266 



VERTEBRATE ZOOLOGY 



TV/I 



encies observable in the latter. The only aortic arch is the right 

 (but there is a reduction of the left arch in many reptiles) ; the heart 

 is completely four chambered (but the crocodile heart is nearly so); 

 the red blood corpuscles are nucleated as in the reptile. 



FORMAL LIST OF CHARACTERS OF A TYPICAL BIRD 



External Characters. Body short and spindle-shaped; head, 

 neck and trunk clearly denned; tail short and broad; horny beak with 



patch of swollen skin 

 at base, called cere; 

 two slit-like oblique 

 nostrils between beak 

 and cere; eye with 

 upper and lower lids 

 and a complete third 

 eyelid or nictitating 

 membrane; auditory 

 aperture behind the 

 eye, without external 

 ear, leading to the 

 tympanum; wings al- 

 ready described; legs 

 covered with scales 

 and armed with claws. 

 Feathers (Fig. 143.) 

 A feather is a mod- 

 ified scale, that arises 

 from a dermal papilla 

 and is at first cov- 

 ered with an epidermal 

 sheath. A typical 

 feather consists of a 



FIG. 143. Feathers of pigeon. A , part of a tail 

 feather; B, filoplume; C, nestling down, cal, cala- 

 mus; inf. umb, inferior umbilicus; rch, rachis; sup. 

 umb, superior umbilicus. (From Parker and Has- 

 well.) 



stiff axial rod or stem, 



of which the basal portion is hollow and forms tne qufflor calamus; 

 the distal part is filled with pith and is called the rachis. The rachis 

 supports the vane or flat part of the feather, which is composed of 

 parallel barbs, each barb divided ii^to numerous barbules along either 

 side, that hoolf themselves to barbules of adjacent barbs and thus 

 help to make a coherent plane out of a series of separate parts. Three 



