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.
1 This is the usually accepted statement: but see p. 53, infra.
2 Gulliver, P. Z. S. 1842, 1869, 1870; Leydig, Fische und Reptilien, 1853, p. 41; Histologic, 1857, p. 324.
3 There are two carotids in all Columbae and in the majority of non-passerine birds, but in no true Passerines.
1 Upon the differences observable in the number and character of these coils Dr. Hans Gadow has based a classification of Birds in two memoirs containing much valuable information, J. Z. xiii. 1879.
2 That however different at first sight the topographical relations of the lungs, liver, and heart may seem to be in Birds and Mammals respectively, they nevertheless are not essentially dissimilar, may be seen from the fact that the technical works on the physical examination of the heart in man speak of the 'difficulty of separating the adjacent edges of the heart and liver' by percussion, and of the dulness produced by the apex of the heart being indistinguishable from that produced by the convex surface of the liver below it. See e.g. French's Diseases of the Liver, New Sydenham Soc. Trans. 1860, p. 30; Walshe, Diseases of the Heart, 1862, p. 42.
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 uropygit) is brought into view1. 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 of its contour lines from those of the gland itself. The oil-gland of the Lamellirostres figured by C. G. Carus, Tab. Anat. Comp. Illust. Pars vii. Tab. 7. fig. 5, furnishes us with a sharp contrast to that of the Pigeon and the Passeres in being very deeply bilobed and in having its anteroposterior axis much longer than its transverse. The Ostrich, Emeu, Cassowary, and Apteryx agree with the cursorial Bustard in lacking this gland. Its presence is not constant in all the species of either Columbae or Psittacidae. It is larger in size in Birds of aquatic than in those of other habits.
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.
In all Birds, and in no other class of animals, will the same description as that given here apply to the nerve-system, to the relations of the muscles of the anterior limb, and to the relations of the aorta to the right bronchus. The peculiarities of the pancreas and duodenum are probably nearly equally distinctive. The crop and the uropygial gland are peculiar to, though not universally found in Birds; but the presence or absence of these two latter structures is explicable probably by reference to the special habits or special needs of the species possessing or lacking them, and is therefore of physiological rather than of morphological importance.
The epidermic skeleton consists, as in all birds, of the horny covering of the bill, of claws to the toes, and scales covering the metatarsalia and toes, and of feathers. Of the latter, the Pigeon possesses two kinds, contour feathers or pennae, and filo-plumes - down-feathers or plumulae being absent. Every penna consists of the following parts: a central axis shaft or scapus divisible into a proximal hollow tube or calamus, and a distal solid white shaft or rachis: of barbs borne upon the rachis and bearing in their turn barbules. Rachis and barbs together make up the vane or vexillum. The calamus is implanted in a follicle of the skin to which small muscular bundles are connected. It has a proximal aperture or inferior umbilicus, and a distal, the superior umbilicus, at its junction with the rachis. The barbules are implanted on the proximal and distal surfaces of the barb forming two series of process pointing obliquely towards the edge of the vane. The distal series of barbules of one barb overlaps the proximal series of the barb beyond, i. e. nearer the tip of the feather. The distal barbules bear on their under surfaces microscopic hooklets, each one of which catches hold of an underlying proximal barbula. The vane thus acquires great solidity.