38 PTERYLOGRAPHY. 



tract runs through between them. I find them similarly arranged in Circi aruyinosus, pyyaryus, 

 and cinerarias, except that each tract tapers off anteriorly into a narrow band. 



In Ocypterus leucorrliynchus (Plate III, fig. 4) I have found on each side four powder-down- 

 tracts, which form continuations or borders of contour-feather-tracts, in a very peculiar manner, 

 such as I have met with in no other bird. One of them lies close to the rhombic saddle of the 

 spinal tract, the second and third accompanying the two sides of the femoral tract ; the fourth 

 is a nearly square appendage to the extremity of the lateral branch of the inferior tract. 



In Crypturus varieyatus the powder-down-feathers are intruded among the lateral feathers of 

 the great saddle of the spinal tract, and form, with them, a mixed tract, the portions of the 

 spinal tract before and behind this spot being much narrower (Plate VII, fig. 12). 



In the Herons (Plate VIII), the only birds in which these powder-down-feathers were pre- 

 viously known, although their nature was not understood, there are always two large tracts on the 

 hinder part of the hips, and two smaller ones near the furcula ; a third pair, composed of narrow 

 bands, in the inguinal region, is not always present. I have found all three pairs in Ardeee 

 cinerea, purpurea, nycticorax, and ralloides ; the third pair was wanting in Ardece stettaris and 

 minuta, and probably does not occur in any of the Bitterns. 



Cancroma cochlcaria agrees with the true Herons in the form and position of its powder- 

 down-tracts (Plate VIII, figs. 13 and 14). In Eurypyya Jtelias (fig. 15), which I was enabled 

 to examine in Paris by C.uvier's kindness, I found only the two large dorsal tracts, like those of 

 the Kites. 1 



CHAPTER VIII. 

 ON THE ANAL OIL-GLAND. 



THE remarkable gland surrounded by the caudal tract, which is generally known by the 

 name of the oil-yland, and which occurs in most birds, has hitherto been but superficially treated 

 by authors, and by no one carefully examined in many birds ; so that, with the exception of what 

 I have myself stated in my contributions to Naumann's ' Naturgeschichte der Vo'gel Deutschlands' 

 and in some other places, scarcely anything special has been published about it. 2 



It is situated above the last vertebrae of the tail, between the tubes of the rectrices ; it is 

 usually covered only by the skin, but sometimes by a tendon, from which the elevator muscles of 

 the tail originate, and it secretes the oleaginous fluid with which birds lubricate their feathers. 



Essentially it consists of two glandular bodies, more or less united together, but always at 

 their posterior extremity ; and as each lobe is broader and rounded anteriorly, and narrow and 

 pointed behind, where they have a common issue, they together present the general form of a heart. 



1 For some additional remarks ou the powder-down-tracts ia various birds, see Appendix, note 

 1. P.L.S. 



2 Something will be found about it in Tiedemanu's ' Zoologie/ ii, p. 135 ; in Cuvier's 

 ' Lejons sur 1' Anatomic Comparee,' v, p. 2GO ; and also in Blainville, ' De 1'OrganiEation des 

 Animaux,' i, p. 104. 



