In the predaceous birds, as the eagles (fig . 48), the oesophagus (a), the crop (b), the infundibu lum (e), and the gizzard (de), are capacious, thin, and membranous, and form a continuous cavity for the prey, from which the indigestible parts can be thrown out in a bolus. In these birds the cceca-coli (g) are very small, sometimes unequal, or wanting. The urinary (ii) and genital organs (kk) enter the cloaca (/) near the anus. The right ventricle of birds has the tricuspid valve in form of a thick strong mus cular fold, and the aorta descends on the right side. The lungs form two undivided, light coloured lobes, fixed by pleurae to the back part of the trunk, the last rings of the trachea form an inferior larynx, the bronchi pass in a mem branous form through the lungs, and the lungs open into large membranOus abdominal air cells, which communicate with the interior of the bones. This extensive aeration of their systemic as well as their pulmonic vessels gives energy to their muscles for their aerial life and their distant migrations, and a high tempera ture to their body for the incubation of the egg. Their plumage and their downy covering are the best suited for their aerial life and their high internal heat. Their organs of generation are double and symmetrical in the male, and 8 generally unsymmetrical in their development in the female. The testes are internal, and the vasa deferentia terminate in the cloaca, where there is sometimes a grooved organ of intro mission. In the female the left ovary and oviduct are developed, the right for the most part atrophiated and useless. The cavity of the cloaca in most birds, as seen in that of the great condor of the Andes (fig. 49), receives the end of the rectum (a), which forms a wide rectal vestibule (b): beneath this lies the part analogous to the urinary bladder (c d). Lower than the urinary sac are found the two openings of the ureters (h h), with the pervious oviduct on the ,left side (f), and the remains of the impervious oviduct (g) on the right side. The bursa Fabricii and the clitoris (when present) are placed more posteriorly in the preputial cavity. The most distinct forms of these gene rative and urinary parts, and the nearest ap proach to the mammalia are seen in the cloaca of the ostrich (fig. 50), where the rectum (a) opens into a wide and distinct rectal vestibule (b), which extends into a large urinary bladder (d). Beneath the urinary bladder is the ure thro-sexual canal (e), into which the two ureters are impregnated internally, their chorion is calcified, and their development is effected by incubation. (See Av Es.)
23. Mammalia, warm and red-blooded ver tebrata, having four cavities of the heart, with a viviparous mode of generation, and possessing mammary glands ; with the lungs free in a distinct thoracic cavity, and generally having the body more or less covered with hair. The bodies of their vertebne unite by flat surfaces, the tympanic bone is fixed, the jaws are gene rally furnished with teeth lodged in deep alveoli, the coracoid bone rarely reaches the sternum, and the posterior extremities, when present, are always attached by the pelvic arch to a solid sacrum. The thoracic and abdominal cavities are separated by a muscular diaphragm. The hemispheres of the brain contain large ventri cles, and rarely want convolutions, the optic lobes are small, concealed, solid, and divided by a transverse sulcus, the commissures of the brain and cerebellum, and the hemispheres of the cerebellum are large. The alimentary canal is of great length, the colon long and wide, with a single ccecum, and sometimes with a vermiform appendix, and the anal open ing is generally distinct from the urinary and genital passages. The tricuspid valve is thin and membranous, the aorta descends on the left side, there is no inferior larynx, the epi glottis is distinct, and the bronchi continue cartilaginous into their ramifications in the lungs. The lungs, generally divided into lobes, move freeely in a distinct thoracic cavity, and have no abdominal cells or perforations on their surface, as in birds. There is always a urinary bladder, and the urethra in the male passes through a tubular penis. The organs of gene ration are double in both sexes, symmetrical in the male, and rarely unsymmetrical, in the female. The oviducts commonly unite at their lower part to form a uterus, in which the ovum becomes again connected with the parent, and is hatched. There are mammary glands open ing externally for lactation during the helpless condition of the young. (See MAMMALIA .) These are the PRIMARY and SECONDARY DIVISIONS of the ANIMAL KINGDOM, the struc ture,. classification, and history of which it is proposed to consider in this Cyclopmdia, under the heads of the several classes as enumerated in the subjoined table.
h le) and the oviducts g) open towards the dorsal and lateral part. The putial cavity (i) is the terminal portion in which the distinct clitoris is here lodged. The ova