GENERAL CHARACTERISTICS 1471 



to secrete oil for the lubrication of the feathers. This gland, which is most de- 

 veloped in aquatic birds, may be absent, and when present may be either naked or 

 crowned with a tuft of feathers. 



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 oth- 

 ers 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 incased 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, dentirostral, 

 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 ap- 

 position, 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 no- 

 tice. 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 want- 

 ing. 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 horney than the quill, 

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

 are 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 hooklets. 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 a typical 

 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 bar- 

 bules and hooklets. Sometimes the ends of such feathers break up into powder, 

 and they may then be spoken of as powder-down feathers. In another type of feather 

 the vane is rudimentary, and the whole structure then becomes more or less hair-like; 



