THE OIL-GLAND. 3!) 



Internally the gland consists of parallel, filiform, closely packed, secretory tubules, which com- 

 mence in caecal extremities, and gradually unite into several stems or sacs, which open either into 

 a common cavity of very variable size or directly at the end of the elongated posterior 

 process. 1 



The oil-gland is largest in the water-birds and in those aerial birds which go into the water ; 

 in the others it is smaller. I find it of the largest proportional size in Pandion, Dysporm, Sterna, 

 and Proccllaria ; the comparatively smallest that I have seen is in Caprimulym europceus. 



It appears to me to be very remarkable that this gland is regularly deficient in certain 

 birds. 2 Thus, I have been unable to find it in any specimen of the Common Bustard (Otis 

 t(trda) examined by me, and also in two of the Little Bustard (Otis tetrax] which were killed 

 near Halle. In the same way it is wanting in Casuarius nova hottandice, of which I was 

 enabled by Cuvier to examine two specimens in Paris, and have since obtained a third for the 

 Academic Collection at Halle ; likewise in the same number of individuals of Casuarius indicus 

 which lately died in Halle, and the skins of which are now set up in the Zoological Museum of 

 the University. The other cursorial birds, to which I have given the name of Platysterna ; such 

 as Strutldo camelus and Rhea americana, are also destitute of the oil-gland. In Aptenodytes, on 

 the contrary, to which I formerly denied the gland, it is really present, but is concealed beneath very 

 stiff feathers. It is wanting, however, in many of the American Parrots, of which I have repeatedly 

 examined fresh specimens, as, for instance, in Psittaci rufirostris, ILLIG., dominicensis, leucocephalus, 

 ochrocephalus, dufresnii, menstruus, imAjjurpureus, and this is the more remarkable as most of 

 the other species, among which are some very nearly allied to those just mentioned, possess very 

 perfect oil-glands. However, other similar apparent anomalies occur, and I may mention, among 

 those with which I am acquainted, the deficiency of the gland in Columba coronata and C. mili- 

 faris, and also in Argus giyanteus, although the nearest relatives of these birds are well provided 

 with it. 



But even leaving these exceptions out of the question, there are many remarkable differences 

 in this organ. I regard it as a difference of great importance whether the elongated issue of the 

 gland is furnished with a circlet of feathers at the end or destitute of it. This circumstance 

 always furnishes a definite group-character it is either an indication of family, or at least a 

 generic character. Thus, I find the circlet of feathers above mentioned in all the Diurnal Birds of 

 Prey (with the exception of CatJiartcs), in the Picinee, Amphiboly, PsittacintE, Lipoglossa, Galli- 

 naccce, and all the Aquatic Birds ; on the other hand, it is deficient "in the Nocturrial Birds of 

 Prey, Passerine, MacrocMres, Cuculincn (except Prodotes s. Indicator), and the Columbints. 



The plumage of the skin over the gland itself stands in near relation to the absence or 

 presence of this circlet of feathers. Thus, it is always wanting in those birds in which the circlet 

 of feathers at the orifice is deficient, but in the opposite case it is usually present ; but even then it 

 presents differences, the feathers sometimes standing close together, sometimes at greater distances 



1 The internal structure of the oil-gland of the Swan has recently been accurately described by 

 Joli. Miiller, in his admirable work on the glands, ' De Glandularum Sccernentium Structura Penitiori,' 

 Lipsise, 1830, fol., p. 41, tab. ii, figs. 1 a, b, 



2 I say regularly, and by no means as a deformity, as in the case of the tailless Domestic Fowls, 

 in which alone, so far as I am aware, this deficiency of the oil-gland has hitherto been noticed. 

 See Reaumur, 'Art de Faire Eclore des Oiseaux Domestiques,' 1752, ii, p. 332; and Tiedemann's 

 ' Zoologic,' loc. cit. supra. 



