GENERAL CHARACTERISTICS. 299 



Ornithologists have devised a number of terms to indicate the different parts 

 of a body of a bird, several of which are located in the accompanying diagram ; 

 the others it will be unnecessary to mention here. It may be observed, however, 

 that the ears of Birds are unprovided with external conchs, merely opening flat on 

 the sides of the head, usually a little behind and below the eyes. The eyes, which 

 are in most cases placed laterally and near the middle of the head, are provided 

 with a third eyelid, or nictitating membrane, which can be drawn obliquely like a 

 shutter over the eyeball, while the proper eyelids remain open ; as may be observed 

 in a captive owl or eagle, when the glistening white membrane will be seen from 

 time to time to sweep across the eye with extreme rapidity. 



The beaks of birds, which, as we have said, are always encased in horn or 



leathery skin, have different terms applied to them, according to their relative 



length and form; the meaning of most of such terms, as fissirostral, dentirost rt 1 /, 



conirostral, etc., being self -apparent. A horny investment is also generally 



present on such portions of the lower part of the legs as are devoid of feathers ; 



although in some cases, as in the ducks, this is replaced by a more or less leathery 



skin. The horny covering of the metatarsus may consist of small pieces, with the 



edges in apposition, as in the plovers, when it is said to be reticulate ; but frequently 



the front surface, as in the fowls, has a number of broad overlapping plates, when 



it is termed scutate. Occasionally each side is invested by a single greave-like 



plate, meeting its fellow in a prominent ridge at the back. 



The feathers of birds being all-important need a somewhat fuller 

 Plumage. . . 



notice. A feather in its most complete state of development consists 



of a main stem, and a secondary stem, or after-shaft; but the latter is frequently 



wanting. The base of the main stem is formed by the hollow horny quill, the 



lower end of which is pointed and inserted into the skin. The upper part of the 



quill passes into the shaft, or rachis, at a point marked by a small aperture termed 



the upper umbilicus. The shaft is four-sided, elastic, pithy, and less horny than 



the quill ; and gradually tapers at its extremity to a fine point. On either side of 



the shaft are the two webs, collectively forming the vane of the feather. Each 



web or half of the vane, one of which is generally considerably wider than the 



other, is composed of a series of flattened plates closely applied to one another, and 



diverging from the shaft at an open angle, each plate terminating in a point. 



These plates form the barbs, and they are held together by barbules, given off in 



the same manner as are the barbs from the stem; while the barbules may again 



give off hooJdets. The after-shaft is, when fully developed, a miniature of the main 



stem, from which it is given off at the junction of the quill with the shaft. Such 



is the structure of atypical feather; but the soft feathers known as down have 



the stem short and weak, or even wanting, while the barbs are soft and not held 



together by fully-formed barbules and booklets. Sometimes the ends of such 



feathers break up into powder, and they may then be spoken of as poivdi r-down 



fathers. In another type of feather the vane is rudimentary, and the whole 



structure then becomes more or less hair-like : to such the term thread-feathers is 



applicable. Finally, the feathers covering tin- body and concealing the underlying 



down are conveniently referred to as the contour-feathi rs. 



Instead of being evenly distributed over the body of a bird, the feathers grow 



