions of a bird's leg, with the 

 exception of the uppermost one, or thigh, do not exactly corre- 

 spond with the subdivisions found in a mammal. The other 

 two sections may be called, for convenience, lower leg and foot, 

 but the ankle-joint between them is not in exactly the same 

 position as in the last-named group. This will be more fully 

 explained in dealing with the endoskeleton. The digits are 

 four in number, instead of five as in a typical mammal, the digit 

 corresponding to the little toe of which being absent. The great 

 toe is turned backwards, and, since the body is supported upon 

 the digits while the metatarsus and tarsus are raised from the 

 ground, the bird may be termed digitigrade. 



Skin and Exoskeleton. — The thin skin is made up, as in a 

 mammal, of epidermis and dermis, and the former gives rise to 

 a well-marked exoskeleton, consisting of beak, claws, scales, and 

 feathers. The beak comes under this heading in so far as the 

 horny sheaths which cover both upper and lower jaws are con- 

 cerned, while the toes are provided with claws, and the feet are 



