STRUCTURE OF BIRDS. 75 



ing. The lateral lines, from which the barbs arise, incline 

 toward the median line of the shaft at this place, as has already 

 been explained, and meet at its commencement. 



At this point there is, in the feathers of a large 

 portion of birds, a plumiform process, or small 

 feather. Fig. 20, 5, which is of the following de- 

 scription : From the fore part of the tube, at the 

 commencement of the shaft, and lying over the 

 aperture by which the internal membrane of the 

 tube escapes, rises a thin lamina, being a con- 

 FiG. 20. tinuation of the substance of the tube. It gra- 



dually narrows, and is continued in the form of a very deli- 

 cate thread, for a greater or less extent. From the sides of 

 this shaft rise two series of barbs, and from the barbs two series 

 of barbules, as in the ordinary feather itself, all the parts being 

 extremely fine, and entirely disunited. The barbules are very 

 much elongated, and loose, resembling in these respects those of 

 the lower part of the webs of feathers in general. This minia- 

 ture feather may be called the accessory feather or plumule. 

 In feathers possessed of this structure, the internal membrane 

 of the tube comes out entire between the accessory feather and 

 the feather properly so called, and is not continued internally 

 along the back of the shaft. 



With respect to this accessory plumule, there is a curious 

 and beautiful gradation among birds, from the diving species, 

 in which it scarcely exists, to the gallinaceous order, in which 

 it is two-thirds of the length of the feather, and the Cassowary 

 and Emu, in which it equals the feather. Previous to the 

 publication of my paper on feathers in the Edinburgh Philoso- 

 phical Journal, the double-feather had been noticed in the 

 Emu, Cassowary, and Ptarmigans ; but in the other birds 

 there mentioned its existence does not seem to have been 

 known. Since that period, however, considerable attention 

 has been paid to the form and texture of feathers, although I 

 am not aware that any person excepting Dr. Richardson and 

 Mr. Audubon have so much as mentioned my observations. 

 In birds possessing this sort of feather, the quills and large 

 tail-feathers, as well as the first row of superior and inferior 



