The Air-sacs in Birds

In most birds, the air-sacs (except the anterior and posterior thoracic, which never communicate with any cavity but that of the lungs) are in connection with a more or less extensively ramified system of air-passages, which may extend through a great many of the bones, and even give off subcutaneous sacs. Thus the interclavicular air-sac generally sends a prolongation into each axilla, which opens into the proximal end of the humerus, and causes the cavity of that bone to be full of air. When the sternum, the ribs, and the bones of the pectoral girdle, are pneumatic, they also receive their air from the interclavicular air-sacs. The cervical air-sacs may send prolongations along the vertebral canal of each side, which supply the bodies of the cervical vertebrae, and communicate with elongated air-chambers in the spinal canal itself. When the dorsal vertebrae are pneumatic, they communicate with the system of the cervical air-sacs. The abdominal air-sacs send prolongations above the kidneys to the sacral vertebrae and to the femora, whence these bones, when they are pneumatic, receive their air.

The pulmonary air-saos and their prolongations do not communicate with the air-cavities of the skull, which receive their air from the tympana and the nasal chambers. In some birds, the air is conducted from the tympanum to the articular piece of the mandible by a special bony tube, the siphonium.

In all Sauropsida, the ureters open directly into the cloaca, which is provided with a urinary bladder in the Lacertilia and the Chelonia, but not in other Reptilia, nor in Aves.

Organs of copulation present themselves under three forms:
1. In the Chelonia, the Crocodilia, and the Ostrich, a simple solid penis, grooved upon its posterior aspect, is attached to the anterior wall of the cloaca, and contains erectile tissue. In the ostrich this penis lies in a sac of the cloaca, into which it can be retracted somewhat as in the Menotremata.

2. In many birds, such as the Rheidae, Casuaridae, Apterygidre, Tinamomorphae; Penelope, and Crax, among the Alectoromorphae; and in many aquatic birds, there is also a single penis attached to the front wall of the cloaca, grooved on its dorsal side, and supported by two fibrous bodies coated with more or less erectile tissue. But the distal end of the penis is invaginated, and the involution held in this position, except during erection, by an elastic ligament.

3. In Lacertilia and Ophidia, two copulatory organs are developed at the sides of tne cloaca. The integument is prolonged inward, on each side, into a blind sac, which lies upon the inferior caudal muscles. The inner surface is often armed with spiny developments of the epidermis, and presents a groove, which is continued on the parietes of the cloaca to the aperture of the vas deferens. The wall of the blind sac contains erectile tissue, and it can be erected or retracted by appropriate muscles.