Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/745

ANATOMY.] BIRDS " In the genus Cinyxis, among the Chclonia, and in some species of Crocodilus (C. acutus, e.g.), the trachea is bent upon itself. Similar flexures attain an extraordinary development in many Birds, and may lie outside the thorax under the integument (Tetrao urogallus, some species of Crax and Penelope) ; in the cavity of the thorax (some Spoonbills) ; under the body of the sternum, in a large chamber hollowed out of the keel (some Swans and Cranes) ; even in a sort of cup formed by the median process of the t ureula (Numida cristata). In the Emeu some of the rings of the trachea are incomplete in front, and bound the aperture of an air-sac which lies in front of the trachea. Some Birds (Aptenodytes, Proccllaria) have the trachea divided by a longitudinal septum, as iu Sphnrgis among the Cliclonia. The tracheal tympanum is greatly enlarged in Ccphaloptcrus, and in many Ducks, Geese, and Divers ; and in these aquatic birds the enlargement is more marked in the males, and is usually symmetrical, the left side being generally the larger." " In Aves the lungs are firmly fixed on each side of the vertebral column, the dorsal surface of each lung being moulded to the super- jacent vertebra? and ribs. The muscular fibres of the diaphragm arise from the ribs outside the margins of the lungs, and form the vertebral column, and end in an aponeurosis upon the ventral surface of the lungs. Each bronchus enters its lung nearer the centre than the anterior edge, and, immediately losing its car tilaginous or bony rings, dilates, and then traverses the lung, gradually narrowing to the posterior edge of that viscus, where it terminates by opening into the posterior air-sac, which gene rally lies iu the abdomen. From the inner side of the bronchus canals are given off, one near its distal end, and others near its entrance into the lung, which pass directly to the ventral surface of the lung, and there open into other air-sacs. Of these there are four. Two, the anterior and the posterior thoracic, lie in the ventral face of the lung in the thorax. The other two are situated in front of its anterior end, and are extra-thoracic. The external and superior is the cervical ; the internal and inferior the inter-clavicular (Macg., vol. ii., p. 17, tig. 107). This last unites into one cavity with its fellow of the opposite lung. Thus there are altogether nine air-sacs ; two posterior or abdominal, four thoracic, two cervical, and one inter-clavicular. Other large canals given off from the bronchus do not end in air-sacs, but those which pass from the inner side of the bronchus run along the ventral surface, and those on the outer side along the dorsal surface of the lung. Here they give off at right angles a series of secondary canals, and these similarly emit still smaller tertiary canals, and thus the whole substance of the lung becomes inter-penetrated by tubuli, the walls of the finest of which are minutely sacculated. The different systems of tubuli are placed in communication by perforations in their walls. In most birds these air-sacs (except the anterior and posterior thoracic, which never communicate with any cavity but that of the lungs) are in communication with a more or less extensively ramified system of air passages, which may extend through a great many of the bones, and even give off subcutaneous sacs. Thus the inter- clavicular air-sac generally sends a prolongation into each axilla, which opens into the proximal end of the humerus, and causes the cavity of that bone to be full of air. When the sternum, the ribs, and the bones of the pectoral girdle are pneumatic, they also receive their air from the inter-clavicular air-sacs. The cervical air-sacs may send prolongations along the vertebral canai of each side ; which supply the bodies of the cervical vertebne, and communicate with elongated air-chambers in the spinal canal itself, ^^ 7 hen the dorsal vertebrae are pneumatic they communicate with the system of the cervical air-sacs. The abdominal air-sacs send prolongations above the kidneys to the sacral vertebrre and to the femora, whence these bones, when they are pneumatic, receive their air. The pulmonary air-sacs and their prolongations do not communicate with the air cavities of the skull, which receive their air from the tympana and the nasal chambers. In some Birds (Passerincc) the air is conducted from the tympanum to the articular piece of the mandible by a special bony tube, the siphonium [the largest of the tympanic