CLASS AVES 



391 



and are covered with feathers; the rest of that joint, called the shank, 

 and the toes are naked and covered by horny epidermal scales. The 

 bare portion of the leg is not muscular, containing only tendons passing 

 from muscles in the upper part of the leg to the toes (Fig. 276). In 

 some birds, as in the owls, however, feathers may even extend over the 

 toes themselves. Birds walk only on their toes (Fig. 267). 



Tibiofarsus 



Tarsomeivftarsus 





p^ Phalanges 



(P- 



B 



Fig. 267. — Leg and wing bones of a pigeon to show the homology between the two and 

 the modification of each. A, wing; B, leg. From a specimen. 



The fleshy part of the tail is small and is concealed by the feathers 

 of the trunk. It bears a number of long tail feathers which when spread 

 often present a considerable area. The tail serves as a rudder and an 

 accessory organ of flight, though it is also used in balancing and in some 

 birds in the support of the body when the bird is clinging to a vertical 

 surface. 



423. Feathers. — A typical feather is composed of a quill set into 

 the skin and a continuation of the quill, the .s7m/?, which bears slender 

 harhs (Fig. 268). The barbs in turn bear still smaller and slenderer 

 lateral projections known as barbules. When the barbs lie parallel 

 and the barbules are hooked together so as to make of the whole a 



