4 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 Mycteria) 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- 
ing 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, which enter more or less directly from the lungs into the 
vertebra 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 Jasseres, of Swifts, Divers, Rails, the Kiwi, 
and of Terns, are solid, and air-cells are restricted chiefly to the 
cranium, many parts of the skeleton of the large fatitw 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 
