The respiratory system has the usual structure seen in birds. The syrinx or lower larynx is simple. The last 3-4 rings of the trachea send towards one another on the ventral surface processes which do not fuse. The two last rings are widely separated dorsally, but they are joined inter se by a cartilaginous rod. The bronchial rings are only half-rings, i. e. are incomplete on the inner surface of the bronchus. The free ends of the two first rings meet dorsally and ventrally, and together with the modified tracheal rings form the tympanum. A thin membrane - the membrana tympaniformis interna - forms the inner wall of the anterior part of each bronchus where the ends of the half-rings are wide apart. Inside the tympanum the mucous membrane is thickened, and there are two cushion-like projections, one on each side. A fold, the membrana semilunaris, projects forwards between the cushions from the point of bifurcation of the trachea. It is supported by the pessulus, a rod of cartilage, sometimes ossified. The Pigeon has but one pair of syringeal muscles - the tracheales laterales.
The syrinx is least developed in the Cathartidae, or American Vultures. It is a distinctive avian structure, and is present in the Ratitae, where it is often said to be absent, and it is best developed in Rhea. The trachea is tied to the sternum by a pair of muscles - the sterno-tracheales.
The structure of the lungs, bronchi and air-sacs, is very similar in all birds. The main bronchus enters the lung, gives off above the spot where the pulmonary artery crosses it four eparterial bronchi and below that spot nine hyparterial. The ultimate branches of the bronchi are long tubes closely packed. Their inner walls carry projecting circular fibrous septa connected by longitudinal septa. The respiratory capillaries are distributed on these septa. An epithelium lining these tubes has not been detected. Other structures to be carefully noted are the costo-pleural muscles, small muscular bundles which spring from the junction of the vertebral and sternal ribs and spread out like a fan in an aponeurosis (i. e. tendinous expansion) which lies on the ventral aspect of the lung between the pleura and the air-sacs. The air-sacs appended to certain bronchi are nine in number and are as follows, - an interclavicular sac lying in front of the trachea and formed by the fusion of two sub-bronchial sacs, the bronchial apertures of which lie just anterior to the spot where the bronchi enter the lungs; two prae-bronchial sacs lying one dorsal to each lobe of the interclavicular sac, with apertures at the anterior border of the lung: two anterior intermediate or thoracic sacs with apertures close behind the point of exit of the pulmonary veins: two posterior intermediate sacs lying one on each side in front of the corresponding posterior or abdominal sacs, with apertures at the postero-external angles of the lungs.
The walls of the sacs are thin, and lined by a pavement epithelium, the ciliated epithelium of the bronchi ending at the spot where they open into the sacs.
The following additional points should be noted in the heart. A thin muscular Eustachian valve protects the entrance of the vena cava inferior. The right auriculo-ventricular valve consists of two muscular flaps, one long and external, the other short, meeting the first at a slight angle and connected to the ventricular wall at the base of the conus of origin of the pulmonary artery, which is long. The left auriculo-ventricular or mitral valve consists of two membranous flaps with chordae tendineae and musculi papillares as in a Mammal. A transverse section across the ventricles shows a very thick walled left ventricle with a thinner walled right ventricle, the cavity of which is crescentic, embracing the left ventricle. Ornitho-rhynchus alone among Mammals shows a similar section. Three semilunar valves guard the base of the aorta and pulmonary artery. The common carotids run up the demi-canal on the ventral aspect of the cervical vertebrae, hidden by the muscles. The jugulars are connected by an anastomosis across the base of the skull, and there is a remarkable subcutaneous venous plexus in the neck. The connections of the veins of the kidney and of the hind limb are characteristic.
The femoral vein coming from the hind limb passes through the kidney between its anterior and middle lobes. Close to the internal border of the kidney it receives on its anterior border the efferent vein of the anterior lobe: on its posterior of the middle and posterior lobes. It then passes on as the common iliac vein, and, joining its fellow, becomes vena cava inferior. Towards the outer border of the kidney the femoral vein gives off an afferent renal vein, which enters and ramifies in the anterior lobe of the kidney; and a second - the renal portal, or hypogastric vein - which traverses the middle and posterior lobes of the kidney, gives off branches in its course, receives the sciatic vein, and then issues from the posterior lobe of the kidney and unites with its fellow. Into the point of union falls the caudal vein: from it issues a large coccygeo-mesenteric vein, which receives veins from the cloaca and large intestine, runs along the mesentery of the large intestine, and joins the portal system. Each lobe of the kidney receives a small artery.
As pointed out by Jourdain, the calibre of the veins is out of all proportion to that of the arteries. . He states that each renal lobule contains a central vein, or rootlet of the efferent vein, and is surrounded by a number of venules derived from the afferent vein. A capillary network connects the two systems of vessels. As there are no valves in the renal portal veins, with rare exceptions, such as certain Ratitae, Bustard, Swan, the blood from the viscera and hind limbs can pass freely either through the iliac veins and thence to the vena cava, or through the coccygeo-mesenteric vein to the hepatic portal system. It can hardly be doubted from these facts that the kidney, as in Reptilia, receives venous blood.