The intestinal canal extends from the gizzard to the cloaca, and is, comparatively speaking, short. The secretions of the liver and pancreas are poured into the small intestine as in Mammals. The commencement of the large intestine is almost always furnished with two long "caeca" or blind tubes, the length of which varies a good deal in different birds (fig. 326, k). They are sometimes wanting (Parrots, &c), or there may be only one; and their exact function is uncertain; though they are most probably connected partly with digestion and partly with excretion. The large intestine is always very short - seldom more than a tenth part of the length of the body - and it terminates in the "cloaca" (fig. 326, cl). This is a cavity which in all birds receives the termination of the rectum, the ducts of the generative organs, and the ureters; and serves, therefore, for the expulsion of the faeces, the generative products, and the urinary secretion.
Respiration is effected in Birds more completely and actively than in any other class of the Vertebrata, and as the result of this, their average temperature is also higher. This extensive development of the respiratory process is conditioned by the the fact that, in addition to true lungs, air is admitted into a greater or less number of the bones, and into a number of cavities - the so-called air-receptacles - which are distributed through various parts of the body, and which are present in all birds except the Apteryx. By this extensive penetration of air into various parts of the body, the aeration of the blood is effected not only in the lungs, but also over a greater or less extent of the systemic circulation as well; and hence in Birds this process attains its highest perfection. The cavities of the thorax and abdomen are not separated from one another by a complete partition, the diaphragm being mostly only present in a rudimentary form. The lungs are two in number, of a bright-red colour, and spongy texture. They are confined to the back of the thorax, extending along each side of the spine, from the second dorsal vertebra to the kidney. They differ from the lungs of the Mammals in not being freely suspended in a pleural membrane. The pleura, on the other hand, is reflected only over the anterior surface of the lungs. The -bronchi, or primary divisions of the windpipe (fig. 327), diminish in size as they pass through the lung, by giving off branches, which, in turn, give off the true air-vesicles of the lung. When the bronchial tubes reach the surface of the lung, they open, by a series of distinct apertures, into a series of "air-sacs." These are a series of membranous sacs formed by the continuation of the lining membrane of the bronchi, and supported by reflections of the serous membrane of the thora-cico-abdominal cavity. There are nine proper air-sacs - two abdominal (the only ones present in some birds, such as the Penguin), two in the hinder part of the thorax, two in the front part of the thorax, two on the sides of the neck, and one between the branches of the furculum. The air-cells not only greatly reduce the specific gravity of birds, and thus fit them for an aerial life, but also assist in the mechanical work of respiration, and must also greatly promote the aeration of the blood.
In connection with the air-receptacles, and as an extension of them, is a series pf cavities occupying the interior of a greater or less number of the bones, and also containing air. In young birds these air-cavities do not exist, and the bones are filled with marrow, as in the Mammals. The extent also to which the bones are "pneumatic" varies greatly in different birds. In the Penguin - which does not fly - all the bones contain marrow, and there are no air-cavities. In the large Running birds (Cursores), such as the Ostrich, the bones of the leg, pelvis, spine, ribs, skull, and sternum, are pneumatic; but the bones of the wings, with the exception of the scapular arch, are without air - cavities, and permanently retain their marrow. All birds which fly, with the singular exception of the Woodcock, have air admitted to the humerus. In the Pelican and Gannet, all the bones of the skeleton, except the phalanges of the toes, are penetrated by air; and in the Horn-bill even these are pneumatic. The functions discharged by the air-cavities of the bones appear to be much the same as those of the air-receptacles - namely, that of diminishing the specific gravity of the body and subserving the aeration of the blood.
Fig. 327. - Lung of Goose (after Owen). a Main bronchus dividing into secondary branches as it enters the lung, these giving off smaller branches, the openings of which are seen on the back of the bronchial tubes: b b Bristles passed from the bronchi through the apertures on the surface of the lung by which the bronchi communicate with the air-receptacles.
The heart in all Birds consists of four chambers, two auricles and two ventricles. The right auricle and ventricle, constituting the right side of the heart, are wholly concerned with the pulmonary circulation; the left auricle and ventricle, forming the left side of the heart, are altogether occupied with the systemic circulation; and no communication normally exists in adult life between the two sides of the heart. In all essential details, both as regards the structure of the heart itself and the course taken by the circulating fluid, Birds agree with Mammals. The venous blood - namely, that which has circulated through the body - is returned by the venae cavae to the right auricle, whence it is poured into the right ventricle. The right ventricle propels it through the pulmonary artery to the lungs, where it is aerated, and becomes arterial. It is then sent back by the pulmonary veins to the left auricle, whence it is driven into the left ventricle. Finally, the left ventricle propels the aerated blood to all parts of the body through the great systemic aorta.