THE POWDER-DOWN TRACTS. 37 



here. Nevertheless, as this gland is sometimes naked and sometimes clothed with a circlet of 

 feathers, at least at its outlets, it may be regarded either as the bearer of a particular tract when 

 the latter is the case, or when the circlet of feathers is wanting as endowed with a peculiar space, 

 and thus its consideration may be combined with that of the plumage of birds. These con- 

 siderations determined me to give a general description of both these objects at the close of the 

 section on General Pterylography. 



CHAPTER VII. 



Or THE POWDER-DOWN-FEATHERS AND THEIR TRACTS. 



IN certain birds belonging to very different groups, there are down-feathers of very remark- 

 able structure, the shafts of which are never completed at their lower extremities, but continue 

 growing out of the persistent follicle, whilst the upper ends of the barbs are broken off. To 

 these feathers I give the name of Powder- or Dust-down-feathers (Puder- or Staubduneri), because 

 they are constantly pouring out a white or bluish dust from the upper open extremity of the 

 follicle which surrounds the shaft, no doubt the dry residue of the fluid from which this feather 

 is formed. 1 



Such down-feathers, which might, in some degree, be regarded as secretory organs, occur 

 particularly in the lumbar region and on the sides of the back, but likewise in other places. In 

 some birds they are scattered all over the body and not collected into tracts, as in Gypaetos barbatus, 

 in which they produce a yellow dust ;" likewise in RJiynchodon subbuteo and some Parrots, such as 

 Psittaci yaleritus, sulpUureus, dufresnii, ochroccpJudus, alexandri, and pondicerianus. On the 

 other hand, in other birds they form very dense tracts, and always on certain spots, which are 

 not usually covered with contour-feathers. 



I have found these definitely limited powder-down-tracts of very different forms, numbers, and 

 positions, in certain Hawks of the sub-genera Elanus and Circus, and also in Ocypterus, Crypturus, 

 Eurypyya, all the Ardece, and Cancroma. 



In Elanus furcatus there is a simple, large, continuous powder-down-tract on the hinder 

 surface of the back and the lumbar region, accompanying the portion of the spinal tract which is 

 situated over the sacral vertebrae and distinctly separated from the scapular portion, and extending 

 forwards far beyond it. 



In Elanus melanopterus and Cymindis uncinata, on the contrary, the powder-down-feathers 

 form two symmetrical tracts on the sides of the pelvis, and the posterior portion of the spinal 



1 May not this dust be produced by the crumbling of the membrane which intervenes between 

 the feather and the matrix, and which is dried and thrown off in proportion as the latter becomes 

 enlarged ? See my observations on the Genesis of Feathers, p. 9 (B.). 



8 In this bird I could not find any true powder-down-feathers; consequently, if it possesses 

 them, they are probably transitory. 



