335 



CHAPTER XV. 



B. SAUKOPSiDA — Coiitinuecl. 

 II. — AvES (Birds). 



BiEDS are oviparous warm-blooded vertebrates, with a complete 

 double circulation, and are covered with feathers. Their body 

 temperature is about 104° F. The feathers are analogous to 

 the hairs in mammals : thej' are dermal outgrowths, formed in 

 sacs from small papillte of dermal origin. A typical feather 

 (fig. 173) consists of the following parts: (1) The "calamus'' 

 or "quill" (C), by which it is inserted in the dermal papilla; 

 part of the quill is hollow, the basal part being spongy in- 

 ternally. (2) The "rachis" [R), which forms the shaft, and 

 which is simply the continuation of the quill : on one side the 

 shaft is grooved, internally it is filled with a soft pithy sub- 

 stance. On each side of the shaft is a so-called " web,'' the 

 two webs forming (3) the " vexiUum " or "vane" (F). The 

 vexiUum is biiilt up of a number of " barbs " {B), the barbs being 

 united by small hook-like " barbules," which attach the barbs 

 together towards the distal part of the feather, but not towards 

 their base. At the junction of the rachis and quill is found 

 a small accessory feather, the "aftershaft" or " hyporachis " ; 

 this may be large or simply reduced to a tuft of " down,'' in 

 which the barbs are disconnected. The feathers vary in 

 structure in different parts of the body. Those of the tail 

 and wings are called " quill -feathers." The longest quiU- 