chain, and having the general anatomical relations of the ichthyic interopercular. ] " RENAL AND REPRODUCTIVE ORGANS. The kidneys of Birds are composed of a number of lobules of unequal sizes, and these are packed in the concavities of the pelvis, in the same manner as the lungs are packed in the regular intercostal spaces of the upper part of the thorax. The ureters, as in the Reptiles, open directly into the cloaca ; but there is no urinary bladder. The bnrsa Fabricii opens into the cloaca above its hinder part. The testes He on each side the foremost lobes of the kidneys. They are very small in mid-winter, and largest by tli3 middle of April. In the embryo Bird there are two oviducts. " The duct of Miiller on the right side (that on the left side with the corresponding ovary generally dis appearing) remains in the female as the oviduct. In the male it is almost entirely obliterated on both sides " {Foster and Balfour, p. 168). INTEGUMENT AND FEATHERS. 1 " The exoskeleton of Birds consists almost entirely of epidermic structures in the form of horny sheaths, scales, plates, or feathers. No Bird possesses dermal ossifications, unless the spurs, which are developed upon the legs and wings of some species, may be regarded as such. The feathers are of various kinds. Those which exhibit the most complicated structure are called pennoe, or contour feathers, because they lie on the surface and determine the contour of the body. In every penna the following parts are to be distinguished : a main stem (scapus) form ing the axis of the feather, and divided into a proximal hollow cylinder, partly imbedded in a sac of the derm, called the calamus, or quill ; and a distal vexillum, or vane, consisting of a four-sided solid shaft, the rachis, which extends to the extremity of the feather, and bears a num ber of lateral processes, the barbs. The calamus has an inferior aperture (umbilicus inferior), into which the vascular pulp penetrates, and a superior aperture (umbilicus superior), situated on the under surface of the feather at the junction of the calamus with the scapus. The barbs are narrow plates, tapering to points at their free ends, and attached by their bases on each side of the rachis. The edges of these barbs are directed upwards and down wards, when the vexillum of the feather is horizontal. The interstices between the barbs are filled up by the barbules, pointed processes, which stand in the same relation to the barbs as the barbs do to the rachis. The barbules themselves may be laterally serrated and termi nated by little hooks, which interlock with the hooks of the opposed barbules. In very many Birds each quill bears two vexilla ; the second, called the after-shaft (hypo- rachis), being attached on the underside of the first close to the superior umbilicus. The after-shaft is generally much smaller than the chief vexillum ; but in some Birds, as the Casuariidw, the two are of equal size, or nearly so. Muscles pass from the adjacent integuments to the feather sac, and by their contraction erect the feather. The other kinds of feathers differ from the pennae, in having the barbs soft and free from one another, when they constitute pennoplumce, or plumulce (down), accord ing as the scapus is much or little developed. When the scapus is very long, and the vexillum very small or rudi mentary, the feather is termed a filopluma. The contour feathers are distributed evenly over the body only in a few Birds, as the Ratitce, the Penguins, and some others. Generally, the pennse are arranged in definitely circumscribed patches or bands, between which the integument is either bare or covered only with down. These series of contour feathers are termed pterylce, and their interspaces apteria. In some Birds, such as the Herons, plumulse of a peculiar kind, the summits of which break off into a fine dust or powder as fast as they are formed, are developed upon certain portions of the integument, which are termed poivder-doicn patches.- 1 This abstract is taken (by the author s permission) from Professor Huxley s Anatomy of Vertebrated Animals, pp. 274, 275. For a full account of these structures, see Nitzsch s Pterylography, translated from the German by Dr P. L. Sclater, F.R.S., Ray Soc., 1867. 2 See Bartlett, "On the Balaeniceps," Proc. Zool. Soc., March 26, 1861, pp. 1-4 ; and Murie " On the Dermal and Visceral Structures of the Kagu, Sun-Bittern, and Boat-Bill," Trans. Zool. Soc., 1871, plate 56, pp. 465-492 ; in this valuable paper the powder-down patches are

also shown in PodargusKnd Cacatwt.