335 



CHAPTEE XV. 



B. SAUKOPSIDA — Continued. 

 II. — AvEs (Birds). 



Birds 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 : they are dermal outgrowths, formed in 

 sacs from small papillae of dermal origin. A typical feather 

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

 or "quill" ((7), by which it is inserted in the dermal papUla; 

 part of the quiH 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-caUed "web," the 

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

 vexUlum is built 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 "quiU- feathers." The longest quiU- 



