In addition to the points already noted in the heart, the right auricle is larger than the left: the left ventricle is of great size, with strong muscular walls, whilst the right has thin walls and embraces the left: the left auri-culo-ventricular valve consists of two membranous flaps with attached chordae tendineae and musculi papillares. The fourth aortic arch on the left side is converted into the innominate artery, instead of forming a second or left systemic aorta as in Reptilia, though its homology with this latter vessel is shown in many birds, especially Accipitrine, by the retention of a compact fibrous prolongation onwards to the dorsal aorta. The aorta appears to divide close to its origin into three great trunks, as it gives off two sub-equal innominate arteries. In birds of powerful flight these arteries are often of larger calibre than the aorta itself. The two carotids run up the ventral aspect of the neck and are sometimes united for a portion of their course, or the aortic origin of one of the two may be lost. There are various arterial plexuses, and one especially is notable on the surface of the abdomen which enlarges during the period of incubation.
There are as a rule two superior venae cavae, but the cardiac extremity of the left is sometimes much reduced or even obliterated. They open separately from each other and from the vena cava inferior into the right auricle, the sinus venosus of Reptilia having disappeared by absorption into the auricle. The two jugular veins are united under the base of the skull by an anastomotic vein. The vena cava inferior is formed by the confluence of the iliac veins. These veins are in their turn formed by the union of the femoral veins with the two efferent renal veins (see p. 56). Lymphatic glands are present, few however in number. A pair of lymph-hearts with striated muscle fibres in their walls are found in the pelvic region of the Ostrich, Goose, Swan, and Stork, and also in the embryo chick in which they subsequently disappear. The valves of the lymphatics are not so numerous as in Mammalia.
The nostrils lie at the tip of the beak in Apteryx: in other birds at its base in front of the orbits. In the Gannets (Sula) they are closed. The larynx consists of two arytenoid cartilages and a cricoid ring. The trachea is always of considerable length: its cartilaginous rings are usually perfect and at least partially ossified: it is often tortuous, forming convolutions beneath the skin and sometimes lodged in the keel of the sternum, e.g. in the Swan: it may be dilated at intervals and its cavity is divided by a longitudinal septum in Aptenodytes and Procellaria (a Penguin and a Petrel). In most birds including the Ratitae (see p. 55) a lower larynx or syrinx is developed at the junction of the trachea with the bronchi. It is produced by modifications of some of the lower tracheal rings only (some American Passeres = Coracomorphae), of the upper bronchial rings only (Steatornis among Caprimulgidae or Goatsuckers, and Crotophaga, a Cuckoo, both from S. America), or, as most usual, of both tracheal and bronchial rings. It has often a complicated structure, and in many singing birds six intrinsic pairs of muscles.
The voice is caused by the varying tensions of an internal and external tympaniform membrane developed, the former on the inner aspect of the incomplete upper bronchial rings, the latter between two of the same rings. In the males of many Ducks the lower tracheal rings are dilated asymmetrically into a resonant cavity with walls partly membranous, partly ossified. The bronchi lose their cartilaginous rings to a great extent when they enter the lung. There are four eparterial bronchia, and usually nine hyparterial. Of the former, the first winds round the trachea and branches in a dorsal and ventral direction: the fourth is rudimentary: but the second and third run towards the inner border of the lungs parallel to one another. The hyparterial bronchia branch on the outer aspect of the lungs dorsally and ventrally. The lungs are fixed to the back of the thorax and are deeply indented by the ribs but not otherwise lobed. The pleura cover only their ventral surfaces on to which pass from the ribs small muscular bundles. The ultimate branches of the bronchia are tubes more or less hexagonal, and containing transverse and longitudinal ridges upon which the respiratory capillaries are distributed.
Certain of the chief bronchia traverse the lungs and open at their surface into air-sacs. Of these there are nine (see PP. 55-6). Processes are prolonged from the anterior and posterior air-sacs into the bones, with the exception of those of the head, which are supplied with air from the nasal fossae and the tympanic cavity. The bones of the head are sometimes, as in Mammalia, the only pneumatic bones: the vertebrae, sternum and humerus come next; whilst in a few birds, e. g. the Toucan, all the bones of the body are stated to be pneumatic. The bones of the fore-arm, of the lower leg, of the hand and foot often retain their medulla. In some instances processes of the air-sacs are prolonged between the muscles and under the skin, e. g. in the Gannet (Sula). The ciliated epithelium of the air passages is replaced by pavement epithelium in the air-sacs and their extensions.
The kidneys are divided into three lobes which fit into the fossae of the ilia. The glands often meet and even fuse posteriorly, and their ventral surface is often marked by the intestines. The ureter arises from the ventral surface, is dilated at its origin, and opens into the cloaca on a papilla placed to the inner side and a little in front of the genital papilla.