Examine in detail some of the feathers. In one of the quill-feathers note the central _stem_ or _shaft_ composed of two parts, a basal hollow _quill_, which bears no web and by which the feather is inserted in the skin, and a longer, terminal, four-sided portion, the _rachis_, which bears on either side a _web_ or _vane_. Each vane is composed of many narrow linear plates, the _barbs_, from which rise (like miniature vanes) many _barbules_. Each barbule bears many fine _barbicels_ and _hamuli_ or _hooklets_. The barbs of the feather are interlocked. How is this effected? The feathers which overlie the whole body and bear the color pattern are called _contour-feathers_. How do they differ from or correspond with the quill-feathers in structure? Soft feathers called _down-feathers_ or _plumules_, cover the body more or less completely, being, however, 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 _vibrissæ_ about the nostrils. In the determination of birds by the use of a classificatory "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 reference 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 careful 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 unfeathered 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. [Illustration: FIG. 133.--Diagrammatic outline of bird's body with names of external parts and regions.]