_The Abdominal Air-sacs._—The two abdominal air-sacs located on each side of the abdominal cavity, when inflated with air, form enormous bladder-like structures. They are related laterally with the abdominal wall and internally with the abdominal viscera. The anterior extremities are in communication with the mesobronchi and are somewhat inflected to pass under the fibrous arches extending from the spine to the pelvis. Anteriorly these sacs adjoin the diaphragm, the testes in the male, and ovary in the female, and to the parietes of the abdomen and those of the pelvis. Below and in front, they rest on a fibrous septum, which in all birds divides the abdominal cavity into two smaller cavities: one anterior, representing the abdomen and containing the liver; the other posterior, representing the pelvis and containing the gizzard and the intestines. The anterior portion overlies the posterior part of the lobes of the liver, the proventriculus, the spleen, and the gizzard. The kidneys are located above these air-sacs. Dorsal to the sacs is also a part of the intestines and in the female the oviduct. The abdominal air-sacs are attached by a ligament-like structure in their medial, their anterior, and their lateral margin. The posterior, the dorsal, and the ventral margin are free. Mesially this attachment is to the mesentery, connecting the left cæcum to the dorsal margin of the gizzard, and also to the mesentery of the proventriculus. The anterior attachment is to the body wall and extends in front of the end of the ovary and the adrenal glands. At the antero-lateral part of the body cavity the attachment extends in a widening band along the lateral side of the ovary and of the oviduct, as far back as the caudal margin of the sac. The lateral attachment is related to the kidney, the dorsal ligament of the oviduct, and the abdominal wall. Each of these abdominal sacs has three extensions: one suprarenal and two femoral. The _suprarenal extension_ leaves the principal sac at the postero-external part of the kidney, extends upward, and forward, and expands over the surface of the kidney. At the internal border of the kidney, this prolongation extends between the transverse processes of the sacral vertebræ, reaches a height of the first dorsal vertebra, forms a triangular canal located above the sacrum in the sacral channel, and is separated from its fellow by a series of corresponding spinous processes. The two _femoral extensions_, an anterior, small, and a posterior, large, extend from the abdominal air-sac at the cotyloid cavity, leave the pelvis through the bony passage occupied by the crural vessels, extend around the coxo-femoral articulation, and terminate in a blind extremity. In some birds, particularly in birds of prey and ostriches, there are prolongations extending into the femur, entering through a foramen at the anterior part of the great trochanter.