50 DESCRIPTIONS OF PREPARATIONS. 



difference is furnished by the prolongation of the lung, by means of its 

 bronchial stem and branches, into air-cells permeating a very large part 

 of the entire body. The largest of these receptacles are the infra-renally 

 placed 'abdominal air-sacs/ the right one of which extends from the 

 posterior border of the lung above and behind the liver, so as, firstly, 

 to interpose itself between the inferior surface of the kidney and the 

 intestines, and, secondly, to stretch beyond the region of the kidney 

 into that of the rectum. The kidney, like the lung of Birds, is shaped 

 conformably with the bones supporting it ; and it is divisible here into 

 three lobes, increasing in size from before backwards in correspondence 

 with the iliac and pelvic surfaces in relation with them. 



The division of the kidney into three lobes is better marked when 

 seen as here from the side than from in front. Even from in front however 

 the anterior lobe may be seen to be more or less limited off from the 

 middle lobe by the great vein from the lower extremity which corresponds 

 to the external iliac of mammals ; and the middle lobe in its turn to be 

 limited off from the posterior by the chief artery of the lower extremity, 

 which is in most Birds, as here, the sciatic, not the external iliac artery. 

 The sciatic artery gives off branches to the two posterior lobes of the 

 kidney, and an arteria renalis superior arising from the aorta mainly 

 supplies the anterior lobe. The veins from the lower limbs are supposed, 

 and with considerable probability, to act as a renal-portal or inferent 

 system, as in the cold-blooded Sauropsida (see infra, p. 56). 



The (rectrices) feathers having been removed from the caudal tract, the 

 anal oil-gland (glandula uropygii) is brought into view 1 . Its duct projects 

 freely, and is apically biperforate and tongue-shaped. It has no circlet 

 of feathers, differing herein from that of the Fowls, the Diurnal Raptores, 

 and all Aquatic Birds, and resembling that of the Nocturnal Raptores and 

 all Passerines. Its outlines pass gradually into those of the bilobed gland 

 mass, and with them make up a heart-shaped contour, the transverse axis 

 of which is somewhat shorter than its anteroposterior. Though the Pigeon 

 resembles the Passeres in the absence of feathers round this duct and upon 

 the skin covering the oil-gland, and herein as in some other particulars 

 comes nearer to that order than do the Gallinaceous Birds, the oil-gland 

 and duct of the Passeres are nevertheless sufficiently distinctive to enable 

 us to distinguish a specimen of the order from one of any other, even in 

 the absence, not merely of the head, but also of the feathers of the whole 

 body. These distinctive characters are the predominance of the transverse 

 over the anteroposterior diameter of the gland, the shortness and apical 

 bluntness of its efferent duct, the thinness of its walls, and the distinctness 



1 For a full account of the uropygial gland, see Nitzsch's Pterylography, Ray Soc. Trans. 1867, 

 pp. 38-42. For the correlations of this gland with certain other structural peculiarities, see Garrod, 

 P. Z. S. 1874, p. 118. 



