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 (1) 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 barbules; and (2) semiplumes (penno-plumae) which have a stiff rachis and soft barbs and lie at the outer margin of the pterylae and are covered by the contour feathers. In a few birds down-feathers exist which grow persistently and break off at the apex; whilst a powder or dust is poured out of the follicle lodging the tube. They are known as powder-down feathers, and either occur scattered all over the body - e. g. in some Parrots, or restricted to limited tracts - e. g. in Ardea.
The tuft of feathers springing from the pollex constitutes the bastard wing. The row of large wing-feathers is termed remiges, and is divisible into a set of primaries and of secondaries attached, the former along the manus, the latter along the ulna. They are covered above and below by the upper and lower wing coverts. The large tail feathers are known as rectrices, and they are covered above and below by the tail coverts. The feathers are not implanted irregularly into the body but along certain tracts or pterylae between which are bare spaces or apteria.
The following special points should be noted in the internal anatomy. In the central nervous system the small olfactory lobes; the cerebral hemispheres pointed in front, broad behind, showing in horizontal section a huge corpus striatum, a lateral ventricle reduced to a narrow chink and thin internal and posterior walls: a small pineal gland, reverted, with walls composed chiefly of fibrous tissue, and its extremity attached to the dura mater: the solid optic lobes widely separate in the middle line where the cerebellum touches the cerebral hemispheres: the cerebellum composed of a large median lobe and two small lateral floccular lobes, the median lobe showing in longitudinal section an arbor vitae as in Mammalia: the well-marked angle between the medulla oblongata and spinal cord. The latter has a large lumbar swelling, in which a mass of neuroglia or substantia reticularis lies immediately dorsal to the central canal, the posterior fissure is widely open and the gap filled by a gelatinous tissue derived from the pia mater. This lumbar swelling was of immense size in the extinct Stegosaurus (Deinosauria). The spinal cord ends with a filum terminale and the posterior nerves form a cauda equina.
The sympathetic system is double in the neck: one part accompanies the vertebral artery and vein and is lodged in the vertebrarterial canal: the other accompanies the carotid arteries on the ventral aspect of the neck. The two parts are connected.
The structure of the eye is peculiar in some points. The sclerotic coat has an anterior conical portion containing a ring of bones, and a posterior spheroidal portion. A pigmented vascular fold of membrane - the pecten - runs obliquely forwards from the entrance of the optic nerve and projects into the vitreous humour. The line of attachment marks the position of the embryonic choroidal fissure. Its capillaries are continuous with those of the optic nerve, and not of the choroid, and are contained within lymphatic sheaths. The nictitating membrane is moved by two special muscles - a quadratus or bursalis, and a pyrami-dalis, which lie at .the back of the eye and take origin from the sclerotic. The former is a square muscle ending in a tendinous border, but the tendon is tubular. Through the tube runs the cord-like tendon of the pyramidalis which is inserted into the lower angle or edge of the nictitating membrane. When the pyramidalis contracts, its tendon is prevented from pressing on the optic nerve over which it runs by the simultaneous contraction of the quadratus.
There is a well-developed Harderian gland for the third eye-lid lying below the eye-ball.
The tongue is of fair size. It has been discovered by Fraisse (Z. A. iv. 1881, p. 310), that in the embryo Duck there are embryonic feathers developed on the tongue which are arrested in development. The crop in the Pigeon is remarkable for its large size and bilateral symmetry. Gadow has distinguished between a true crop with glandular walls and a 'Haut' or 'Schlund' crop with non-glandular walls. The former exercises a chemical action on the food and occurs in the Fowl and Pigeon and in their congeners; whilst the latter exercises no such action and is simply a storehouse for food swallowed, e. g. in many Ducks, Cassowary, etc. But the researches of Hasse (Zeitschrift fur Rationelle Medizin, xxiii. 1865) proved long ago that the upper part of the oesophagus and the crop itself are non-glandular in the Pigeon, whereas the portion of the oesophagus below the crop, like the proventriculus, is provided with glands. The crop, and the upper as well as the lower part of the oesophagus, are lined by a many-layered epithelium - the lower cells of which are granular and plump, but as they pass to the surface become flattened out yet not cornified. If the surface of the crop is scraped a small amount of a whitish liquid can be collected at all times.
The amount is greatly increased in both sexes for about the first eight days after the hatching of the young, which are fed with the so-called 'pigeon's milk' regurgitated by the parent bird into the mouth of the young. It is a milky liquid containing cheese-like solid morsels. Hasse found that at this time the epithelium of the upper parts of the oesophagus and of the crop, but especially its side parts was much thickened, the bloodvessels dilated and full of blood. He also found that the cells of the epithelium undergo rapid division: are granular, and contain abundant fatty granules: that they are set free in masses which break down partially. The remnants form the cheese-like morsels, whilst the fatty cells set free give the liquid a milky look. The cells in the masses retain their nuclei, those set free have either lost them or show them undergoing fatty degeneration. The 'milk' collects within the crop whence it is expelled by the action of two muscles which spring from the upper part of the clavicles and are inserted into the skin ven-trally. The physiological properties of the fluid do not appear to have been fully investigated. It is doubtful, perhaps, whether the small amount of it present at times other than the breeding season has any chemical effect on the food.