Page:First course in biology (IA firstcourseinbio00bailrich).pdf/394

 by pressing together the two sides of a split in the vane? Does the web separate at the same place when pulled until it splits again?

, enlarged.

The hollow part of the shaft of a quill feather is called the quill. The part of the shaft bearing the vane is called the rachis (rā-kis). The vane consists of slender barbs which are branches of the shaft (II, Fig. 292). As the name indicates (see dictionary), a barb resembles a hair. The barbs in turn bear secondary branches called barbules, and these again have shorter branches called barbicels (III, Fig. 292). These are sometimes bent in the form of hooklets (Fig. 292, III), and the hooklets of neighboring barbules interlock, giving firmness to the vane. When two barbules are split apart, and then reunited by stroking the vane between the thumb and finger, the union may be so strong that a pull upon the vane will cause it to split in a new place next time.

, enlarged.

There are four kinds of feathers, (1) the quill feathers, just studied; (2) the contour feathers (I, Fig. 292), which form the general surface of the body and give it its outlines; (3) the downy feathers (Fig. 293), abundant on