266 



VERTEBRATE ZOOLOGY 



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 Chakactees of a Typical Bird 



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

 neck and trunk clearly defined; 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 hds 

 and a complete third 

 eyelid or mcUtating 

 membrane; auditory 

 aperture behind the 

 eye, without external 

 ear, leading to the 

 tjanpanum; 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 dennal papilla 

 and is at first cov- 

 ered with an epidermal 

 sheath. A typical 

 feather consists of a 

 stiff axial rod or stem, 

 of which the basal portion is hollow and forms the quill or 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 into numerous barbulcs along either 

 side, that hook themselves to barbulcs of adjacent barbs and thus 

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



Fig. 143. — Feathers of pigeon. A, part of a tail 

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

 mus; M)/. uinh, inferior umbilicus; 7t/!, rachis; sup. 

 umb, superior umbilicus. (From Parker and Has- 



well.) 



