AIR-SACKS 



in the Duck, which in the Fowls, Gulls, Gannets, and some others, 

 communicates with the next pair. In the Stork, Flamingo, and 

 Screamer each sac is elongated and divided into numerous smaller 

 cells. Frequently these sacs extend far up the neck, even into the 

 head, and small side branches may enter any of the neighbouring 

 organs, such as the inside of the vertebrae, the carotid and vertebral 

 canals, the cervical muscles, the cranial cavities, and others. Some- 

 times they form large inflatable sacs on the throat, as, for instance, 

 in the Prairie-fowls. 



2. A pair of subbronchial or interclavicular sacs. They are 

 united into one sac in Storks, communicate with each other in Ducks, 

 are subdivided into a number of smaller sacs in the Swan and in the 

 Screamer : in Vultures they take the large crop between them. 

 Lateral extensions accompany the large blood-vessels and form axil- 

 lary cells penetrating ultimately the humerus and other bones of the 

 wing ; other secondary cells penetrate the large pectoral muscles 

 {e.g. in Myderia) or enter the body and the keel of the sternum. 



3, 4. A pair of anterior and posterior intermediate sacs, 

 extending more or less far into the abdominal cavity, covering 

 chiefly the lower portions of the lungs and the liver, occasionally 

 subdivided, being filled through several openings at the external 

 edge of the lungs, and 'sometimes continued into the lateral parts of 

 the sternum. 



5. A pair of abdominal sacs. These are the largest, extend- 

 ins: with irregular subdivisions between the intestines into the 

 pelvis, and penetrating the femur together with the rest of the bones 

 of the sacrum, and the legs. 



Besides these principal air-sacs, there exist numerous smaller 

 cells, Avhich enter more or less directly from the lungs into the 

 vertebrae and ribs, between the muscles, underneath the skin and 

 other parts, thus making the skeleton, and sometimes the greater 

 part of the body, pneumatic. The air-sacs do not enter the bones 

 before a considerable portion of the marrow has been absorbed , 

 an extremely small hole in the bone is sufficient for their entrance ; 

 the cavity of hollow bones is ultimately lined with the thin mem- 

 brane of the air-sac. Generally the skeleton is most pneumatic in 

 large birds that fly well, like Vultures, Storks, Swans, Pelicans ; 

 less so in small birds, and least in heavy or little-flying water-birds. 

 However, there are many exceptions. While, for instance, most 

 of the bones of many Passeres, of Swifts, Divers, Eails, the Kiwi, 

 and of Terns, are solid, and air-cells are restricted chiefly to the 

 cranium, many parts of the skeleton of the large liatitx are very 

 pneumatic. 



The greatest development of pneumatic cells exists in the 

 Screamers and Hornbills, in which even the fingers and toes, in fact, 

 any part of the skeleton, are hollow, and large subcutaneous air-sacs 



