COMMON PIGEON. 51 



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. 



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 booklets, each one of which catches hold of an 

 underlying proximal barbula. The vane thus acquires great solidity. The filo- 

 plumes are closely associated with the pennae from which they differ in having 

 a slender shaft with but a trace of the tube and a rudimentary vane composed 

 of a few barbs bearing simple and disunited barbules. In many birds an after- 

 shaft or hyporachis arises close to the superior umbilicus, and resembles when well 

 developed, e. g. in Ratitae or gallinaceous birds, a second feather. It varies much 

 in size and in the character of its vane, and is sometimes absent as in the Pigeon. 

 The distal barbules sometimes carry 'barbicels, structures which resemble the hook- 

 lets minus the terminal hook. 



Other varieties of feathers not found in the Pigeon are (i) the down-feathers 

 or plumulae, which lie beneath and between the contour-feathers, and have either 

 a simple soft rachis bearing soft barbules or a tube with a crown of soft bar- 



E 2 



