86 GENERAL ORNITHOLOGY. 



without any ineans of holding together, the feather would have uo texture or consistency — 

 there would be no true web; therefore they are connected by means of 



(5) Barbules or radii (Lat. radius, a ray ; pi. radii). Just as the rhachis bears its two 

 series of barbs, so does each barb bear two series of processes or plates of the second order. 

 These barbules, as they are called (Lat., diinin. of barba), or radii, are to the barbs exactly 

 what the barbs are to the main shaft, and are similarly given off from both sides of the thick 

 upper border of a barb ; they make the vane truly a web — that is, they so connect the barbs 

 together that some slight force is required to pull them apart. Barbules are variously shaped, 

 but generally flat sidewise, to pack together closely, with an upper and under edge at base, 

 rapidly tapering to a slender thready end; and are long enough for each one to reach obliquely 

 over several barbules of the next barb. Their number on most feathers is very great ; a feather 

 with a few hundred barbs to each web may have several hundred thousand barbules. All the 

 structures thus far described may be seen by the naked eye or with a simple pocket lens ; but 

 a microscope is required to make out the minute structures by means of which the barbules 

 confer consistency on the bai'bs. These are the 



(6), (7) Barbicels or cilia (barbicel, another dimin. of Lat. barba ; and cilium, an eye- 

 lash; pi. cilia), and booklets or hamuli (Lat. hamulus, a little hook, dimin. of hamus, 

 a hook; pi. hamuli). Both of these minute structures are simply a sort of fringe to a 

 barbule, as if the end and part of the lower edge of the barbule were frayed out, and only 

 differ from each other in that barbicels are plain hair-like processes, while hamuli are hooked 

 at the end ; they are not found on all feathers, nor on all parts of any feathers. There are 

 countless millions of barbicels and hamuli on the main feathers of every bird which has smooth 

 webby surface plumage and well-formed wings and tail ; but their absence characterizes all 

 neossoptiles, all supplementary feathers, and all the downy or hairy under plumages to be 

 presently noticed. Barbicels occur on both anterior and posterior rows of barbules, though 

 rarely on the latter; booklets are confined to anterior series of barbules, which, as we have 

 seen, overlie the posterior rows, forming a diagonal mesh-work. The purpose of this beauti- 

 ful structure is evident ; barbules are interlocked, and the whole made a web ; for each booklet 

 of one barbule catches hold of a barbule from the next barb in front, any barbule thus holding 

 on to as many barbules of the next barb as it has booklets ; while, to facilitate this interlock- 

 ing, barbules have a thickened or folded-over upper edge of the right size for booklets to grasp. 

 The aiTaugement is shown in fig. 22, where a, a, a, a, are four barbs in transverse section, 

 viewed from the cut surfaces, with their anterior, b, b, b, b, and posterior, c, c, c, c, barbules, 

 the former bearing the booklets which catch over the edge of the latter. 



Types of Feathery Structure. — But all feathers do not answer the above complete 

 description. The aftershaft may be wanting, as we have seen. Booklets may not be devel- 

 oped, as frequently happens. Barbicels may be few or entirely lacking. Barbules may be 

 similarly deficient, or so defective as to be only recognized by their position and relations. 

 Even barbs may be few or lacking on one side of the shaft, or on both sides, as in certain bristly 

 or hair-like styles of feathers. Finally the main stem may be a mere filament, without obvious 

 distinction of calamus and rhachis. Consideration of these and other modifications of feather- 

 structure has led to recognition of three types or plans : 1. The perfectly feathery, plumoiis, or 

 pennaceous (Lat. pluma, a plume, or penna, a feather fit for writing with ; fig. 23), as above 

 described. 2. The downy or plumulaceous (Lat. plumula, a little plume, a down-feather), 

 when the stem is short and weak, with soft rhachis and barbs, long slender thready barbules, 

 little knots in place of barbicels, uo booklets, and consequently no smooth webbing. 3. The 

 hairy, bristly, or filoplumaceous (Lat. filum, a thread), with a very long, slender stem, rudi- 

 mentary or very small vanes composed of fine cylindrical barbs and barbules, if any, and no 

 barbicels, knots, or booklets. There is no abrupt definition between tliese types of structure; 



