BRANCH CHORD AT A; CLASS AVES: THE BIRDS 329 



mostly hidden by the contour-feathers ; the barbs of these 

 are sometimes not borne on a rachis, but arise as a tuft 

 from the end of the quill. Certain other feathers which 

 have an extremely slender stem and usually no vane, 

 except a small terminal tuft of barbs, are called thread- 

 feathers, or filoplumules. They are rather long, but are 

 mostly hidden by the contour-feathers. In certain birds 

 they stand out conspicuously, as the vibrissce about the 

 nostrils. 



In the determination of birds by the use of a classifica- 

 tory "key" (see p. 359 it is necessary to be familiar 

 with the names applied to the various external regions of 

 the body and plumage, and with the terms used to denote 

 the special varying conditions of these parts. By refer- 

 ence to figure 133 the names of the regions or parts most 

 commonly referred to may be learned. A full account 

 of all of the external characters with definitions of the 

 various terms used in referring to them may be found in 

 Coues's " Key to North American Birds." 



TECHNICAL NOTE. Pull the feathers from the body, being care- 

 ful not to tear the skin. 



In the fish and toad, already studied, the head is closely 

 joined to the trunk. How is it with the bird ? Observe 

 that the knee of the sparrow is covered by feathers and 

 that it is the ankle which extends down as the bare un- 

 feathered part to the digits. How many digits have the 

 feet of the bird ? How are they arranged ? 



Internal structure (fig. 132). TECHNICAL NOTE. With a 



pair of scissors cut just beneath the skin anteriorly from the cloacal 

 opening to the angle of the lower jaw. Pin the sparrow on its back 

 by the wings, feet, and bill. Push back the skin from both sides 

 and pin out. 



Note the large powerful pectoral muscles. Note a hard 

 median projection of bone, the stern urn, which is a large 



